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International Journal of
Molecular Sciences
Review
The Chemistry of the Ketogenic Diet: Updates and
Opportunities in Organic Synthesis
Michael Scott Williams * and Edward Turos *
Department of Chemistry, University of South Florida, Tampa, FL 33620, USA
* Correspondence: mswilliams@usf.edu (M.S.W.); eturos@usf.edu (E.T.)
Abstract: The high-fat, low-carbohydrate (ketogenic) diet has grown in popularity in the last decade
as a weight loss tool. Research into the diet’s effects on the body have revealed a variety of other
health benefits. The use of exogenous ketone supplements to confer the benefits of the diet without
strict adherence to it represents an exciting new area of focus. Synthetic ketogenic compounds are of
particular interest that has received very little emphasis and is an untapped area of focus for chemical
synthesis. In this review, we summarize the chemical basis for ketogenicity and opportunities for
further advancement of the field.
Keywords: ketogenic diet; ketosis; keto; epilepsy; exogenous ketone supplements; acetoacetate;
β-hydroxybutyrate
1. Introduction
The ketogenic diet, a high-fat, low-carbohydrate diet, has a long history of use begin-
ning primarily as a treatment option for epilepsy [1,2]. However, it is only in the last few
Citation: Williams, M.S.; Turos, E. decades that the diet has been popularized among the general public. Renewed clinical
The Chemistry of the Ketogenic Diet: interest in the diet and its emerging popularity as a weight-loss tool have led to a larger
Updates and Opportunities in scope of research into the diet’s effects on the body and the discovery of a broad range of
Organic Synthesis. Int. J. Mol. Sci. physical, biochemical, and cosmetic benefits.
2021, 22, 5230. https://doi.org/
Even with expanding research interest in the ketogenic diet, most prior reported
10.3390/ijms22105230
studies have been done in clinical settings or in biology labs [3,4]. Emerging research
shows that certain ketogenic supplements can confer some of the benefits of the ketogenic
Academic Editor: Roberto Bei
diet while following a normal diet [5,6]. Difficulty in adherence to the ketogenic diet is
often cited as a reason why it is abandoned due to limited food options. If these obstacles
Received: 29 April 2021
Accepted: 12 May 2021
can be overcome, a wide array of possibilities may open up. Yet, very little work has
Published: 15 May 2021
been done so far in expanding the options to do this, particularly from the chemistry side.
This review will cover the history and health-related aspects of the ketogenic diet and its
Publisher’s Note: MDPI stays neutral
biochemical basis as well as highlight some exciting opportunities in organic synthesis to
with regard to jurisdictional claims in
devise routes to potential new ketogenic compounds as components or supplements of the
published maps and institutional affil- ketogenic diet.
iations.
2. History of the Ketogenic Diet
Dietary fasting has been a common societal practice in various religions for thousands
of years, and as a treatment for epilepsy for over a century. Much of the early reported
Copyright: © 2021 by the authors.
information is based on the personal experience of cultists and physicians instead of
Licensee MDPI, Basel, Switzerland.
scientifically motivated clinical trials. In 1911, two French physicians Guelpa and Marie
This article is an open access article
published a report detailing their use of fasting to treat epileptic seizures, in which six of
distributed under the terms and their 26 patients showed a reduction in the severity of their seizures or the rate of seizure
conditions of the Creative Commons occurrence [7]. No further details were provided. At the American Medical Association
Attribution (CC BY) license (https:// convention in 1921, Geyelin presented a report of a controlled study involving 36 epileptic
creativecommons.org/licenses/by/ patients who used fasting as a treatment plan [8]. The results showed that 80% of these
4.0/). patients experienced a decrease in the number of seizures. Despite this promising outcome,
Int. J. Mol. Sci. 2021, 22, 5230. https://doi.org/10.3390/ijms22105230 https://www.mdpi.com/journal/ijmspatients who used fasting as a treatment plan [8]. The results showed tha
patients experienced a decrease in the number of seizures. Despite this p
come, fasting saw sporadic use among physicians to try to control epilepti
Int. J. Mol. Sci. 2021, 22, 5230 2 of 18
no formal clinical trials were done and its use as a long-term treatment met
ously not viable.
Around the same time, research into the use of diet modification to co
fasting saw sporadic use among physicians to try to control epileptic seizures, but no
mellitus
formal was
clinical developing.
trials were done andThe difference
its use between
as a long-term treatmenttype 1 and
method type 2 diabet
was obviously
derstood
not viable. at that time and so there was no distinction made. Newburgh
Around the same time, research into the use of diet modification to control diabetes
ported the use of a high-fat, low-carbohydrate diet to manage diabetics in
mellitus was developing. The difference between type 1 and type 2 diabetes was not
1921, Woodyatt
understood published
at that time a review
and so there article onmade.
was no distinction the modification
Newburgh and Marshof diet to co
mellitus
reported thebased
use of a on his own
high-fat, research diet
low-carbohydrate andto that
manageof diabetics
Philip in Shaffer, who
1920 [9–11]. In sough
1921, Woodyatt published a review article on the modification of diet to control diabetes
relationship between acetoacetic acid and glucose in vitro [12,13]. It had lon
mellitus based on his own research and that of Philip Shaffer, who sought to study the
that diabetic
relationship betweenpatients often
acetoacetic acidhad an increased
and glucose concentration
in vitro [12,13]. of known
It had long been acetoacetic
droxybutyric
that acid,
diabetic patients andhad
often acetone in their
an increased blood (ketonemia)
concentration of acetoacetic and
acid, urine
(R)-β- (ketou
hydroxybutyric acid, and acetone in their blood (ketonemia) and urine (ketourea) based on
research done by Gerhardt in 1865 [14]. These three compounds are collecti
research done by Gerhardt in 1865 [14]. These three compounds are collectively known as
theketone
the ketone bodies
bodies (Figure(Figure
1). 1).
Figure
Figure 1. The Ketone
1. The Bodies.Bodies.
Ketone
This increase in blood concentrations of the ketone bodies had been shown by Geel-
mudyen This increase
in 1897 in blood
to be caused concentrations
by an increase of the
in the metabolism ketone
of fatty acids,bodies had been sh
due to diabetic
patients’
mudyen in 1897 to be caused by an increase in the metabolism of[15].
inability to metabolize glucose at a rate fast enough to meet caloric needs fatty aci
This can be dangerous for those with diabetes because unchecked ketonemia can lead
betic
to patients’
a state we now know inability to metabolize
as diabetic glucose
ketoacidosis, where excessat ketone
a ratebodies
fast enough
acidify theto mee
[15]. This
blood, leadingcanto be dangerous
nausea, weakness,for andthose with diabetes
even death. because aunchecked
Woodyatt suggested diet based keton
on Newburgh and Marsh’s research where the amount
to a state we now know as diabetic ketoacidosis, where excess ketone of carbohydrates consumed did bod
not exceed the quantity the body could metabolize, and to supplement the diet with fats.
blood,
This leading
prevented the to nausea,
buildup weakness,
of sugar in the blood and even death. one
(hyperglycemia), Woodyatt
of the causessuggested
of a
Newburgh
ketonemia. and Marsh’s
Woodyatt research
believed that over time,where the
resting theamount
pancreas of wouldcarbohydrates
increase the cons
body’s capability to metabolize carbohydrates and
exceed the quantity the body could metabolize, and to supplementa normal diet could be resumed. Thethe diet
discovery of insulin in 1921 and its use to treat diabetes mellitus starting in 1922 decreased
prevented
the the treatment
need for other buildupoptions,
of sugar but in
thethe blood
interest (hyperglycemia),
in high-fat, low-carbohydrate onediets
of the cau
mia. Woodyatt
continued to grow [16]. believed that over time, resting the pancreas would incre
Later that same
capability to metabolize year, Wildercarbohydrates
published a report and basedaonnormal
Geyelin’sdiet
use of fastingbe
could as aresume
treatment for epilepsy and suggested that it may be due to the patients’ ketonemia [17].
ery of insulin
Referencing Shaffer’sin work,
1921Wilder
and its use toinducing
suggested treat diabetes
ketonemia not mellitus
throughstarting
fasting, butin 1922
need for
through other low-carbohydrate
a high-fat, treatment options, diet he but thetointerest
referred in high-fat,
as the “ketogenic low-carbohyd
diet”. Peterman,
one of his colleagues,
tinued to grow [16]. formulated the optimized amounts of macronutrients for the diet,
as being no more than 15 g of carbohydrates a day, 1 g of protein per kilogram of body
weight,Later
and the that same caloric
remaining year, Wilder
deficit madepublished
up of fats. aThis
report based
is nearly on Geyelin’s
identical to the us
a treatment
modern ketogenicfordiet,
epilepsy and
a 4:1 ratio suggested
of fats that itand
to carbohydrates may be due
protein withto the patients’ k
a maximum
allowance
Referencing of 50 gShaffer’s
of carbohydrates
work,perWilder
day. Wilder released a inducing
suggested report shortly after this on not th
ketonemia
three of his patients who saw a sharp decrease in the rate of seizures [18].
but Further
through a high-fat,
studies low-carbohydrate
over the next few years corroborated dietthe heusereferred to as the
of the ketogenic diet“ketog
terman,
as oneforofepilepsy
a treatment his colleagues,
[19]. Peterman formulated
reported his ownthe observations
optimized inamounts 1924 acrossof macr
two
the diet, as being no more than 15 g of carbohydrates a day, treatments.
studies on the use of the ketogenic diet compared to a variety of other 1 g of protein p
In the first, nine out of 13 patients using only the ketogenic diet as treatment for their
body weight, and the remaining caloric deficit made up of fats. This is nea
epilepsy were free of seizures for up to a year [1]. In the second study, 19 out of 37 were
the modern
seizure-free ketogenic
for up to two yearsdiet, a 4:1
[20]. ratio
In 1927, of fats
Talbot to carbohydrates
reported a study of 200 childrenand protein
where
mumthe ketogenic diet
allowance ofprovided
50 g ofcomplete symptomatic
carbohydrates perrelief among
day. a thirdreleased
Wilder and partiala repor
this on three of his patients who saw a sharp decrease in the rate of seizure
Further studies over the next few years corroborated the use of the ke
a treatment for epilepsy [19]. Peterman reported his own observations in 1ketogenic diet provided complete symptomatic relief among a third and partial
Int. J. Mol. Sci. 2021, 22, 5230 3 of 18
ment to three quarters [21]. Helmholz reported the use of the diet across 144
patients. Forty-six patients were seizure-free, and 34 saw a decrease in the rates
occurrence [22].
improvement to three quarters [21]. Helmholz reported the use of the diet across 144
The use of medication to treat epilepsy was gaining popularity around th
epileptic patients. Forty-six patients were seizure-free, and 34 saw a decrease in the rates of
well occurrence
seizure (Figure 2). Phenobarbital was brought to market as an anti-convulsant in
[22].
was
Theoneuseof
of the treatment
medication options
to treat Peterman
epilepsy compared
was gaining popularity toaround
the ketogenic
this time asdiet [1,23
well (Figure 2). Phenobarbital was brought to market as an anti-convulsant
toin was synthesized in 1908 and shown to be useful for preventing seizures in in 1912 and was
one of the treatment options Peterman compared to the ketogenic diet [1,23]. Phenytoin
Carbamazepine and sodium valproate were both brought to market in 1962 [2
was synthesized in 1908 and shown to be useful for preventing seizures in 1936 [24].
are still usedand
Carbamazepine and are currently
sodium valproate on
werethe World
both broughtHealth Organization
to market Model
in 1962 [25,26]. All List of
areMedicines.
still used and The
are emergence of novel
currently on the anticonvulsant
World Health Organization medications
Model List ofcaused
Essentialthe keto
Medicines. The emergence of novel anticonvulsant medications caused
to decrease in popularity as a treatment option, possibly because it was though the ketogenic
diet to decrease in popularity as a treatment option, possibly because it was thought
dicinal options would eliminate behavioral changes altogether [27]. Nonethele
that medicinal options would eliminate behavioral changes altogether [27]. Nonetheless,
thetogenic
ketogenicdiet
dietnever
nevercompletely disappeared.
completely disappeared. TheThe difficulty
difficulty in developing
in developing a widely a widely
medicinal
accepted treatment
medicinal protocol
treatment protocoltoto control epileptic
control epileptic seizures
seizures led toled to its sporadic
its sporadic use. use
Figure
Figure2. Common Anticonvulsant
2. Common Medications.
Anticonvulsant Medications.
Johns Hopkins Hospital in particular seemed to use the diet for epilepsy treatment
more oftenJohns
thanHopkins
most health Hospital in particular
care centers, seemed
publishing reviews andtoreports
use the diet
on its for epilepsy
efficacy in
themore
1980soften than[27–29].
and 1990s most Thehealth
causecare centers,
of the publishing
diet’s subsequent reviews
upswing and reports
in popularity is on i
at in
least partially due to the efforts of Jim Abrahams, a film director
the 1980s and 1990s [27–29]. The cause of the diet’s subsequent upswing in pwhose son Charlie’s
seizures were treated successfully at Johns Hopkins using the ketogenic diet. His son’s
is at least partially due to the efforts of Jim Abrahams, a film director whose son
story was documented on Dateline NBC in 1994 and he started a foundation called the
seizures
Charlie were treated
Foundation to spreadsuccessfully
awareness andat Johns
fund Hopkins
research into theusing thediet
ketogenic ketogenic
[30]. It diet.
storybecause
is likely was documented
of this increaseon in Dateline NBC
publicity that in 1994and
popularity and he started
interest a foundation
in the diet grew,
leading to a surge of research in the past twenty-five years [31]. This
Charlie Foundation to spread awareness and fund research into the ketogenic research has helped to d
expand the benefits of the ketogenic diet beyond its use as a treatment for epilepsy [32].
is likely because of this increase in publicity that popularity and interest in the d
While early use of the ketogenic diet to treat epilepsy was done without a full un-
leading to
derstanding of a surge
the of research
biochemistry throughin which
the past twenty-five
ketone years [31].
bodies are generated andThis research h
utilized,
to expand
research thepast
over the benefits
centuryofhasthe ketogenic
further dietthese
elucidated beyond its use as a treatment for epil
pathways.
While early use of the ketogenic diet to treat epilepsy was done without a f
3. Competing Energy Sources in the Body
standing of the biochemistry through which ketone bodies are generated and
Energy for the body comes from the digestion and metabolic breakdown of macronu-
research over the past century has further elucidated these pathways.
trients (fats, carbohydrates, and protein) in the diet. The body prioritizes carbohydrate
3. Competing Energy Sources in the Body
Energy for the body comes from the digestion and metabolic breakdown
nutrients (fats, carbohydrates, and protein) in the diet. The body prioritizes carband in muscles, chemically converting them through hydrolysis to glucose, which is oxi-
Int. J. Mol. Sci. 2021, 22, x FOR PEER REVIEW
datively
Int. J. broken
Mol. Sci. 2021, 22, 5230 down through glycolysis to S-acetyl coenzyme A (or S-acetyl CoA). 4 of S-
18
Acetyl CoA is needed to fuel the Krebs cycle, which completes the conversion of upstream
carbon sources to carbon dioxide and adenosine triphosphate (ATP), the primary source
of energy in cells (Figure and in
3).muscles,
metabolism chemically
due to their converting
availability from the dietthem through
and from stored hydrolysis toliver
glycogen in the glucose,
and whic
in muscles, broken
datively chemically converting
down them through
through hydrolysis
glycolysis to glucose,
to S-acetyl which is oxidatively
coenzyme A (or S-acetyl C
broken down through glycolysis to S-acetyl coenzyme A (or S-acetyl CoA). S-Acetyl CoA
Acetyl CoA is needed to fuel the Krebs cycle, which completes the conversion of u
is needed to fuel the Krebs cycle, which completes the conversion of upstream carbon
carbontosources
sources to carbon
carbon dioxide dioxide triphosphate
and adenosine and adenosine
(ATP),triphosphate (ATP),
the primary source the primar
of energy
ofcells
in energy in 3).
(Figure cells (Figure 3).
Figure 3. Adenosine Triphosphate (ATP).
Fats can likewise be broken down hydrolytically to fatty acids that metabolically pro-
duce S-acetyl CoA forFigure
Figure Adenosine
the 3.Krebs
3. AdenosineTriphosphate
cycle. Triphosphate
This occurs through a repetitive β-oxidation process
(ATP). (ATP).
in the mitochondrion ofFats liver
can cells
likewise(mostly).
be brokenNormally, these two
down hydrolytically pathways
to fatty to S-acetyl
acids that metabolically
produceFats can likewise
S-acetyl CoA for be broken
the Krebs down
cycle. hydrolytically
This occurs through to
a fatty acids
repetitive that metabolic
β-oxidation
CoA are complementary and carefully regulated metabolically. However, when pushed
duce S-acetyl
process CoA for the Krebs
in the mitochondrion of liver cycle. This occurs
cells (mostly). through
Normally, thesea two
repetitive β-oxidation
pathways to
beyond this regulation by
S-acetyl
dietary consumption
CoA are complementary
or by
andcells
the
carefully
body’s
regulated
immediate
metabolically.
energy
However,
needs,
when
in the mitochondrion of liver (mostly). Normally, these two pathways to
the pathways can become
pushed competitive.
beyond this Indeed,
regulation by by
dietary limiting
consumption the
or availability
by the
CoA are complementary and carefully regulated metabolically. However, whenbody’s of carbohy-
immediate energy
needs,
drates in the diet and the the pathways can become
accumulation ofbystoredcompetitive.
glucose Indeed,
(glycogenby limiting the availability
reserves), the body of
beyond this in
carbohydrates regulation
the diet and thedietary consumption
accumulation or by the
of stored glucose body’sreserves),
(glycogen immediatethe energ
can switch to the usebody
of dietary
the pathways or stored
can switch tocan become
the use
fats to produce
competitive.
of dietary
the S-acetyl
or stored fatsIndeed,
to produce
CoA
bythelimitingneeded for the
the needed
S-acetyl CoA availability
for of c
Krebs cycle (Figure 4).
drates in the diet and the accumulation of stored glucose (glycogen reserves), t
the Krebs cycle (Figure 4).
can switch to the use of dietary or stored fats to produce the S-acetyl CoA needed
Krebs cycle (Figure 4).
Figure 4. Metabolic Sources of S-Acetyl CoA.
Figure 4. Metabolic Sources of S-Acetyl CoA.
Figure
4. 4. Metabolic
Dietary Fats Sources of S-Acetyl CoA.
4. Dietary Fats
While the ratio of macronutrients in the ketogenic diet has remained relatively un-
While the ratio changed
ofDietary
4. macronutrients
sinceFats in theinketogenic
its development 1921, there is diet
some has remained
variety within therelatively
dietary fatsun-
we
consume.
changed since its development Fats are lipophilic,
in 1921, largely water-insoluble substances that can be extracted from
While the ratio ofthere is some variety
macronutrients in the within
ketogenic thediet
dietary fats we relati
has remained
living cells with common organic solvents, such as hexane or diethyl ether, and depending
consume. Fats are lipophilic,
changed
on
largely
since its
their structures,
water-insoluble
candevelopment in substances
1921, there
appear as oils, semi-solids,
that canvariety
is some
or greasy waxes
be extracted
at roomwithin
from
the dietary
temperature.
living cells with common organic
consume.
Collectively, Fats
thesesolvents,
are suchlargely
lipophilic,
organic-soluble as hexane
compounds or be
diethyl
water-insoluble
can referredether, and depending
substances
to commonly that canand
as fats, be extract
include
on their structures, can a wide
appear
living cells asrange
with ofsemi-solids,
oils, structural
common typesor
organic thatgreasy
includesuch
solvents, long as
waxes chain
atester
room
hexane derivatives,
diethyl such
or temperature.
ether,as and de
triacylglycerides, branched chain compounds comprised of a wide assortment of terpenes
on their structures,
Collectively, these organic-soluble can appear
compounds as be
can oils,referred
semi-solids, or greasy waxes
to commonly as fats,at and
room temp
and terpenoids, such as the steroids (e.g., cholesterol) and their fatty esters. Figure 5
Collectively,
include a wide rangeillustrates some these
of structural types
examplesorganic-soluble
that include
of common compounds
long
dietary chain ester
lipids. can be referred to such
derivatives, commonly as as f
includechain
triacylglycerides, branched a widecompounds
range of structural
comprised typesofthat
a wideinclude long chain
assortment ofester derivatives
terpenes
and terpenoids, suchtriacylglycerides,
as the steroids (eg., branched chain compounds
cholesterol) and their comprised
fatty esters. of Figure
a wide assortment
5 illus- of t
and terpenoids, such as the steroids (eg., cholesterol) and their fatty esters. Figur
trates some examples of common dietary lipids.
trates some examples of common dietary lipids.Int. J. Mol. Sci. 2021, 22, x FOR PEER REVIEW 5 of 18
Int. J. Mol. Sci. 2021, 22, 5230 5 of 18
Int. J. Mol. Sci. 2021, 22, x FOR PEER REVIEW 5 of 18
Figure 5. Examples
Figure 5.Figure 5.of
Examples ofCommonofDietary
Common
Examples Dietary
Common Fats.
Fats.
Dietary Fats.
Dietary
Dietary fats
fats areprimarily
are primarily triacylglycerides
triacylglycerides (triglycerides),
triacylglycerides (triglycerides),
(triglycerides),a glycerol backbone
aa glycerol
glycerol bound
backbone
backbone bound
bound
to three
to three carboxylic
three carboxylic
carboxylicacids acids with
acidswith long
withlong aliphatic
longaliphatic
aliphatic chains
chains
chains (fatty
(fatty acids)
acids)
(fatty via ester
via via
acids) ester bonds.
bonds.
ester Mono-
Monoacyl-
bonds. Mono-
acylglycerides
glycerides and andand diacylglycerides
diacylglycerides are are also
also naturally
naturally occurring,but
occurring, but not as abundant inin the
acylglycerides diacylglycerides are also naturally occurring, not
but asnotabundant
as abundant in
the diet (Figure 6).
diet (Figure
the diet 6). 6).
(Figure
Figure 6. Dietary Acylglyceride Fats.
The triacylglycerides found in the diet are composed of three long-chain fatty acids
Figure 6.6. Dietary
Dietary Acylglyceride
Acylglyceride Fats.
Figure
(between 14 and 22 carbons)Fats. and may be identical or different in the carbon chain length
or in the presence of unsaturation or oxygenation in the chain [33]. The number of alkenes
The triacylglycerides found in the diet are composed
in the chain can vary as well, from being saturated,composed
of
of three
monounsaturated, threeorlong-chain
long-chain fatty
fatty acids
polyunsaturated. acids
(between
(between 14
14 and
and 22
22 carbons)
carbons) and
and may
may be
beidentical
identical or different
or different in the
in
The alkene geometry can likewise be either E or Z, although nearly all examples in nature carbon
the carbonchain
chainlength or
length
in
or the
in presence
the presence of unsaturation
of unsaturation or oxygenation
or oxygenation in the
in chain
the [33].
chain The
[33].
are Z. The length of the carbon chain, and the location or the type of alkene(s) within the Thenumber
number of alkenes
of in
alkenes
the chain
in the chain
fatty can
estercan vary
vary
chain, as well,
as well,
largely from being
from being
determine saturated,
saturated,
the physical monounsaturated,
and tomonounsaturated, or
or
some extent the biochemicalpolyunsaturated.
prop-
Theerties
alkene geometry
geometry
of the can
can likewise
triglyceride, fullybe
likewise
with be either
either E
saturated E or
or Z,or
chains although
although nearly
nearly
those having all
all examples
E-alkenesexamples
being morein
in nature
nature
are Z. The
The length
lengthof
conformationally-rigid ofthe
thecarbon
carbonchain,
and leadingchain,and
to andthe
compounds the location
with
location or or
higherthethe
melting
typetype of alkene(s)
points. Plants
of alkene(s) andwithin
within the
the animals
fattyfatty that chain,
esterester
chain, live in cold
largelylargelyenvironments
determine
determine the produce a and
the physical
physical greaterand
to abundance
extentof
to some
some triglycerides
extent
the andprop-
the biochemical
biochemical
other fatty lipids that remain withfluid at lowsaturated
temperatures,
properties
erties of theoftriglyceride,
the triglyceride,
with fully fully
saturated chains ordepending
chains or those
those havingonhaving
the required func-being
E-alkenes
E-alkenes being more
moretion and physical state.
conformationally-rigid and leading to compounds with higher melting points. Plants
conformationally-rigid and leading to compounds with higher melting points. Plants and
and The fatty
animals that acid
live concentrations
in cold in triacylglycerides
environments produce also vary
a greater greatly based
abundance on the
of triglycerides
animals
sourcethat liveThe
of fat. in cold environments
concentrations produce
of specific fatty a greater
acids abundance
in different sourcesof of
triglycerides
fat can be and
and
other other
fattyfatty
lipids lipids that remain
that remain fluid attemperatures,
low temperatures, depending theonrequired
the required
analyzed by cleaving the fattyfluid
acidsat low
from triacylglycerides and depending
converting onthem into their func-
function
tion and and physical
physical state.state.
The
The fatty acidconcentrations
fatty acid concentrations in triacylglycerides also also
in triacylglycerides vary vary
greatly based based
greatly on the on
source
the
of fat. The concentrations of specific fatty acids in different sources of fat can be
source of fat. The concentrations of specific fatty acids in different sources of fat can beanalyzed
by cleaving
analyzed by the fatty the
cleaving acids from
fatty triacylglycerides
acids and converting
from triacylglycerides them into
and converting themtheir
into ester
their
forms. Using these methods, the primary fatty acids in different fats has been studied
(Figure 7) [33,34].Sci. 2021,
Int. J.22, x FOR
Mol. Sci.PEER
2021,REVIEW
22, x FOR PEER REVIEW 6 of 18
ester
Int. J. Mol. Sci. 2021, forms. Using
22, 5230 these
ester methods,
forms. Usingthe primary
these fatty acids
methods, in different
the primary fats
fatty has been
acids studied fats
in different has
6 of 18 been
(Figure 7) [33,34].
(Figure 7) [33,34].
Figure 7. Major Fatty Acids in Common Foods.
Figure
Figure
These concentrations7. Major
7. Major FattyFatty
can also Acids
Acidsbe in Common
in Common
affected Foods. Foods.
by environmental factors within specific
dietary fat sources [35].
These concentrations can also be affected by environmental factors within specific
The long dietary These
alkyl chains concentrations
found
fat sources in most dietary
[35]. can also be useful
fats are affected by environmental
biofuels factors within
for cells, as their
carbons are in dietary
a higher fat
The long reduction
sources
alkyl chainsstate
[35]. (typically
found in most-2 or -3)fats
dietary than
are those
useful in carbohydrates
biofuels for cells, as their
(around 0). Consequently,
carbons are in fats
a store
higher about
reduction twice
statethe amount
(typically -2 of
The long alkyl chains found in most dietary fats are useful
or potential
-3) than energy
those in within
biofuels for cells
carbohydrates
their structures, which
(around is
0). released
Consequently,when converted
fats store in
about cells
twice to CO
the 2.
amount
carbons are in a higher reduction state (typically -2 or -3) than those of potential energy within
in carbo
their structures, which is released when converted in cells to CO2 .
(around 0). Consequently, fats store about twice the amount of potential energ
5. Biochemistry of the Ketogenic Diet
5.their structures,
Biochemistry which
of the is released
Ketogenic Diet when converted
in cells to CO2.
The biochemistry behind the ketogenic diet is relatively simple. The ketogenic diet
The biochemistry behind the ketogenic diet is relatively simple. The ketogenic diet
attempts to emulate the
attempts tobody’s
emulateresponse
thethe
body’sto response
starvationto or fasting by
starvation eliminating
or fasting carbohy- carbo-
by eliminating
5. Biochemistry of Ketogenic Diet
drates as the provider of the
hydrates as S-acetyl CoA,
provider of and thusCoA,
S-acetyl the and
body´s
thussource of energy.
the body´s sourceWhen car- When
of energy.
The biochemistry
bohydrates arecarbohydrates
unrestricted in the diet, behind
the the
glucose ketogenic
present in diet
cells is
or relatively
in the blood simple.
are unrestricted in the diet, the glucose present in cells or in theen-
is bloodThe
is ketog
attempts
zymatically converted to emulate
to pyruvate
enzymatically converted the
through body’s response
the glycolytic
to pyruvate to starvation
through thepathway
glycolytic(Figure
pathwayor fasting
8).(Figure 8).by eliminating
drates as the provider of S-acetyl CoA, and thus the body´s source of energy. W
bohydrates are unrestricted in the diet, the glucose present in cells or in the blo
zymatically converted to pyruvate through the glycolytic pathway (Figure 8).
Figure 8. Metabolism of Glucose.
Figure 8. Metabolism of Glucose.
The glycolysis The glycolysisgenerates
pathway pathway generates
energy energy
in the inform
the form of adenosinetriphosphate
of adenosine triphosphate (ATP).
The pyruvate molecule is then converted to S-acetyl CoA to be used as fuel for the Krebs
(ATP). The pyruvate molecule is then converted to S-acetyl CoA to be used as fuel for the
cycle, the body’s primary process for generating energy in the form of ATP (and its direct
Krebs cycle, the body’s
Figure
link to the8.primary
Metabolism process
mitochondrial of for generatingchain).
Glucose.
electron-transport energy in the form of ATP (and its
direct link to the mitochondrial
When carbohydrates electron-transport
are abundantly chain).
available in the food we eat, the increase in
When carbohydrates
blood glucose are
The glycolysis abundantly
concentrationpathway available in
generates
signals the the energy
pancreas food weineat,
to secrete thetheform
insulin, aincrease in
of adenosine
protein hormone triph
blood glucosethat concentration
stimulates signals
the the
cellular pancreas
absorption to
of secrete
glucose insulin,
from the a protein
blood.
(ATP). The pyruvate molecule is then converted to S-acetyl CoA to be used as fu hormone
Insulin-dependent
that stimulatesglucose
Krebs
uptakeabsorption
the cellular is largely responsible
cycle, the body’sofprimary glucosefor excess
from theglucose
process blood.
for
storage in the liver and
Insulin-dependent
generating energy inglu-
skeletal
the form of ATP
muscle as glycogen, and for the conversion of glucose to glycerol
cose uptake is largely responsible for excess glucose storage in the liver and skeletal mus- (along with glycerol
direct
from link
thefor to the of
hydrolysis mitochondrial
triglycerides) toelectron-transport
be chain).of triacylglycerides
cle as glycogen, and the conversion of glucose to used in the
glycerol biosynthesis
(along with glycerol from
that endWhen carbohydrates
up being stored in adiposeare abundantly
tissue [36]. Asavailable
a result ofin theprocesses,
these food webloodeat, the in
the hydrolysis of triglycerides) to be used in the biosynthesis of triacylglycerides that end
glucose
blood concentration
glucose is maintained
concentration at a steady
signals state of around
the processes,
pancreas blood 100–140
to secrete mg/dL of
insulin, blood,
up being stored in adipose tissue [36]. As a result of these glucose con- a protein h
that stimulates
centration is maintained the cellular
at a steady state of absorption
around 100–140 of glucose
mg/dL from the blood.
of blood, and uponInsulin-depend
cose uptake is largely responsible for excess glucose storage in the liver and skele
cle as glycogen, and for the conversion of glucose to glycerol (along with glyceInt.
Int.J.J.Mol.
Mol.Sci.
Sci.2021,
2021,22,
22,x5230
FOR PEER REVIEW 77ofof18
18
the
andburning
upon the ofburning
glucose of through
glucosethe Krebsthe
through cycle, ultimately
Krebs decreasesdecreases
cycle, ultimately to a point to where
a point
glucagon (another protein hormone) is released by the pancreas
where glucagon (another protein hormone) is released by the pancreas to orchestrate to orchestrate the break-
the
down of stored
break-down of glycogen to release
stored glycogen more glucose
to release into theinto
more glucose blood. These glycogen
the blood. stores
These glycogen
typically last forlast
stores typically about a day.aWhen
for about moremore
day. When carbohydrates
carbohydrates are consumed,
are consumed, more insulin
more is
insulin
released
is releasedto accelerate
to accelerate thethe
uptake
uptakeof of
glucose
glucose totoreplenish
replenishglycogen
glycogenstores,
stores,andandthetheprocess
process
repeats
repeatsitself
itself[37].
[37].AnAnaverage
average180-pound
180-pound manman typically stores
typically around
stores aroundone pound
one poundof gly- of
cogen in the liver (giving about a day’s worth of energy) but around
glycogen in the liver (giving about a day’s worth of energy) but around 40 times that 40 times that amount
inamount
fat deposited inside of adipocytes
in fat deposited (fat cells).(fat cells).
inside of adipocytes
However,
However,ififglycogen
glycogenstores
storesarearedepleted
depletedand andnono carbohydrates
carbohydratesare areconsumed,
consumed,the the
body
bodywill willbegin
begintotobreak
breakdown
downboth bothdietary
dietarytriacylglycerides
triacylglyceridesand andtriacylglycerides
triacylglyceridescon- con-
tained
tainedininadipose
adiposecellscellsthrough
throughthe theprocess
process of of lipolysis
lipolysis (Figure
(Figure 9)9) [38].
[38]. The
Thefirst
firststep
stepin in
lipolysis
lipolysisisisthetheremoval
removalofofthe thefirst
firstfatty
fattyacid,
acid,ininthis
thiscase
casestearic
stearicacid,
acid,from
fromthethetriglyceride.
triglyceride.
ItItrequires
requiresaaunique
uniquelipase
lipasecalled
calledadipose
adipose triglyceride
triglyceride lipase.
lipase. Although
Although therethere are three di-dia-
acylglycerol
cylglycerol stereoisomers, adipose triglyceride lipase lipase shows
shows aapreference
preferencefor forhydrolyzing
hydrolyzing
theester
the esteratatthe
thesn-2
sn-2position
position[39].
[39].Similarly,
Similarly,hormone-sensitive
hormone-sensitivelipase lipasehydrolyzes
hydrolyzesdiacyl-
diacyl-
glycerolwith
glycerol withaapreference
preferencefor for1,3-diacylglycerols.
1,3-diacylglycerols.Monoacylglycerol
Monoacylglycerollipase lipasehydrolyzes
hydrolyzesthe the
finalfatty
final fattyacid
acidtotogenerate
generateglycerol
glycerolandandaanetnettotal
totalof
ofthree
threefatty
fattyacids.
acids.These
Theseenzymes
enzymesare are
relativelynonspecific
relatively nonspecificbecause
becauseof ofthe
thelarge
largevariety
varietyof ofunique
uniquetriacylglcerides
triacylglceridesin inthe
thediet.
diet.
Figure 9.ofLipolysis
Figure 9. Lipolysis of Stored Triacylglycerides.
Stored Triacylglycerides.
Dietaryfats
Dietary fatsare
arebroken
brokendowndownby byaa similar
similar method.
method. Lingual
Linguallipases
lipasessecreted
secretedininthe
the
mouth and gastric lipases in the stomach break down some triacylglycerides
mouth and gastric lipases in the stomach break down some triacylglycerides into mono- into mono-
anddiacylglycerides.
and diacylglycerides.BileBilesalts
saltsininthe
thesmall
smallintestine
intestineemulsify
emulsifyfats
fatsand
andallow
allowpancreatic
pancreatic
lipase to do the same. Free fatty acids and the mono- and diacylglycerides
lipase to do the same. Free fatty acids and the mono- and diacylglycerides are then are then absorbed
ab-
by intestinal mucosal cells and combined to form triacylglycerides that can
sorbed by intestinal mucosal cells and combined to form triacylglycerides that can be dis- be dissolved in
lipoprotein complexes called chylomicrons. Chylomicrons are able to
solved in lipoprotein complexes called chylomicrons. Chylomicrons are able to carry in-carry insoluble fats
throughfats
soluble thethrough
bloodstream. They are absorbed
the bloodstream. by liver,
They are adipose,
absorbed and muscle
by liver, adipose,cells
andwhere the
muscle
triacylglycerides are hydrolyzed by lipoprotein and hepatic triglyceride
cells where the triacylglycerides are hydrolyzed by lipoprotein and hepatic triglyceridelipases to generate
free fatty
lipases acids. free fatty acids.
to generate
The fatty acids released either through lipolysis or through the digestion of dietary
The fatty acids released either through lipolysis or through the digestion of dietary
fats are carried through the blood by albumin until they are absorbed by cells and used as
fats are carried through the blood by albumin until they are absorbed by cells and used as
an energy source. The glycerol released by the breakdown of triacylglcyerides can be used
an energy source. The glycerol released by the breakdown of triacylglcyerides can be usedInt. J. Mol. Sci. 2021, 22, x FOR PEER REVIEW 8 of 18
Int. J. Mol. Sci. 2021, 22, 5230 8 of 18
to synthesize new triacylglycerides or can be converted in the liver to D-glyceraldehyde-
to synthesize new triacylglycerides or can be converted in the liver to D-glyceraldehyde-3-
3-phosphate and enterthe
phosphate and enter thegluconeogenesis
gluconeogenesis pathway
pathway to generate
to generate glucose,
glucose, butwill
but this thisnot
will not
fully meet
fully the
meet thecaloric
caloricrequirements ofthe
requirements of thebody
body (Figure
(Figure 10).10).
Figure
Figure 10. 10. Metabolic
Metabolic Conversion
Conversion of Glycerol
of Glycerol to Glucose.
to Glucose.
Instead, the fatty acids released by lipolysis are absorbed by the cells and converted
Instead, the fatty acids released by lipolysis are absorbed by the cells and converted
into S-acetyl CoA through β-oxidation. This process is a repeating set of steps that converts
intothe
Int. J. Mol. Sci. 2021, 22, x FOR PEER S-acetyl
REVIEW CoA through β-oxidation. This process is a repeating set of steps9that of 18 con-
β-carbon of the fatty acid into a carbonyl group and then cleaves off an S-acetyl CoA,
verts the β-carbon of the fatty acid into
shortening the fatty acid (Figure 11). a carbonyl group and then cleaves off an S-acetyl
CoA, shortening the fatty acid (Figure 11).
The first step in fatty acid β-oxidation is the thioesterification by coenzyme A, which
assists in transport to the mitochondrial outer membrane. Inside the mitochondrion, the
first step is a regioselective dehydrogenation reaction. Next, the alkene is hydrated to gen-
erate a hydroxyl group at the β-carbon. This addition is stereoselective for the S-enantio-
mer. The penultimate step is the NAD+ oxidation of the secondary alcohol to a ketone.
Finally, S-acetyl CoA is cleaved, and the resulting fatty acid chain is two carbon atoms
shorter. The cycle repeats itself until the fatty acid is fully converted to S-acetyl CoA with
some minor differences for unsaturated fatty acids and those with odd-numbered carbon
chain lengths.
Figure 11.
Figure β-Oxidation of a Fatty Acid.
11. β-Oxidation
The S-acetyl CoA generated by cells through β-oxidation can directly enter the Krebs
cycle and provide energy in the form of ATP. For most of the cells in the body, this process
could supply their energy needs until triacylglyceride stores are depleted. However, be-
cause fatty acids cannot cross the blood–brain barrier, they cannot be absorbed and usedInt. J. Mol. Sci. 2021, 22, 5230 9 of 18
The first step in fatty acid β-oxidation is the thioesterification by coenzyme A, which
assists in transport to the mitochondrial outer membrane. Inside the mitochondrion, the
first step is a regioselective dehydrogenation reaction. Next, the alkene is hydrated to
generate a hydroxyl group at the β-carbon. This addition is stereoselective for the S-
enantiomer. The penultimate step is the NAD+ oxidation of the secondary alcohol to a
ketone. Finally, S-acetyl CoA is cleaved, and the resulting fatty acid chain is two carbon
atoms shorter. The cycle repeats itself until the fatty acid is fully converted to S-acetyl CoA
with some minor differences for unsaturated fatty acids and those with odd-numbered
carbon chain lengths.
The S-acetyl CoA generated by cells through β-oxidation can directly enter the Krebs
cycle and provide energy in the form of ATP. For most of the cells in the body, this process
could supply their energy needs until triacylglyceride stores are depleted. However,
because fatty acids cannot cross the blood–brain barrier, they cannot be absorbed and used
by cells in the brain. The brain uses approximately 20% of the energy the body requires,
which necessitates an effective method for providing lasting energy when carbohydrate
consumption is low [3,40].
The body overcomes this obstacle by converting S-acetyl CoA into the three ketone
bodies (Figure 1), water-soluble compounds that can enter the blood, cross the blood–
brain barrier, and be absorbed by neuronal cells in the brain before being converted to
S-acetyl CoA to fuel the Krebs cycle. While they are presented here in their acid forms,
these two acids are deprotonated at physiological pH and referred to as acetoacetate and
(R)-β-hydroxybutyrate [41].
The metabolic process of generating ketone bodies from S-acetyl CoA occurs through
ketogenesis, and takes place primarily in the liver (Figure 12). The first step is the Claisen-
type condensation of two molecules of S-acetyl CoA to form S-acetoacetyl CoA. This is
followed by an aldol addition of another S-acetyl CoA to the β-carbonyl with subsequent
hydrolysis of one of the coenzyme A thioesters to generate a carboxylic acid. The S-acetyl
CoA is cleaved by HMG-CoA lyase to generate acetoacetic acid, which can subsequently
be reduced by 3-hydroxybutyrate dehydrogenase to generate (R)-β-hydroxybutyric acid.
Decarboxylation of acetoacetic acid, as a means to release acetone and carbon dioxide, can10 of 18
Int. J. Mol. Sci. 2021, 22, x FOR PEER REVIEW
occur spontaneously as well, although this is primarily the method through which excess
ketone bodies are removed as waste.
S-AcCoA
CoA-SH H2O CoA-SH
O O O
O HO O
S-CoA S-CoA HMG-CoA
Thiolase HO S-CoA
synthase
S-acetyl CoA S-acetoacetyl CoA 3-hydroxy-3-methylglutaryl CoA
(2 moles)
HMG-CoA
lyase S-AcCoA
NAD
NAD+ H/H+
OH O O O
OH 3-hydroxybutyrate OH
dehydrogenase
(R)-ß-hydroxybutyric acid acetoacetic acid
FigureFigure 12. Ketogenesis
12. Ketogenesis of Ketone
of Ketone Bodies
Bodies from
from S-AcetylCoA.
S-Acetyl CoA.
While S-acetoacetyl CoA could be hydrolyzed to acetoacetic acid directly, there is
While S-acetoacetyl CoA could be hydrolyzed to acetoacetic acid directly, there is
evidence that the expression of HMG-CoA synthase plays a role in limiting the rate at
evidence that the expression of HMG-CoA synthase plays a role in limiting the rate at
which ketogenesis occurs [42]. The ketone bodies produced metabolically in the liver are
released into the blood where they can be absorbed by other cells and used to synthesize
S-acetyl CoA through the process of ketolysis (Figure 13). First, NAD+ oxidation converts
(R)-β-hydroxybutyric acid to acetoacetic acid, which undergoes thioesterification with co-(R)-ß-hydroxybutyric acid acetoacetic acid
Figure 12. Ketogenesis of Ketone Bodies from S-Acetyl CoA.
Int. J. Mol. Sci. 2021, 22, 5230 While S-acetoacetyl CoA could be hydrolyzed to acetoacetic acid directly,
10 of 18
evidence that the expression of HMG-CoA synthase plays a role in limiting th
which ketogenesis occurs [42]. The ketone bodies produced metabolically in the
released
which into the
ketogenesis blood
occurs where
[42]. they bodies
The ketone can beproduced
absorbed by other cells
metabolically in theand
liverused
are to sy
S-acetylinto
released CoAthe through thethey
blood where process
can beof ketolysis
absorbed (Figure
by other cells13).
and First,
used toNAD + oxidation
synthesize
S-acetyl CoA through the process of ketolysis (Figure 13). First, NAD + oxidation converts
(R)-β-hydroxybutyric acid to acetoacetic acid, which undergoes thioesterification
(R)-β-hydroxybutyric acid to acetoacetic acid, which undergoes thioesterification with
enzyme A to generate S-acetoacetyl CoA. S-Acetoacetyl CoA is cleaved by coen
coenzyme A to generate S-acetoacetyl CoA. S-Acetoacetyl CoA is cleaved by coenzyme
Athrough
through aaretro-Claisen
retro-Claisen reaction
reaction to afford
to afford twoCoA
two S-acetyl S-acetyl CoAthat
molecules molecules
can directly that can
enterthe
enter the Krebs
Krebs cycle.
cycle.
Figure
Figure Ketolysis
13.13. of theofKetone
Ketolysis Bodies. Bodies.
the Ketone
6. Metabolic Effect of Ketolysis Versus Glycolysis
6. Metabolic Effect
Bypassing the of Ketolysis
traditional pathways Versus Glycolysis
of releasing energy through glycolysis in favor of
using ketone
Bypassing bodiesthe
has traditional
a profound effect on the body.
pathways While the energy
of releasing completethrough
mechanism is
glycolysis
not fully understood, bypassing carbohydrate metabolism pathways
of using ketone bodies has a profound effect on the body. While the complete mein the brain can lead
to a decreased incidence or even the elimination of epileptic seizures [6].
is not fully understood, bypassing carbohydrate metabolism pathways in the b
The state of having elevated ketone bodies in the blood is called ketosis. Typical
lead to
ketone a decreased
body incidence
concentrations or even
during dietary the elimination
ketosis are between 0.5–3of epileptic
mM, whileseizures
a normal [6].
The
diet will givestate of body
ketone having elevated ketone
concentrations bodies
of less than 0.3 mMin[43].
the Achieving
blood is called ketosis. Ty
and keeping
the body in a state of ketosis is the goal of the ketogenic diet, as any
tone body concentrations during dietary ketosis are between 0.5–3 mM, while a carbohydrates above
an absolute bare minimum will trigger the release of insulin and rapidly decrease the rate
of generation of ketone bodies.
It is important to note that this is different than diabetic ketoacidosis, a potentially life-
threatening condition that occurs primarily in type I diabetics where unchecked ketogenesis
can cause ketone body concentrations to rise to 10mM and beyond, overwhelming the
body’s acid-base buffering system and causing the blood to turn acidic [44]. Insulin acts as
an inhibitor for lipolysis, β-oxidation, and ketogenesis and prevents the build-up of ketone
bodies to unsafe levels during dietary ketosis [43]. Even for those with type 2 diabetes
where insulin sensitivity in the cells is decreased, diabetic ketoacidosis is rare since these
catabolic processes are very sensitive to insulin. However, type I diabetics are at risk for
diabetic ketoacidosis since the hormone to stop the generation of ketone bodies is present
in low amounts or absent entirely.Int. J. Mol. Sci. 2021, 22, 5230 11 of 18
7. Benefits of the Ketogenic Diet
The use of the ketogenic diet as a treatment option for epilepsy is well-studied, but
research has shown that there are other benefits. The most popular is its efficacy as a
weight-loss tool. Statistical analysis of fourteen studies comparing weight loss using the
ketogenic diet or low-fat diets showed the ketogenic diet caused greater reductions in body
weight [45]. The diet has also been shown to be a more effective method for weight loss
compared to a normal diet at similar caloric deficits in trials that range in length from
three months to two years [46,47]. It is possible that the initial rapid weight loss when
initiating the diet comes from the body’s attempts to use glycerol from triglycerides to
overcome a lack of dietary carbohydrates, leading to a decrease in adipose fat stores and
thus weight. Converting glycerol into glucose through the gluconeogenesis pathway is
also a very energy-demanding process, so the body is using excess energy before it begins
to adapt to using ketone bodies as a primary energy source.
Tangentially related to the benefits of weight loss, the ketogenic diet has been shown
to improve and even reverse insulin resistance in those suffering from type 2 diabetes or
for those who are at risk of becoming diabetic [48,49]. Increased insulin resistance has
been shown to lead to higher conversion of glucose into triglycerides that can lead to heart
disease [50,51]. For obese patients, the ketogenic diet has been shown to decrease total
cholesterol and triglyceride concentrations in the blood, which can decrease the risk of
cardiovascular and metabolic diseases [52–54].
Alongside the more well-studied benefits of the diet, there is emerging evidence
indicating benefits for other diseases and neurological disorders [55–57]. Patients with
Parkinson’s disease have shown improved scores on the Unified Parkinson’s Disease Rating
Scale after four weeks on a ketogenic diet [58]. The infusion of (R)-β-hydroxybutyric acid
was shown to protect against dopaminergic neurodegeneration induced by neurotoxins
that mimic the effects of Parkinson’s disease and Alzheimer’s disease [59,60]. Mouse
models have been used to show that the ketogenic diet can decrease the concentration of
amyloid-β in the brain, a risk factor for developing Alzheimer’s disease [61]. The ketogenic
diet has also demonstrated in mice models to slow the degradation of motor neurons due
to amyotrophic lateral sclerosis (ALS) [62].
It has long been recognized that cancer cells consume glucose at a much higher rate
than normal cells [63,64]. Increased glycolysis promotes excessive proliferation in cancer
cells [65]. However, in vitro studies have shown that a lack of glucose can cause apoptosis
in malignant cells [66–68]. Certain tumors are unable to utilize ketone bodies as a source of
energy due to a decrease in ketolysis enzyme activity [69,70]. The use of the ketogenic diet
to effectively “starve” certain types of tumors could be an adjuvant treatment alongside
more traditional forms of cancer treatment. In mice, the ketogenic diet has been shown to
slow the rate of tumor growth and improve the effects of radiation treatment [71–73]. There
are a few published reports of similar outcomes in human patients, but no formal clinical
testing has yet been done [74,75]. While much of this evidence is far from conclusive, it
shows the wide range of possibilities the diet holds, which justifies and indeed creates
opportunities for further research.
8. Limitations of the Ketogenic Diet
Most of the long-term effects of the ketogenic diet are based on reports from the
children with epilepsy who were treated by it. Hyperlipidemia has been seen in a majority
of children treated with the traditional ketogenic diet, although this can be avoided by
adjusting the types of fats consumed [76,77]. Kidney stones are seen in approximately 10%
of children on the ketogenic diet [78,79]. There is concern that a strict ketogenic diet may
affect growth rates, possibly due to protein or overall calorie restrictions [80,81]. There
is some evidence that the ketogenic diet can cause osteopenia, a condition where bone
mass is lost, possibly due to vitamin D and calcium deficiencies [82,83]. Research into the
long-term effects of the ketogenic diet on healthy adults is sparse, but the recent popularity
of the diet makes it likely that these studies will be done.Int. J. Mol. Sci. 2021, 22, 5230 12 of 18
One of the more common criticisms of the ketogenic diet is that it is difficult to
maintain. Clinical studies from the diet’s earliest use in the 1920s to treat epilepsy and
more recent studies on weight loss commonly report patients that were unable to follow
the diet [20–23,45–47]. A single meal with too high a proportion of carbohydrates will
cause the body to fall out of ketosis and revert back to using carbohydrates as its main
energy source [84,85]. Long-term adherence to the diet can be low due to lack of discipline,
adverse gastrointestinal effects, or palatability issues.
Modified versions of the ketogenic diet have been developed to try and address these
concerns. The MCT (medium-chain triglyceride) diet in particular has seen wide use. It
focuses on using naturally-occurring triacylglycerides consisting of medium-chain fatty
acids (between 6 and 12 carbons) that do not require active transport into cells and are
instead absorbed by the liver directly. This increases the rate at which dietary triglycerides
can be converted to ketone bodies, but research has shown that a greater percentage of fatty
acids in medium-chain triglycerides are metabolized in the liver and can be used to generate
ketone bodies compared to longer fatty acids that are partially used in the synthesis of
new triacylglycerides to be stored in adipose tissue [86]. Medium-chain triglycerides
have been shown to have a greater effect on ketone body concentration compared to
long-chain triglycerides [87]. Because of this, the MCT diet allows for a greater amount
of carbohydrates and proteins in the diet and medium-chain triglycerides are considered
to be more “ketogenic”; however, it still suffers from many of the same drawbacks as the
traditional ketogenic diet, including a reliance on a strict dietary regiment.
Improving the effectiveness and sustainability of the ketogenic diet using alternative
natural sources of fat does not address many of the primary limiting factors of the diet.
Preliminary research into the use of synthetic compounds to induce ketosis could better
address these factors.
9. Synthetic Ketogenic Compounds as an Alternative Path to Ketosis
Originally, it was believed that the benefits of the ketogenic diet were due to the
decrease in glucose metabolism; however, more recent studies have shown that some
of these benefits are instead caused by an increase in ketone body concentrations in the
blood [88,89]. In the past, this distinction was irrelevant since a normal diet will prevent
the body from generating ketone bodies at a rate high enough to maintain a state of ketosis.
However, three synthetic ketogenic compounds have been shown to increase ketone body
Int. J. Mol. Sci. 2021, 22, x FOR PEER REVIEW 13 of
concentration in the blood to the point where dietary ketosis can be maintained without a
change from a normal diet (Figure 14).
Figure14.14.
Figure Synthetic
Synthetic Ketogenic
Ketogenic Compounds.
Compounds.
(±(±)-1,3-Butanediol
)-1,3-Butanediol acetoacetate diester
acetoacetate was synthesized
diester from (±)-1,3-butanediol
was synthesized and
from (±)-1,3-butanediol an
t-butyl acetoacetate via a transesterification, published in 1995 (Figure 15) [90].
t-butyl acetoacetate via a transesterification, published in 1995 (Figure 15) [90].
O O
O(±)-1,3-Butanediol acetoacetate diester was synthesized from (±)-1,3-butaned
t-butyl
Figure 14.acetoacetate via aCompounds.
Synthetic Ketogenic transesterification, published in 1995 (Figure 15) [90].
Int. J. Mol. Sci. 2021, 22, 5230 13 of 18
O O
(±)-1,3-Butanediol acetoacetate diester was synthesized from (±)-1,3-butanediol and
t-butyl acetoacetate via a transesterification, published in 1995 (Figure 15) [90].
O
O O O O
tBuOH
t-butyl acetoacetate
(2 moles)
O O O O
O O
tBuOH
t-butyl acetoacetate 140oC
OH(2 moles)
O O O O
(±)-1,3-butanediol acetoacetate diester
140oC
OH OH O
(±)-1,3-butanediol (±)-1,3-butanediol acetoacetate diester
OH
Figure 15. Synthesis of (±)-1,3-Butanediol Acetoacetate Diester.
(±)-1,3-butanediol
Figure 15.
15. Synthesis
FigureThisSynthesis
compoundofof(±
(±)-1,3-Butanediol
)-1,3-Butanediol Acetoacetate
Acetoacetate
is metabolically Diester.
Diester.
hydrolyzed to generate two equivalents of a
tic acid
This and (±)-1,3-butanediol, which can beto oxidized to generate β-hydroxybuty
This compound
compoundisismetabolically
metabolicallyhydrolyzed
hydrolyzed togenerate
generate two
two equivalents
equivalents of ace-
of acetoace-
(Figureacid
toacetic 16)and
[91,92]. The oxidation
(±)-1,3-butanediol, of the
which can (R)-enantiomer occurs
be oxidized to generate in the liver. (S)-β-H
β-hydroxybutyric
tic acid and (±)-1,3-butanediol, which can be oxidized to generate β-hydroxybutyric acid
acid (Figure
butyric 16)is[91,92].
acid The oxidation
not naturally of the (R)-enantiomer
occurring, occurs in
but its metabolism in the liver. (S)-β-
perfused rat livers h
(Figure 16) [91,92].
Hydroxybutyric acidThe oxidation
is not naturallyofoccurring,
the (R)-enantiomer occurs in
but its metabolism in perfused
the liver.rat
(S)-β-Hydroxy-
livers
shown acid
butyric to generate ketone occurring,
is not naturally bodies through
but its an unknown
metabolism in pathway
perfused rat[93].
livers has been
has been shown to generate ketone bodies through an unknown pathway [93].
shown to generate ketone bodies through an unknown pathway [93].
Figure16.16.
Figure Oxidation
Oxidation of (R)-β-1,3-Butanediol
of (R)-β-1,3-Butanediol in the
in the Liver. Liver.
Figure 16. Oxidation of (R)-β-1,3-Butanediol in the Liver.
Racemic 1,3-butanediol acetoacetate diester was shown to increase ketone body con-
Racemic
Racemic
centrations in the 1,3-butanediol
1,3-butanediol
blood of both aacetoacetate
acetoacetate
pig and dog. diester
diester
More was wasstudies
shown
recent shown into
to increase increase
rats ketone ketone
indicate body
it bo
con-
centrations
induces ininthe
a rapid
centrations theblood
blood
increase of both a pig
andand
inof(R)-β-hydroxybutyric
both a pig dog.
dog.acid
More More
recentrecent
concentration studies
in the
studies rats in
inblood andrats
a indica
indicate it in-
ducesaarapid
decrease
duces inrapid
blood increase
glucose in
increase in (R)-β-hydroxybutyric
concentration, indicative of acid
(R)-β-hydroxybutyric acid
ketosis [94].concentration
Importantly,
concentration in the and
triglyceride
in the blood blood an
a de-
and cholesterol
creaseininblood
crease levels
bloodglucose did
glucose not change to
concentration,
concentration, a statistically relevant
indicative
indicative level
of ketosis
of ketosis over 28 days of daily
[94]. Importantly,
[94]. Importantly, trig
triglyceride
administration [6].
and Acholesterol
and cholesterol levels did not change to a statistically relevant level over 28 days
levels did not change to a statistically relevant
similar compound, (R)-3-hydroxybutyl (R)-3-hydroxybutanoate, was synthesized
level over 28 days of daily
administration
administration [6].
[6].
by Clarke and Veech in 2010 [95]. A lipase-mediated transesterification reaction between
AAsimilar
similarcompound,
(R)-1,3-butanediol compound, (R)-3-hydroxybutyl
(R)-3-hydroxybutyl
and ethyl (R)-3-hydroxybutanoate (R)-3-hydroxybutanoate,
(R)-3-hydroxybutanoate,
generates was synthesized
a stereospecific monoester was synt
by
thatClarke
can beand Veech
hydrolyzed into2010 [95].
generate A lipase-mediated
(R)-β-hydroxybutyric transesterification
acid
by Clarke and Veech in 2010 [95]. A lipase-mediated transesterification reaction and reaction
(R)-1,3-butanediol, betweenb
which can be oxidized in the liver to give a second equivalent of (R)-β-hydroxybutyric acid
(Figure 17) [96].
It has been shown in adult humans that daily administration of (R)-3-hydroxybutyl
(R)-3-hydroxybutanoate can safely induce ketosis while on a normal diet over 28 days of
administration [97,98].
A final set of ketogenic compounds that have been used to induce ketosis are salts
generated from racemic β-hydroxybutyrate. Most commonly, these are sodium, potassium,
and calcium salts (Figure 18). In studies in rats fed a normal diet and supplemented
with these salts, a small increase in ketone body concentration in the blood was observed.
Mineral salts of β-hydroxybutyrate can induce ketosis, but the amounts required can lead
to negative gastrointestinal effects and issues due to high levels of sodium [99].You can also read
