![Changes in [βHB] under various physiological states Plasma [KB] is <0.1 mM in the postprandial state when consuming high CHO or high protein meals, and rises upward after an overnight fast and with ketogenic dieting, prolonged fasting, starvation, and pathological states of ketoacidosis. After prolonged aerobic exercise, post-exercise ketosis (0.3 to 2.0 mM) may ensue depending on intensity and duration of exercise, aerobic fitness and nutrition status. The circulating KB ratio of βHB:AcAc is generally ß1:1 to 3:1, but during the aforementioned nutritional states can rise six-to tenfold, such that [KB] primarily reflects changes in [βHB]. An optimal concentration range for βHB to improve performance after exogenous ketone ingestion is proposed as ß1 to 3 mM, with concentrations ranging from ß1 to 5 mM reported after ketone ester (KE) ingestion. See text for further details.](https://www.researchgate.net/profile/Brendan-Egan/publication/309885871/figure/fig1/AS:436653622861826@1481117744934/Changes-in-bHB-under-various-physiological-states-Plasma-KB-is-01-mM-in-the_Q320.jpg)
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J Physiol 000.00 (2016) pp 1–15 1
The Journal of Physiology
TOPICAL REVIEW
Metabolism of ketone bodies during exercise and training:
physiological basis for exogenous supplementation
Mark Evans1,KarlE.Cogan
1and Brendan Egan1,2
1Institute for Sport and Health, School of Public Health, Physiotherapy and Sports Science, University College Dublin, Belfield, Dublin, 4, Ireland
2School of Health and Human Performance, Dublin City University, Glasnevin, Dublin, 9, Ireland
Reduced CHO availability
Prolonged fasting
Ketogenic diet
HMGCS
HMGCL
ACAT
AcAc
AcAc
FFA
Exogenous ketones
HB
HB
TCA cycle
Ac-CoA
ACAT
OXCT
BDH
ATP
HDAC
MPS
Lactate
Glycogen
IMTG Performance
Recovery and
adaptation?
BDH
Mark Evans and Karl Cogan are graduate students at the Institute for Sport and Health,
University College Dublin, Ireland. Mark received his MSc in Sport Nutrition from
Liverpool John Moores University in 2015. Karl received his MSc in Biotechnology from
University College Dublin in 2013. Their research explores optimising nutrition strategies
for performance and recovery in athletes with specific interest in ketone bodies and protein
hydrolysates, respectively. Brendan Egan PhD is Senior Lecturer in Sport and Exercise
Physiology at Dublin City University’s School of Health and Human Performance, and
Visiting Associate Professor at University College Dublin. His research group investigates
the molecular regulation of skeletal muscle function, adaptation and performance across the
life course with special interest in the synergy between nutrition and exercise interventions ranging from athletes to older adults. All three authors are
accomplishedsportsmenintheirownright,andcurrentlyinvolvedintheprovisionofsportssciencesupporttoteamsportathletes.
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2016 The Authors. The Journal of Physiology C
2016 The Physiological Society DOI: 10.1113/JP273185
2 M.Evansandothers J Physiol 000.00
Abstract Optimising training and performance through nutrition strategies is central to
supporting elite sportspeople, much of which has focused on manipulating the relative intake of
carbohydrate and fat and their contributions as fuels for energy provision. The ketone bodies,
namely acetoacetate, acetone and β-hydroxybutyrate (βHB), are produced in the liver during
conditions of reduced carbohydrate availability and serve as an alternative fuel source for peri-
pheral tissues including brain, heart and skeletal muscle. Ketone bodies are oxidised as a fuel
source during exercise, are markedly elevated during the post-exercise recovery period, and the
ability to utilise ketone bodies is higher in exercise-trained skeletal muscle. The metabolic actions
of ketone bodies can alter fuel selection through attenuating glucose utilisation in peripheral
tissues, anti-lipolytic effects on adipose tissue, and attenuation of proteolysis in skeletal muscle.
Moreover, ketone bodies can act as signalling metabolites, with βHB acting as an inhibitor of
histone deacetylases, an important regulator of the adaptive response to exercise in skeletal muscle.
Recent development of ketone esters facilitates acute ingestion of βHB that results in nutritional
ketosis without necessitating restrictive dietary practices. Initial reports suggest this strategy
alters the metabolic response to exercise and improves exercise performance, while other lines of
evidence suggest roles in recovery from exercise. The present review focuses on the physiology
of ketone bodies during and after exercise and in response to training, with specific interest in
exploring the physiological basis for exogenous ketone supplementation and potential benefits
for performance and recovery in athletes.
(Received 4 August 2016; accepted after revision 24 October 2016; first published online 10 November 2016)
Corresponding author B. Egan: School of Health and Human Performance, Dublin City University, Glasnevin, Dublin
9, Ireland. Email: brendan.egan@dcu.ie
Abstract figure legend Acetoacetate (AcAc) and β-hydroxybutyrate (βHB) are ketone bodies produced in hepatic
mitochondria during conditions of reduced carbohydrate availability and serve as an alternativefuel source for peripheral
tissues including skeletal muscle. Elevations in βHB can result from endogenous production i.e. ketogenesis, but also by
ingestion of exogenous ketone supplements such as ketone salts or ketone esters. Ketogenesis from free fatty acids (FFA)
involves sequential reactions of Ac-CoA acetyltransferase (ACAT ), hydroxymethylglutaryl CoA synthase (HMGCS), and
hydroxymethylglutary-CoA lyase (HMGCL). The end product of ketogenesis is AcAc, the majority of which is reduced to
βHB by 3-hydroxybutyrate dehydrogenase (BDH) before entering the circulation. Upon uptake into peripheral tissues,
βHB is oxidised to AcAc. Reactions of succinyl-CoA:3-oxoacid CoA transferase (OXCT) and ACAT ultimately produce
acetyl CoA (Ac-CoA), which enters the TCA cycle for ATP synthesis. The metabolic actions of βHB include altered fuel
selection during exercise through attenuating glycogen utilisation, lowering lactate production and increasing reliance
on intramuscular triglyceride (IMTG). Additionally, βHB may regulate adaptive processes in skeletal muscle by acting as
a signalling metabolite inhibiting histone deacetylases (HDAC), or through positive effects on muscle protein synthesis
(MPS). Ketone ester supplements facilitate acute ingestion of βHB resulting in nutritional ketosis, which, through these
mechanisms, may alter exercise metabolism, improve exercise performance, and influence recovery and the adaptive
response to exercise.
Abbreviations AcAc, acetoacetate; AcAc-CoA, acetoacetyl CoA; ACAT, acetyl-CoA acetyltransferase; βHB,
β-hydroxybutyrate; BDH, 3-hydroxybutyrate dehydrogenase; CHO, carbohydrate; CPT1, carnitine palmitoyltransferase;
FFA, free-fatty acid; HDAC, histone deacetylase; HMG-CoA, hydroxymethylglutaryl-CoA; HMGCL, HMG-CoA lyase;
HMGCS, HMG CoA synthase; KB, ketone body; KE, (R)-3-hydroxybutyl (R)-3-hydroxybutyrate ketone monoester;
MCT, monocarboxylate transporter; OXCT, succinyl-CoA:3-oxoacid CoA transferase; PDH, pyruvate dehydrogenase;
PEK, post-exercise ketosis; PFK, phosphofructokinase; PGC-1, peroxisome proliferator-activated receptor gamma
coactivator1;SLC,soluteligandcarrier;TCA,tricarboxylicacid.
Introduction
Overthepastcentury,exercisephysiologistshave
appreciated the role of carbohydrate (CHO) and fat in
energy provision to exercising skeletal muscle. Much of
the work examining the metabolic response to exercise
and the impact of exercise on metabolic regulation
and adaptive responses to training has focused on the
relative contribution of these fuels (Egan & Zierath,
2013). Optimising training and nutrition strategies by
manipulating the relative intakes of these macronutrients
is central to supporting elite sports performance (Cermak
& van Loon, 2013; Bartlett et al. 2015; Burke, 2015).
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J Physiol 000.00 Ketone bodies and exercise 3
An alternative fuel source to CHO and fat are ketone
bodies (KBs), namely acetoacetate (AcAc), acetone, and
β-hydroxybutyrate (βHB), which are produced in the liver
during physiological states and nutritional manipulations
that result in reduced CHO availability, most commonly
during prolonged fasting, starvation, and ketogenic (very
low CHO (5%), low protein (15%), high fat (80%))
diets (Robinson & Williamson, 1980; Laffel, 1999). This
relative glucose deprivation and concomitant elevation in
circulating free-fatty acids (FFAs) results in the production
of KBs to replace glucose as the primary fuel for peripheral
tissues such as the brain, heart and skeletal muscle in these
states.
Aside from a role as an alternative fuel source, KBs exert
a range of metabolic effects including attenuating glucose
utilisation in peripheral tissues, anti-lipolytic effects on
adipose tissue, and potential attenuation of proteolysis in
skeletal muscle (Robinson & Williamson, 1980). KBs are
utilised by working muscle during exercise (Fery & Balasse,
1986, 1988), and the capacity to take up and oxidise
KBs during exercise is higher in exercise-trained skeletal
muscle (Winder et al. 1975). Despite these observations,
in addition to a glucose sparing action (Maizels et al. 1977)
and potential to lower the exercise-induced rise in plasma
[lactate] (Fery & Balasse, 1988), the potential performance
benefits of KBs when provided as an exogenous fuel
source has received little attention, but has been post-
ulated (Cox & Clarke, 2014; Pinckaers et al. 2016). Apart
from a role as an alternative fuel source, KBs may act
as signalling molecules to regulate gene expression and
adaptive responses (Shimazu et al. 2013; Zou et al.
2016). Moreover, therapeutic roles for KBs have long
been proposed in a variety of disease states including
aberrant glucose metabolism, genetic myopathies, hypoxic
states and neurodegenerative pathologies (Veech, 2004).
For therapeutic effects, exogenous ketones are ingested
in the form of βHBsaltsorketoneesterstoproduce
acute (0.5 to 6 h) nutritional ketosis (Clarke et al.
2012; Kesl et al. 2016), but a surge in interest in KBs as
a performance aid for athletes arose when ketone ester
supplementation was confirmed in professional cycling
(Abraham, 2015; Pinckaers et al. 2016). Moreover, a recent
report provides the first evidence for acute nutritional
ketosis achieved by ketone ester ingestion to alter the
metabolic response to exercise and enhance exercise
performance (Cox et al. 2016). Aspects of ketogenic
diets, ketogenesis and ketone body metabolism have been
reviewed elsewhere (Robinson & Williamson, 1980; Laffel,
1999; Paoli et al. 2013), so the present review will focus
on the physiology of ketone bodies during and after
exercise and in response to training, with specific inter-
est in exploring the physiological basis for exogenous
supplementation and potential benefits for performance
and recovery in athletes.
Overview of ketone body metabolism
Ketone bodies in circulation. Plasma [KB] reflects the
balance between hepatic production (‘ketogenesis’) and
peripheral breakdown and utilisation (‘ketolysis’) in
extra-hepatic tissues, both of which are under various
levels of control as detailed in previous reviews (Robinson
& Williamson, 1980; Laffel, 1999). Ketogenesis is an
evolutionarily conserved adaptive response playing a
critical role in survival during an energy crisis by providing
a substrate for brain, which cannot utilise FFAs as
afuelsource.AcAc,acetone,andβHB comprise the
KBs, although βHB is not technically a ketone because
the ketone moiety has been reduced to a hydroxyl
group. AcAc and βHB are short-chain, four carbon
organic acids that act as FFA-derived circulating sub-
strates to provide energy to extra-hepatic tissues, whereas
the contribution of acetone, readily generated by the
spontaneous decarboxylation of AcAc, to energy provision
is negligible. Plasma [KB] is <0.1 mMin the post-
prandial state, whereas hyperketonaemia is accepted
as [KB] exceeding 0.2 mM(Robinson & Williamson,
1980). Various states of CHO restriction, depletion
and dysregulation produce hyperketonaemia to different
degrees (Fig. 1).
Ketogenesis. The primary substrate for ketogenesis is
FFAs liberated from adipose tissue. Ketogenic amino
acids, namely leucine, lysine, phenylalanine, isoleucine,
tryptophan, and tyrosine also serve ketogenesis, but are
likely contribute to less than 5% of circulating KBs
(Thomas et al. 1982). The rise in FFAs is consequent
to the stimulation of lipolysis as a result of declines
in plasma glucose and insulin that are characteristic of
reduced CHO availability. Factors stimulating ketogenesis
include an elevated glucagon-to-insulin ratio and decline
in hepatic glycogen concentration, while reduced blood
flowtotheliverorelevationsin[KBs]suppressketogenesis
(Robinson & Williamson, 1980; Laffel, 1999). Ketogenesis
involves a series of sequential reactions beginning with
acetyl CoA (Ac-CoA) and acetoacetyl CoA (AcAc-CoA),
and ending with the liberation of AcAc (Fig. 2). Some
AcAc is exported, but the majority is reduced to βHB
in an NAD+–NADH-coupled near equilibrium reaction
catalysed by 3-hydroxybutyrate dehydrogenase (BDH), in
which the equilibrium constant favours βHB formation.
These KBs are transported into the circulation via the
solute ligand carrier (SLC) protein 16A (SLC16A)f amily of
monocarboxylate transporters (MCTs) in mitochondrial
and sarcolemmal membranes.
Ketolysis in extra-hepatic tissues. In peripheral
tissues, KBs, primarily in the form of βHB, enter
the mitochondrial matrix again via MCT1-mediated
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4 M.Evansandothers J Physiol 000.00
transport. βHB is re-oxidised to AcAc via BDH after
which sequential reactions result in the generation of two
molecules of Ac-CoA (Fig. 2). These are incorporated into
the TCA cycle via citrate synthase for terminal oxidation
and production of ATP, which in skeletal muscle
contributes to fuelling muscular work (Fery & Balasse,
1986, 1988). Succinyl-CoA:3-oxoacid CoA transferase
(OXCT) is essential for ketolysis in extra-hepatic tissues,
with very low abundance in hepatocytes explaining
the lack of ketolytic activity in these cells (Robinson &
Williamson, 1980).
Activity of OXCT is highest in heart and kidney,
followed by skeletal muscle and the brain (Robinson &
Williamson, 1980), but because skeletal muscle accounts
for 40% of body mass in adult humans, this organ
accounts for the highest fraction of total KB metabolism at
rest (Balasse & Fery, 1989; Laffel, 1999). Beginning almost
50 years ago, models using various durations of fasting,
and combined with primed constant infusion of radio-
labelled either AcAc or βHB tracers and arteriovenous
difference measures to quantify KB turnover, established
that skeletal muscle is a major site of ketolysis at rest
(Hagenfeldt & Wahren, 1968; Owen & Reichard, 1971;
Wahren et al. 1984; Elia et al. 1990; Mikkelsen et al. 2015).
Skeletal muscle has a high affinity to KBs, but because of
low circulating concentrations under normal conditions,
the contribution to energy provision in muscle is less than
5%, and FFAs are the main source of energy provision in
the post-absorptive state. The relationship between ketone
oxidation and [KB] is curvilinear such that contribution
to energy provision in skeletal muscle rises to 10% after
an overnight fast (Hagenfeldt & Wahren, 1968; Owen &
Reichard, 1971), 20% to 50% after 72 h of fasting (Owen
& Reichard, 1971; Elia et al. 1990), but declines to 15%
after 24 days of starvation (Owen & Reichard, 1971). Thus,
skeletal muscle demonstrates saturation kinetics for the
KB concentration–oxidation relationship, with saturation
likely between 1 and 2 mMas demonstrated by fasting of
various durations (compiled in Balasse & Fery, 1989) or
step-wise βHB infusion (Mikkelsen et al. 2015).
Effect of aerobic exercise training on enzymes
of ketogenesis and ketolysis
Adaptations to exercise training reduce perturbations to
homeostasis during subsequent bouts of exercise, and
thereby enhance resistance to fatigue. Central to these
effects are enhanced respiratory capacity and contractile
parameters, and importantly adaptations that contribute
towards maximising delivery and utilisation of circulating
substrates (reviewed in Egan & Zierath, 2013). Therefore,
if KBs make a meaningful contribution to energy provision
during exercise, it is pertinent to explore analogous
regulation in skeletal muscle. Training-induced changes in
expression and activities of enzymes of ketolysis in skeletal
muscle have not been described in humans, but differences
in KB metabolism during and after exercise between
trained and untrained individuals have been reported
(Johnson et al. 1969; Johnson & Walton, 1972; Rennie et al.
1974; Rennie & Johnson, 1974a). The general pattern is for
attenuation in trained individuals of the post-exercise rise
in [KB], but this is influenced by nutritional manipulation
and relative exercise intensity, the latter of which has often
been poorly controlled (see later sections).
Nevertheless, circulating concentrations reflect the
balance between ketogenesis and ketolysis, these
differences may be explained by the factors influencing
one or both. For ketogenesis, data are limited but suggest
that in exercise-trained rodents enzymatic activity of
BDH or ACAT (Winder et al. 1974), or HMGCS (Askew
et al. 1975) is unaltered in liver, and, in fact, the over-
all activity of the ketogenic pathway may be lower (El
0.0
Post-prandial
Post-exercise ketosis
3 to 7 day fasted
Ketogenic diet
Starvation
Diabetic ketoacidosis
Overnight fasted
24 h fasted
2.0
4.0
6.0
8.0
10.0
12.0
14.0
Plasma βHB concentration
(mM)
Optimal range for acute
nutritional ketosis?
Concentration
range
achieved by KE
ingestion
Figure 1. Changes in [βHB] under various
physiological states
Plasma [KB] is <0.1 mMin the postprandial state
when consuming high CHO or high protein
meals, and rises upward after an overnight fast
and with ketogenic dieting, prolonged fasting,
starvation, and pathological states of
ketoacidosis. After prolonged aerobic exercise,
post-exercise ketosis (0.3 to 2.0 mM) may ensue
depending on intensity and duration of exercise,
aerobic fitness and nutrition status. The
circulatingKBratioofβHB:AcAc is generally
1:1 to 3:1, but during the aforementioned
nutritional states can rise six- to tenfold, such
that [KB] primarily reflects changes in [βHB]. An
optimal concentration range for βHB to improve
performance after exogenous ketone ingestion is
proposed as 1to3m
M, with concentrations
ranging from 1to5m
Mreported after ketone
ester (KE) ingestion. See text for further details.
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J Physiol 000.00 Ketone bodies and exercise 5
Midaoui et al. 2006) compared to untrained rodents. In
these rodent models of intense aerobic exercise training,
the activities of the ketolytic enzymes BDH, OXCT and
ACAT are higher in trained skeletal muscle (Winder
et al. 1974, 1975; Askew et al. 1975; Beattie & Winder,
1984). This coincides with two- to threefold higher ex
vivo rates of βHB and AcAc oxidation in gastrocnemius
muscle homogenates presented with concentrations of
both βHBandAcAcat0.1and0.5m
M(Winder et al. 1973,
1975).
In terms of muscle fibre type, enzymatic activities of
BDH, OXCT and ACAT are all highest in typ e I fibres,inter-
mediate in type IIA fibres, and lowest in type IIB fibres of
rats (Winder et al. 1974). BDH is essentially undetectable
in type IIB muscle fibres, and across the fibre types
BDH activity is much lower than activities of OXCT and
ACAT (Winder et al. 1974). Although OXCT is essential
for ketolysis, BDH activity is, therefore, potentially rate
limiting in skeletal muscle. When rats performed 12 weeks
of treadmill running, compared to sedentary rats BDH
activity was almost threefold higher in type I fibres, but
sixfold higher in type IIA fibres of trained skeletal muscle,
resulting in levels comparable to the type I fibres (Winder
et al. 1974). OXCT activity was 26% higher in type I,
and approximately twofold higher in type IIA and IIB
fibres, whereas ACAT activity was 40% to 45% higher in
all three fibre types in trained skeletal muscle (Winder et al.
1974). Similarly, in skeletal muscle from mice with 8 weeks
of access to running wheels, the difference compared to
sedentary mice was greater for BDH mRNA expression
βHB βHB
Liver Skeletal muscle
FFAFA CoA
TCA cycle
Ac-CoA AcAc-CoA
HMG-CoA
AcAc AcAc AcAc
Acetone
βHB
β oxidation
ACAT AcAc-CoA Ac-CoA
ACAT
+
+
+
+
HMGCS
HMGCL
OXCT
BDH BDH
MCT MCT
Figure 2. Metabolic pathways of ketone body metabolism in liver and skeletal muscle
Ketogenesis: FFAs are converted to fatty acyl CoA (FA-CoA), enter hepatic mitochondria via CPT1-mediated
transport and undergo β-oxidation to acetyl CoA. Sequential reactions of condensation of Ac-CoA molecules to
acetoacetyl CoA (AcAc-CoA) by mitochondrial thiolase activity of Ac-CoA acetyltransferase (ACAT), generation of
hydroxymethylglutaryl-CoA (HMG-CoA) by hydroxymethylglutaryl CoA synthase (HMGCS), and decomposition of
HMG-CoA, liberating AcAc and Ac-CoA, in a reaction catalysed by HMG-CoA lyase (HMGCL). AcAc is the central
KB, and some will be exported to the circulation but the majority is reduced to βHB in an NAD+–NADH-coupled
near equilibrium reaction catalysed by BDH, in which the equilibrium constant favours βHB formation. Ketolysis: The
only metabolic fate of βHB is inter-conversion with AcAc, and upon entry into peripheral tissues it is re-oxidised
to AcAc. Covalent activation of AcAc by CoA is catalysed by succinyl-CoA:3-oxoacid CoA transferase (OXCT)
resulting in generation of AcAc-CoA. This near equilibrium reaction exchanges CoA between succinate and AcAc,
with succinyl-CoA acting as a CoA donor. Because the free energy released by hydrolysis of AcAc-CoA is greater
than that of succinyl-CoA, the equilibrium of this reaction thermodynamically favours the formation of AcAc. Two
molecules of Ac-CoA are liberated by thiolytic cleavage of AcAc-CoA by ACAT, after which Ac-CoA is incorporated
into the TCA cycle. Protein content and enzyme activity that are higher in exercise-trained skeletal muscle are
indicated by the green cross (+).
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6 M.Evansandothers J Physiol 000.00
(twofold higher than sedentary) compared to differences
in OXCT and ACAT mRNA expression (30% to 50%
higher) (Svensson et al. 2016). These changes in ketolytic
enzymes are localised to the working muscle given the
absence of change after training in the heart (Askew et al.
1975), kidney and brain (Winder et al. 1974).
In terms of KB transport into skeletal muscle, similarly
to the ketolytic enzymes, MCT1 protein expression is
highest in type I fibres, poorly expressed in type II fibres,
and correlates well with muscle oxidative capacity (Bonen,
2001). Elevated MCT1 protein expression after exercise
training is well-established for human skeletal muscle,
and increases occur in an intensity-dependent manner
(Thomas et al. 2012). Using a rodent perfused hindlimb
model, the capacity for uptake of KBs in skeletal muscle at
1m
Meach of βHB and AcAc was higher in an aerobically
trained group of rats, with uptake of total KB, AcAc and
βHB 33%, 27% and 53% higher, respectively, compared
to untrained rats (Ohmori et al. 1990). Similarly, βHB
clearance during a βHB tolerance test is higher in mice
given 8 weeks of running wheel access, or with enhanced
oxidative capacity consequent to skeletal muscle over-
expression of PGC-1α, a transcriptional co-activator and
master regulator of mitochondrial biogenesis in adaptive
responses such as exercise training (Svensson et al. 2016).
In both conditions, this coincides with elevated expression
of MCT1 and the ketolytic enzymes in skeletal muscle.
Therefore, the uptake and utilisation of KBs in skeletal
muscle is likely to be greatest in those individuals that are
highly trained with a high proportion of type I muscle
fibres and a high oxidative capacity in skeletal muscle.
Ketone body metabolism during exercise
The existing literature on fuel selection during exercise has
focused almost exclusively on utilisation of CHO and fat,
but skeletal muscle has the ability to resynthesize ATP from
other substrates including protein, lactate and KBs (Fery &
Balasse, 1986, 1988; Mazzeo et al. 1986; Wagenmakers et al.
1991). With increasing exercise intensity, the contribution
of substrates to energy provisions shifts from blood-borne
FFAs and glucose to increased reliance on intramuscular
fuel stores, namely intramuscular triglyceride (IMTG) and
muscle glycogen, such that at moderate to high intensities
(>75% ˙
VO2max) of exercise, muscle glycogen is the main
source of energy provision (van Loon et al. 2001). This
pattern is readily altered by nutritional manipulation such
as CHO loading and acute CHO ingestion resulting in
increased CHO utilisation (Bosch et al. 1996), glycogen
depletion resulting in increased contribution of protein to
energy provision (Wagenmakers et al. 1991), and habitual
high fat consumption resulting in increased contribution
of fat to energy provision (Volek et al. 2016). Clearly,
skeletal muscle is a major site of ketolysis under fasting
conditions, but central to the rationale for exogenous
ketone supplementation must be the observations that
ketolysis increases during exercise, makes a meaningful
contribution to energy provision, and can alter patterns of
substrate utilisation.
The pioneering work of Hagenfeldt, Wahren and
colleagues (Hagenfeldt & Wahren, 1968, 1971; Wahren
et al. 1984) and Fery, Balasse and colleagues (Balasse
et al. 1978; Fery & Balasse, 1983, 1986, 1988) established
that KB disposal into human skeletal muscle is elevated
as much as fivefold during exercise. This is generally
reflected by a drop in [KB] soon after the onset of
exercise, primarily βHB, concomitant with increases in
KB oxidation in skeletal muscle and elevated metabolic
clearance rate (MCR). MCR is a measure of the ability
of tissues to remove ketones from the blood, analogous to
arteriovenous difference per unit time, but when measured
during exercise is taken to represent an index of the
ability of exercise to stimulate the capacity of working
muscles to extract and utilise ketones (Fery & Balasse,
1983; Balasse & Fery, 1989). Because the stoichiometry of
KB oxidation yields respiratory quotients of 1.00 and 0.89
for AcAc and βHB, respectively (Frayn, 1983), calculation
of oxidation rates for KBs from whole-body gas exchange
data has not been routinely performed using methods
that determine the relative contribution of CHO and
fat oxidation. However, a recent attempt has been made
(Cox et al. 2016) based on methods and assumptions
described for KB utilisation during ketogenesis (Frayn,
1983). Previous to this, oxidation rates for KBs have
historically been derived from arteriovenous differences
of radiolabelled KBs across working muscles with rates
calculated as a fraction of O2consumption or CO2
production (Hagenfeldt & Wahren, 1968; Balasse et al.
1978).
Like CHO and fat utilisation, KB metabolism during
exercise is influenced by a variety of factors including
metabolic status (Wahren et al. 1984; Fery & Balasse,
1986), training status (Johnson & Walton, 1972; Rennie
et al. 1974; Beattie & Winder, 1985), and the intensity of
exercise (Cox et al. 2016). Given the aforementioned fibre
type-specific differences for activities of ketolytic enzymes,
the muscle fibre type profile of the working muscle is also
likely to be an important determinant of ketolysis during
exercise. However, the most important determinant of KB
metabolism during exercise is the degree of ketonaemia,
and the method by which this is achieved, i.e. of end-
ogenous or exogenous origin.
Ketone body metabolism during exercise under
conditions of endogenous ketosis. Like KB metabolism
in resting skeletal muscle, the relationship between
concentration and oxidation or MCR is curvilinear
(reviewed in Balasse & Fery, 1989). At low ketonaemia
(<1.0 mM)suchasthatproducedbyanovernight
fast, resting MCR is as much as fourfold greater than
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J Physiol 000.00 Ketone bodies and exercise 7
during prolonged fasting (Fery & Balasse, 1983). During
prolonged exercise of low-to-moderate intensity after an
overnight fast, MCR increases by 50% to 75% (Fery
& Balasse, 1983, 1986), which indicates that working
muscle has an increased capacity to extract ketones from
blood compared to rest. However, when ketonaemia
exceeds 2.5 mMsuch as that achieved by greater than
72 h of fasting, the exercise-induced rise in MCR
is abolished (Fery & Balasse, 1986). Therefore, when
ketosisisachievedbyprolonged(>72 h) fasting there
is a negligible contribution of KB oxidation to energy
provision (Hagenfeldt & Wahren, 1971; Fery & Balasse,
1986), but after an overnight fast, the contribution ranges
from 2 to 10% (Balasse et al. 1978; Fery & Balasse,
1983; Wahren et al. 1984). Under these conditions,
the majority of energy provision in working muscle is
from CHO and fat as classically described (van Loon
et al. 2001). Moreover, unlike CHO and fat, there is
progressive attenuation of the oxidation of KBs with
rising ketonaemia, and thus the mobilisation of KBs
is not the factor limiting oxidation in skeletal muscle.
This attenuation of exercise-stimulated MCR suggests
either that above a threshold concentration the capacity
for skeletal muscle to oxidise KBs becomes saturated,
and/or that hyperketonaemia itself is a self-inhibitory
factor (Balasse & Fery, 1989). Mechanistically, this is
likely to be mediated either through the inhibition
of OXCT by elevated AcAc, and/or via FFA-mediated
inhibition of ketolysis (Robinson & Williamson, 1980).
This regulation is critical in the starvation response
because the capacity of the liver to produce KBs closely
matches the requirements of the brain to utilise KBs as an
energy source (Robinson & Williamson, 1980). Therefore,
excessive oxidation by working muscle would threaten
survival, whereas its inhibition spares circulating substrate
for the brain (Hagenfeldt & Wahren, 1971; Fery & Balasse,
1983).
Methods of exogenous ketone supplementation
producing acute nutritional ketosis
Investigating effects of ketosis on skeletal muscle
metabolism has been typically achieved by endogenous
ketosis using fasting of various durations (Balasse & Fery,
1989), or by exogenous ketosis produced by either ketone
salt ingestion (Johnson & Walton, 1972), or infusion of
AcAc or βHB (Fery & Balasse, 1988; Mikkelsen et al.
2015). Endogenous ketosis may also be achieved by
CHO restriction, particularly by ketogenic diets (Paoli
et al. 2013). The practical relevance for athletes seeking
performance gains of metabolic responses generated
from prolonged fasting is negligible, whereas benefits of
ketogenic dieting for performance with a high intensity
component are equivocal (Burke, 2015). This has led
to the exploration of exogenous ketone ingestion as a
means to achieve acute nutritional ketosis. Importantly,
because endogenous ketosis results in concomitant
elevations in FFAs and alterations in glucose, insulin
and counter-regulatory hormones, isolating the metabolic
effects specific to KBs has proved challenging. Therefore,
exogenous ketone supplementation is a means to address
these questions and explore potential for performance and
therapeutic benefits.
Oral administration of KBs in their free acid form is
expensive and ineffective at producing ketosis, so buffering
the free acid form with sodium/potassium/calcium salts
has been explored and these compounds are commercially
available. These too are relatively ineffective at increasing
[βHB], but may be improved by co-ingestion with
medium chain triglycerides (C:8, C:10), at least in rats
(Kesl et al. 2016). However, ingestion of large quantities of
KB salts is impractical due to resulting gastrointestinal
distress, and potentially undesirable consequences of
cation overload or acidosis (Veech, 2004).
The development of ketone esters provides an
alternative method to increase [βHB], which is
well-tolerated in rodents and humans (Clarke et al.
2012; Cox et al. 2016; Kesl et al. 2016). Two
prominent ketone esters in the published literature are
the R,S-1,3-butanediol acetoacetate diester (Kesl et al.
2016) and the (R)-3-hydroxybutyl (R)-3-hydroxybutyrate
ketone monoester (Clarke et al. 2012; Cox et al. 2016).
Acute ingestion of either ester can result in short-term
(0.5 to 6 h) nutritional ketosis indicated by [βHB]
>1m
M(Clarke et al. 2012; Kesl et al. 2016). For the
ketone monoester, ingestion at a dose of 573 mg (kg body
mass (BM))−1resulted in [βHB] of 3mMafter 10 min
and rising to 6m
M30 min after ingestion (Cox et al.
2016). Nutritional ketosis is therefore achieved without the
impracticality of prolonged fasting or ketogenic dieting.
Ketone body metabolism during exercise under
conditions of exogenous ketosis. The aforementioned
self-inhibitory effect of rising ketonaemia underscores
a key methodological issue when considering KB
metabolism in skeletal muscle, namely the method of
achieving ketosis. While fasting of various durations is
a widely used model of ketosis, acute nutritional ketosis
relevant to sports performance would be achieved with
replete glycogen stores, and in the absence of prolonged
elevations in FFAs and [KB] that would be likely to
impair KB oxidation rates through these mechanisms.
To our knowledge, only two studies have addressed
this convincingly by examining effects of exercise on
KB metabolism without interference from the various
hormonal and metabolic perturbations associated with
prolonged fasting or diabetes (Fery & Balasse, 1988; Cox
et al. 2016).
In the former study (Fery & Balasse, 1988), infusion
of sodium AcAc after an overnight fast achieved [KB]
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8 M.Evansandothers J Physiol 000.00
of 6mM(βHB 3.5 mM,AcAc2.5 mM)atthe
onset of 2 h of exercise at 52% ˙
VO2max.Notably,AcAc
did not change during exercise whereas βHB declined
throughout exercise, to be reduced by 2m
Mat the
end of exercise. This coincided with a progressive rise
in MCR throughout exercise, peaking at 75% higher
than rest at the end of exercise. In contrast, this effect
was abolished with similar ketonaemia in 3–5 day fasted
participants. Importantly, although the inhibition of
KB oxidation by hyperketonaemia is present during
exogenous ketosis, an ‘auto-amplification’ was noted that
is not present in fasting ketosis, i.e. the initial rise in MCR
induced by exercise causes a reduction in concentration
which, in turn, provokes a further rise in MCR and so
on. Additionally, the threshold concentration at which
hyperketonaemia inhibits MCR was higher in exogenous
ketosis than in fasting ketosis. However, in terms of
contribution to energy provision, this ultimately only
resulted in a 2% contribution over the 2 h exercise bout.
Nevertheless, plasma [lactate] did not rise during exercise
after AcAc infusion compared to a 1m
Mrise in the
fasted participants, which suggests that despite a modest
contribution to energy provision, exogenous ketosis can
impact on metabolic processes during exercise.
Despite this promise, these data remained largely iso-
lated for almost 30 years with the exception of a couple
of obscure reports that admittedly did recapitulate the
effects of βHB to alter the metabolic response to very
intense exercise in rats (Kamysheva & Ostrovskaia, 1980),
and ischaemic exercise in humans (Lestan et al. 1994).
The latter report, in fact, supported the ability of a modest
elevation in βHB (0.5 mM) via infusion of sodium βHB
to reduce the plasma lactate response to exercise in an
ischaemic forearm model. However, w ith the development
of the (R)-3-hydroxybutyl (R)-3-hydroxybutyrate ketone
monoester (KE), a comprehensive investigation of sub-
strate metabolism in highly trained athletes in the presence
of acute nutritional ketosis has recently been published
(Cox et al. 2016).
In one of a series of experiments, ingestion of KE
resulted in acute nutritional ketosis indicated by [βHB]
of 3m
Mafter 10 min and rising to 6mM30 min after
ingestion. During exercise lasting 45 min at either 40% or
75%Wmax,[βHB] was 2and3mM, respectively, lower
than ketosis produced after ingestion at rest. This provided
the first evidence of intensity-dependent disposal of βHB
during exercise. Moreover, based on expired air analysis
adjusted for oxidation of KBs, βHB oxidation contributed
18% and 16% of oxygen consumption to energy provision
at the respective intensities. The larger than previously
reported contribution of βHB oxidation probably reflects
the fact that the participants in these experiments were
highly trained cyclists, therefore with a greater capacity
of skeletal muscle to uptake and oxidise KBs. Moreover,
this model is markedly different to the fasting-induced
ketonaemia and the associated self-inhibitory regulation
so making direct comparisons are difficult. In a separate
exercise bout lasting 60 min at 75% ˙
Wmax and with similar
[βHB] after KE ingestion, the rise in plasma [lactate] was
blunted by 2to3m
M(50% reduction) compared
to ingestion of an isocaloric CHO drink. Subsequent
experiments with ingestion of the KE demonstrated
inhibition of glycolytic metabolism, sparing of muscle
glycogen, reduced deamination of branched-chain amino
acids, and increased reliance on IMTG during exercise
(Cox et al. 2016). Lastly, after a 60 min pre-load at
75% ˙
Wmax, cycling performance in a 30 min time-trial
was improved by 2% (411±162 m; mean ±SEM,
n=8) with KE +CHO compared to isocaloric CHO
ingestion. The KE +CHO fuelling strategy combined
KE (40%; 573 mg (kg BM)−1) with CHO (60%) and
elevated [βHB] to between 1.5 and 3 mMthroughout.
Importantly, the KE +CHO condition provided CHO
at a minimum rate of 1.2 g min−1, consistent with an
optimal CHO-based fuelling strategy (Burke, 2015). Taken
together, these data suggest that acute nutritional ketosis
by consumption of exogenous ketones has dramatic effects
on skeletal muscle metabolism during exercise, and can
confer a performance benefit to elite athletes (Fig. 3).
The positive findings notwithstanding, potential adverse
effects should be considered for any performance aid
prior to adoption. Side-effects of KE ingestion have been
reported in humans (Clarke et al. 2012). Specifically,
in a repeated dose design over 5 days, adverse effects
such as flatulence, nausea, diarrhoea and dizziness were
reported in five out of twenty-four participants at doses
ranging from 420 to 1071 mg (kg BM)−1. Such issues
were prevalent in almost all participants when the dose
was increased to 2142 mg (kg BM)−1per day, indicating a
possible upper limit of tolerability in adults (Clarke et al.
2012). Therefore, these data combined with the dosing
strategy associated with exercise performance benefits
should be used to guide future investigations on ergogenic
potential.
Ketone body metabolism after exercise: post-exercise
ketosis
Despite the aforementioned decline in [KB] at the onset
of exercise, this pertains to situations where exercise
has begun during hyperketonaemia (Balasse et al. 1978;
Fery & Balasse, 1983, 1988; Cox et al. 2016). In the
post-absorptive state, the pattern generally observed is for
[KB] to rise gradually during prolonged exercise up to 0.2
to 0.4 mM, after which time post-exercise ketosis (PEK) of
0.3 to 2.0 mMis observed for several hours into recovery
(Koeslag, 1982). Explained in terms of plasma kinetics,
at cessation of exercise, the rate of appearance of KBs
increases coincident with a decrease in MCR relative to
rates present during exercise. MCR remains above resting
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values for several hours after exercise, but ketogenesis
exceeds ketolysis during this period.
On a mechanistic level, regulation probably resides at
several sites including malonyl CoA-mediated regulation
of fat transport into hepatocytes via CPT-1 in addition to
availability of Ac-CoA for ketogenesis, and oxaloacetate
for the TCA cycle as classically described for ketogenic
regulation. Because oxaloacetate is a product of pyruvate
formed during glycolysis, reductions in glycolytic flux
with low glycogen content after intense exercise result
in oxaloacetate moving to cytoplasm for preferential use
in gluconeogenesis, which allows diversion of Ac-CoA
towards ketogenesis during the post-exercise period rather
than to citrate synthesis for the TCA cycle. Additionally,
the actions of insulin and glucagon exert a strong influence
through activation and inhibition, respectively, of Ac-CoA
carboxylase (ACC), which catalyses the synthesis of
malonyl CoA from Ac-CoA. When liver glycogen becomes
depleted and glucagon:insulin ratio is elevated, the
synthesis of malonyl CoA is reduced, thereby relieving the
inhibition of fat transport into hepatocytes, and resulting
in elevated levels of Ac-CoA. These regulatory mechanisms
are acutely sensitive to nutrient manipulations before
and after exercise and to aerobic exercise training, given
their respective influences on substrate availability and
utilisation during exercise.
Modulation of post-exercise ketosis by aerobic exercise
training and nutrition intervention. An attenuation of,
or abolished, post-exercise ketosis has been consistently
observed in rodents and humans in response to aerobic
exercise in trained versus untrained individuals (Johnson
et al. 1969; Johnson & Walton, 1972; Rennie et al.
1974), or after a period of exercise training (Rennie &
Johnson, 1974a; Beattie & Winder, 1984, 1985; Adams
& Koeslag, 1988, 1989; Ohmori et al. 1990). The
aforementioned enhanced ketolytic capacity and down-
regulation of ketogenic capacity by training may play a
role in these observations, but the majority of this work has
been performed in comparisons, with the absolute exercise
intensity and duration being the same for comparisons
(reviewed in Koeslag, 1982). This is problematic because
the relative exercise intensity is the key determinant of the
metabolic and hormonal response to acute exercise, e.g.
catecholamine responses, FFA mobilisation and glycogen
utilisation among others. When trained and untrained
participants have performed exercise at a similar relative
intensity, PEK is blunted but not abolished in trained
individuals (Rennie et al. 1974). Moreover, in rodents
when exercise is completed to exhaustion, i.e. the trained
rats exercise for longer than untrained, [βHB] is twofold
higher at the exercise cessation in the trained group (Askew
et al. 1975). These divergent findings are likely to be due to
the degree of liver glycogen depletion that occurs (Adams
& Koeslag, 1988), inasmuch as higher levels of resting
liver glycogen and attenuated rates of depletion are a
consequence of training (Baldwin et al. 1975).
Therefore, PEK is strongly influenced by nutrition
manipulation. High CHO feeding prior to exercise
attenuates PEK regardless of training status (Rennie &
Glycogenolysis
Glycolytic flux
AT lipolysis
Protein oxidation
Lactate
HDAC
NLRP3
Performance
IMTG use
Proteolysis
MPS
Oxidative stress
Gene expression
Inflammation
Recovery and
adaptation?
βHB
Figure 3. βHB as a metabolic regulator and
signalling metabolite
Effects of elevating βHB through acute nutritional
ketosis may be mediated by acute regulation of
substrate utilisation that may enhance
performance, and/or possibly through regulation of
recovery and adaptive processes related to
inflammation, oxidative stress and changes in gene
expression. See text for further discussion. AT,
adipose tissue; HDAC, histone deacetylase; IMTG,
intramuscular triglyceride; MPS, muscle protein
synthesis.
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10 M. Evans and others J Physiol 000.00
Johnson, 1974b;Askewet al. 1975; Koeslag et al. 1980),
and CHO restriction increases PEK (Impey et al. 2016).
Glucose ingestion at 2 h into recovery (Koeslag et al.
1982; Carlin et al. 1987) and alanine during recovery
(Koeslag et al. 1980, 1985; Carlin et al. 1987) attenuate
PEK, but the glucose effect is not seen when glucose
is ingested immediately after exercise. Alanine ingestion
increases mitochondrial [oxaloacetate] in liver, thereby
allowing condensation with Ac-CoA and diversion away
from ketogenesis. This suggests that the early PEK response
is determined by the extent of liver glycogen depletion
and reduced glycolytic flux, whereas several hours into
recovery it is under regulation by insulin and [FFA] related
to nutrition intake.
Metabolic consequences of post-exercise ketosis during
recovery: a role for exogenous ketones as a recovery
aid? The physiological role for PEK is likely to favour
the replenishment of muscle glycogen, consistent with
classically described metabolic actions of ketosis in the
sparing of protein and CHO stores during times of
low CHO availability. During the post-exercise recovery
period, in contrast to the reliance on CHO metabolism
during exercise, muscle glycogen resynthesis has a high
metabolic priority and is facilitated by an increase in
fat oxidation and sparing of CHO sources for energy
provision (Kiens & Richter, 1998). A priority for muscle
glycogen resynthesis over liver glycogen resynthesis is
suggested to occur because in ancestral terms, a depleted
liver is less of a hindrance to intense exertion than
depleted muscle (Adams & Koeslag, 1988). To this end,
the priority for muscle glycogen resynthesis is observed
even during CHO restriction (Adams & Koeslag, 1989),
and is achieved through non-CHO sources such as lactate
and alanine being used for hepatic gluconeogenesis and
redistribution to skeletal muscle (Fournier et al. 2002).
The contribution of PEK may be via the ability of KBs to
inhibit glycolysis and increase the conversion of glucose
to glycogen as demonstrated in rat skeletal muscle in vitro
(Maizels et al. 1977), and a perfused heart model in dogs
(Laughlin et al. 1994). This effect is likely to be mediated
by inhibition of PDH and phosphofructokinase (PFK) by
elevations in Ac-CoA and citrate formation, respectively,
as a consequence of metabolism of AcAc in mitochondria
(Randle et al. 1964; Maizels et al. 1977; Laughlin et al.
1994; Kashiwaya et al. 1997).
This raises the possibility that an optimal post-exercise
recovery milieu exists that includes both CHO and
ketones to enhance recovery of muscle glycogen. This is
not possible by conventional nutrition strategies because
elevations in glucose, lactate and alanine ultimately
limit ketogenesis and PEK. The suggestion is that
the co-ingestion of exogenous ketones and CHO in a
recovery protocol can confer a metabolic advantage.
This hypothesis remains to be tested rigorously, but a
preliminary report describes a 33% increase in glucose
disposal and 50% increase in muscle glycogen content after
2 h of recovery when nutritional ketosis (5m
MβHB)
is superimposed on a hyperglycaemic (10 mMglucose)
clamp in well-trained military servicemen (Holdsworth
et al. 2016).
Repletion of muscle glycogen is only one component
of post-exercise recovery, and nutrition strategies for
recovery include protein ingestion, with the aim to limit
muscle protein breakdown and enhance muscle protein
synthesis (MPS). KBs have protein sparing effects in
skeletal muscle as indicated by reduced alanine release
during starvation (Sherwin et al. 1975), and reduced
leucine oxidation (Nair et al. 1988). In the latter study,
this coincided with a 10% increase in MPS measured
by fractional synthesis rate and occurred with [βHB]
of 2m
Machieved via sodium βHB infusion. This
raises the possibility that acute nutritional ketosis can
complement current strategies for optimising MPS in
the post-exercise period. Additionally, because low CHO
stores during exercise lead to elevated rates of protein
oxidation (Wagenmakers et al. 1991), exogenous ketone
supplementation may provide both a fuel source and
contribute to protein sparing and recovery during training
in CHO-restricted states commonly practiced by athletes
(reviewed in Bartlett et al. 2015). Together with the
preliminary data for muscle glycogen resynthesis, this
suggests that post-exercise recovery is another application
where elite athletes may benefit from exogenous ketone
supplementation, and where future research is warranted.
Effects beyond fuelling: βHB as a HDAC inhibitor
As investigative techniques in molecular biology evolve,
so too does our appreciation of how complex integrative
signalling networks regulate skeletal muscle adaptation
in response to stimuli such as nutrient manipulation
and exercise training (Egan & Zierath, 2013). Pre-
viously considered relatively inert outside their primary
metabolic function, numerous substrates and metabolites
are emerging as important regulators of intracellular
signalling and tissue adaptation (Hashimoto et al. 2007;
Gao et al. 2009; Morton et al. 2009; Roberts et al.
2014). Noteworthy for the present review is the recent
identification of AcAc as a regulator of skeletal muscle
satellite cell proliferation and muscle regeneration (Zou
et al. 2016), and βHB as an inhibitor of HDACs (Shimazu
et al. 2013) and the NLRP3 inflammasome (Youm et al.
2015). The latter observations are a consequence of βHB,
in essence, acting as a signalling metabolite to regulate
gene expression and metabolic processes (Fig. 3).
Histone acetyltransferases (HATs) and HDACs are
enzymes that facilitate the addition or removal,
respectively, of acetyl moieties from specific lysine
residues on histones and target proteins (McKinsey
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et al. 2001). In general, hyperacetylation of histone tails
induces transcriptional activation while hypoacetylation
is associated with transcriptional repression. Class IIa
HDACs (HDAC4, -5, -7 and -9) are highly expressed in
skeletal muscle (McKinsey et al. 2001) and their function is
responsive to both aerobic endurance exercise in humans
(McGee et al. 2009; Egan et al. 2010) and nutritional
intervention in rodents (Gao et al. 2009; Shimazu
et al. 2013). An acute bout of aerobic exercise increases
class IIa HDAC phosphorylation and subsequent nuclear
exclusion, thus inhibiting HDAC-mediated repression of
specific exercise-responsive genes such as GLUT4 and
PGC-1α(McGee & Hargreaves, 2004; McGee et al. 2009;
Egan et al. 2010). This suggests that compounds that
inhibit or disrupt HDACinhibition could be used to mimic
or enhance adaptations to exercise.
Regulation of HDAC activity by nutrients including
butyrate and βHB has also been established (Gao et al.
2009; Shimazu et al. 2013). Butyrate, a short chain fatty
acid formed via the fermentation of indigestible dietary
fibres by microbial species in the gut, is a potent inhibitor
of HDAC activity (Gao et al. 2009). Mice supplemented
with sodium butyrate are resistant to diet-induced
obesity, and have elevations in markers of skeletal muscle
mitochondrial biogenesis analogous to exercise effects
(Gao et al. 2009). βHB is structurally similar to butyrate,
and although not as potent as butyrate, also inhibits HDAC
class I and II activity in a dose-dependent manner and
supressed oxidative stress responses (Shimazu et al. 2013).
Importantly, HDAC inhibition by βHB both in vitro and in
vivo is evident at physiologically relevant concentrations
of βHB,i.e.1to4m
M, which is similar to those attained
during fasting, PEK and exogenous ketone ingestion
(Fig.1;Clarkeet al. 2012; Kesl et al. 2016). However,
although the inhibitory effects were observed in multiple
tissues, they remain to be confirmed in skeletal muscle. If
confirmed,itwillbeintriguingtoexplorewhether,apart
from the aforementioned ergogenic effects, exogenous
ketone supplementation complements exercise-mediated
adaptive changes associated with modulating HDAC
function (Fig. 3).
Exogenous ketone supplementation for athletes:
cautionary notes and future directions
Despite a strong physiological basis for a variety of
benefits for performance and recovery, the relatively
recent availability of exogenous ketones and thus far only
one peer-reviewed paper examining exercise metabolism,
performance and nutritional ketosis, means that much
more research remains to be performed (Pinckaers et al.
2016). The central tenet is that the combination of
fuel sparing and improved energetic efficiency during
acute nutritional ketosis confers performance benefits
(Fig. 3). Alterations in fuel selection during steady-state
exercise have been demonstrated, which indicate reduced
glycolytic flux, sparing of CHO and increased contribution
of IMTG and βHB to energy provision (Cox et al.
2016). Whether this sparing of CHO, in fact, manifests as
impaired CHO utilisation remains to be determined. The
mechanistic basis for CHO sparing by exogenous ketones
is presently proposed as inhibition of glycolytic flux via
inhibition of PDH and PFK by increases in NADH:NAD+,
acetyl-CoA:CoA ratio or citrate. In theory, this could be
problematic for sports that rely heavily on contributions
from glycolytic pathways, or a range of sports that are inter-
mittent and/or require periods of high intensity ‘bursts’
on a moderate intensity background. This is analogous to
the lack of performance benefits for most athletes under-
taking low CHO, high fat diets (Burke, 2015). In fact,
impaired performance during high intensity efforts has
been observed under such conditions (Havemann et al.
2006), and may be explained by sustained attenuation
of PDH activity (Stellingwerff et al. 2006). Whether the
same effects are observed with acute nutritional ketosis
given that this is a very different metabolic milieu,
especially in the context of exercise, remains to be
explored.
The metabolic consequences of inhibition of adipose
tissue lipolysis by KBs also warrants further exploration,
given that this process is an important contributor to
circulating FFAs, and therefore to the contribution of fat
oxidation to energy provision during long duration, sub-
maximal exercise. Nutritional ketosis achieved by either
AcAc infusion (Fery & Balasse, 1988) or KE ingestion
(Cox et al. 2016) inhibits the lipolytic effect of exercise,
i.e. the amount of lipid-derived substrates available for
working muscle is reduced. In the latter study, this did
not manifest as increased glycogenolysis and/or glucose
utilization, despite these usually being accelerated by the
inhibition of FFA availability (van Loon et al. 2005). In
fact, glycogenolysis was attenuated and IMTG utilisation
was increased in the KE experiments (Cox et al. 2016),
suggesting differential regulation to that achieved by
nicotinic acid administration (van Loon et al. 2005).
However, in each of the experimental conditions with KE,
the duration of exercise was between 45 and 120 min
at moderate intensity (Cox et al. 2016). Recently, the
inhibition of lipolysis via nicotinic acid impaired cycling
time-trial performance in long (120 min), but not shorter
(60 and 90 min) duration efforts (Torrens et al. 2016).
Thus, even in events with high CHO dependence (80
to 95% of energy provision), inhibition of lipolysis
may impair endurance performance, particularly in long
duration activities analogous to professional cycling
or triathlon. Clearly, the many nodes of metabolic
regulation influencing skeletal muscle fuel selection that
are altered by nutritional ketosis need to be fully elucidated
before sports-specific ergogenic strategies can be
advised.
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Improved energetic efficiency is an often-cited potential
benefit of acute nutritional ketosis (Veech, 2004; Cox
& Clarke, 2014). In this model, exogenous ketones
may provide thermodynamic advantages over CHO and
fat, because the available free energy to perform work,
the free energy of ATP hydrolysis (GATP ), is greater
with KBs, and require less oxygen per mole of carbon
to oxidise. Support for this hypothesis comes from a
perfused working rat heart model where adding KBs
to the perfusate supressed glycolytic flux, and increased
hydraulic efficiency (expressed as work in J (mol O2
consumed)−1) by 28% (Sato et al. 1995; Kashiwaya et al.
1997). In practical terms, if the same effect occurs in
skeletal muscle, this would translate as a higher power
output for the same oxygen consumption (i.e. improved
muscular efficiency) during exercise with nutritional
ketosis, but this remains unexplored at present.
Because KBs serve as a substrate for the brain, and
therapeutic uses for KBs have been proposed for cognitive
enhancement and neurodegenerative pathologies (Veech,
2004), the central nervous system (CNS) may be another
target for performance-enhancing effects of nutritional
ketosis. Although speculative at present, effects related to
motor recruitment, perceived exertion, pacing strategies,
skill execution, reaction time, and decision-making will be
interesting for future research, in addition to the proposed
role for the CNS in regulating performance beyond effects
related to skeletal muscle metabolism (Noakes, 2011).
As with any ergogenic aid or nutrition strategy,
optimising dosing strategies including quantity and timing
will be important. Given the saturation kinetics of KB
oxidation by skeletal muscle and curvilinear relationship
between oxidation and plasma concentrations, it is likely
that there is an optimal range for performance benefits.
At present, we speculate that this exists between 1 and
3m
MβHB. As with many ergogenic acids, more is
unlikely to be better and may even be deleterious given the
potential for acidosis at higher [KB], and aforementioned
gastrointestinal distress and other side-effects sometimes
observed with KE, so careful consideration should be given
to these issues.
In conclusion, although data are preliminary, acute
nutritional ketosis achieved by exogenous ketone
supplementation has the potential to alter fuel selection
during exercise and confer performance benefits. This
is most likely to be the case in trained individuals who
have a greater capacity to take up and oxidise KBs
during exercise as a result of training. Additionally, a
strong physiological basis exists that suggests potential
benefits for supporting training and recovery. While much
work remains to be performed, particularly in relation to
sport-specific strategies, this promises to be an exciting
topic for scientists, practitioners and athletes alike for the
coming years.
References
Abraham R (2015). Ketones: Controversial new energy drink
could be next big thing in cycling. Cycling Weekly.
Adams JH & Koeslag JH (1988). Carbohydrate homeostasis and
post-exercise ketosis in trained and untrained rats. JPhysiol
407, 453–461.
Adams JH & Koeslag JH (1989). Glycogen metabolism and
post-exercise ketosis in carbohydrate-restricted trained and
untrained rats. QJExpPhysiol74, 27–34.
Askew EW, Dohm GL & Huston RL (1975). Fatty acid and
ketone body metabolism in the rat: response to diet and
exercise. JNutr105, 1422–1432.
Balasse EO & Fery F (1989). Ketone body production and
disposal: effects of fasting, diabetes, and exercise. Diabetes
Metab Rev 5, 247–270.
Balasse EO, Fery F & Neef MA (1978). Changes induced by
exercise in rates of turnover and oxidation of ketone bodies
in fasting man. J Appl Physiol Respir Environ Exerc Physiol 44,
5–11.
Baldwin KM, Fitts RH, Booth FW, Winder WW & Holloszy JO
(1975). Depletion of muscle and liver glycogen during
exercise. Protective effect of training. Pflugers Arch 354,
203–212.
Bartlett JD, Hawley JA & Morton JP (2015). Carbohydrate
availability and exercise training adaptation: too much of a
good thing? Eur J Sport Sci 15, 3–12.
Beattie MA & Winder WW (1984). Mechanism of
training-induced attenuation of postexercise ketosis. Am J
Physiol Regul Integr Comp Physiol 247, R780–R785.
Beattie MA & Winder WW (1985). Attenuation of postexercise
ketosis in fasted endurance-trained rats. Am J Physiol Regul
Integr Comp Physiol 248, R63–R67.
Bonen A (2001). The expression of lactate transporters
(MCT1 and MCT4) in heart and muscle. Eur J Appl Physiol
86, 6–11.
Bosch AN, Weltan SM, Dennis SC & Noakes TD (1996). Fuel
substrate kinetics of carbohydrate loading differs from that
of carbohydrate ingestion during prolonged exercise.
Metabolism 45, 415–423.
Burke LM (2015). Re-examining high-fat diets for sports
performance: did we call the ‘nail in the coffin’ too soon?
Sports Med 45 (Suppl. 1), S33–49.
Carlin JI, Olson EB Jr, Peters HA & Reddan WG (1987). The
effects of post-exercise glucose and alanine ingestion on
plasma carnitine and ketosis in humans. JPhysiol390,
295–303.
Cermak NM & van Loon LJ (2013). The use of carbohydrates
during exercise as an ergogenic aid. Sports Med 43,
1139–1155.
Clarke K, Tchabanenko K, Pawlosky R, Carter E, Todd King
M, Musa-Veloso K, Ho M, Roberts A, Robertson J,
Vanitallie TB & Veech RL (2012). Kinetics, safety and
tolerability of (R)-3-hydroxybutyl (R)-3-hydroxybutyrate in
healthy adult subjects. Regul Toxicol Pharmacol 63,
401–408.
Cox PJ & Clarke K (2014). Acute nutritional ketosis:
implications for exercise performance and metabolism.
Extrem Physiol Med 3, 17.
C
2016 The Authors. The Journal of Physiology C
2016 The Physiological Society
J Physiol 000.00 Ketone bodies and exercise 13
Cox PJ, Kirk T, Ashmore T, Willerton K, Evans R, Smith A,
MurrayAJ,StubbsB,WestJ,McLureSW,KingMT,Dodd
MS, Holloway C, Neubauer S, Drawer S, Veech RL, Griffin JL
& Clarke K (2016). Nutritional ketosis alters fuel preference
and thereby endurance performance in athletes. Cell Metab
24, 256–268.
Egan B, Carson BP, Garcia-Roves PM, Chibalin AV,
Sarsfield FM, Barron N, McCaffrey N, Moyna NM, Zierath
JR & O’Gorman DJ (2010). Exercise intensity-dependent
regulation of peroxisome proliferator-activated receptor γ
coactivator-1 αmRNA abundance is associated with
differential activation of upstream signalling kinases in
human skeletal muscle. JPhysiol588, 1779–1790.
Egan B & Zierath JR (2013). Exercise metabolism and the
molecular regulation of skeletal muscle adaptation. Cell
Metab 17, 162–184.
El Midaoui A, Chiasson JL, Tancrede G & Nadeau A (2006).
Physical training reverses the increased activity of the hepatic
ketone body synthesis pathway in chronically diabetic rats.
Am J Physiol Endocrinol Metab 290, E207–E212.
Elia M, Wood S, Khan K & Pullicino E (1990). Ketone body
metabolism in lean male adults during short-term
starvation, with particular reference to forearm muscle
metabolism. Clin Sci (Lond) 78, 579–584.
Fery F & Balasse EO (1983). Ketone body turnover during and
after exercise in overnight-fasted and starved humans. Am J
Physiol Endocrinol Metab 245, E318–E325.
Fery F & Balasse EO (1986). Response of ketone body
metabolism to exercise during transition from
postabsorptive to fasted state. Am J Physiol Endocrinol Metab
250, E495–E501.
Fery F & Balasse EO (1988). Effect of exercise on the disposal of
infused ketone bodies in humans. J Clin Endocrinol Metab
67, 245–250.
FournierPA,BrauL,FerreiraLD,FairchildT,RajaG,JamesA
& Palmer TN (2002). Glycogen resynthesis in the absence of
food ingestion during recovery from moderate or high
intensity physical activity: novel insights from rat and
human studies. Comp Biochem Physiol A Mol Integr Physiol
133, 755–763.
Frayn KN (1983). Calculation of substrate oxidation rates
in vivo from gaseous exchange. JApplPhysiol55,
628–634.
Gao Z, Yin J, Zhang J, Ward RE, Martin RJ, Lefevre M, Cefalu
WT & Ye J (2009). Butyrate improves insulin sensitivity and
increases energy expenditure in mice. Diabetes 58,
1509–1517.
Hagenfeldt L & Wahren J (1968). Human forearm muscle
metabolism during exercise. 3. Uptake, release and oxidation
of beta-hydroxybutyrate and observations on the
beta-hydroxybutyrate/acetoacetate ratio. Scand J Clin Lab
Invest 21, 314–320.
Hagenfeldt L & Wahren J (1971). Human forearm muscle
metabolism during exercise. VI. Substrate utilization
in prolonged fasting. Scand J Clin Lab Invest 27,
299–306.
Hashimoto T, Hussien R, Oommen S, Gohil K & Brooks GA
(2007). Lactate sensitive transcription factor network in L6
cells: activation of MCT1 and mitochondrial biogenesis.
FASEB J 21, 2602–2612.
Havemann L, West SJ, Goedecke JH, Macdonald IA, St Clair
Gibson A, Noakes TD & Lambert EV (2006). Fat adaptation
followed by carbohydrate loading compromises
high-intensity sprint performance. J Appl Physiol (1985) 100,
194–202.
Holdsworth D, Cox PJ & Clarke K (2016). Oral ketone body
supplementation accelerates and enhances glycogen
synthesis in human skeletal muscle following exhaustive
exercise [Abstract]. In Proceedings of the Physiological Society.
London, UK.
Impey SG, Hammond KM, Shepherd SO, Sharples AP, Stewart
C, Limb M, Smith K, Philp A, Jeromson S, Hamilton DL,
Close GL & Morton JP (2016). Fuel for the work required: a
practical approach to amalgamating train-low paradigms for
endurance athletes. Physiol Rep 4, e12803.
Johnson RH & Walton JL (1972). The effect of exercise upon
acetoacetate metabolism in athletes and non-athletes. QJ
Exp Physiol Cogn Med Sci 57, 73–79.
Johnson RH, Walton JL, Krebs HA & Williamson DH (1969).
Metabolic fuels during and after severe exercise in athletes
and non-athletes. Lancet 2, 452–455.
Kamysheva VA & Ostrovskaia RU (1980). [Effect of sodium
hydroxybutyrate on the ammonia level in the rat
muscles under physical exercise]. Biull Eksp Biol Med 89,
25–27.
Kashiwaya Y, King MT & Veech RL (1997). Substrate signaling
by insulin: a ketone bodies ratio mimics insulin action in
heart. Am J Cardiol 80, 50a–64a.
Kesl SL, Poff AM, Ward NP, Fiorelli TN, Ari C, Van Putten AJ,
Sherwood JW, Arnold P & D’Agostino DP (2016). Effects of
exogenous ketone supplementation on blood ketone,
glucose, triglyceride, and lipoprotein levels in
Sprague-Dawley rats. Nutr Metab (Lond) 13,9.
Kiens B & Richter EA (1998). Utilization of skeletal muscle
triacylglycerol during postexercise recovery in humans. Am J
Physiol Endocrinol Metab 275, E332–E337.
Koeslag JH (1982). Post-exercise ketosis and the hormone
response to exercise: a review. Med Sci Sports Exerc 14,
327–334.
Koeslag JH, Levinrad LI, Lochner JD & Sive AA (1985).
Post-exercise ketosis in post-prandial exercise: effect of
glucose and alanine ingestion in humans. JPhysiol358,
395–403.
Koeslag JH, Noakes TD & Sloan AW (1980). Post-exercise
ketosis. JPhysiol301, 79–90.
Koeslag JH, Noakes TD & Sloan AW (1982). The effects of
alanine, glucose and starch ingestion on the ketosis
produced by exercise and by starvation. JPhysiol325,
363–376.
Laffel L (1999). Ketone bodies: a review of physiology,
pathophysiology and application of monitoring to diabetes.
Diabetes Metab Res Rev 15, 412–426.
Laughlin MR, Taylor J, Chesnick AS & Balaban RS (1994).
Nonglucose substrates increase glycogen synthesis in vivo
in dog heart. Am J Physiol Heart Circ Physiol 267,
H219–H223.
Lestan B, Walden K, Schmaltz S, Spychala J & Fox IH (1994).
β-Hydroxybutyrate decreases adenosine triphosphate
degradation products in human subjects. JLabClinMed
124, 199–209.
C
2016 The Authors. The Journal of Physiology C
2016 The Physiological Society
14 M. Evans and others J Physiol 000.00
McGee SL, Fairlie E, Garnham AP & Hargreaves M (2009).
Exercise-induced histone modifications in human skeletal
muscle. JPhysiol587, 5951–5958.
McGee SL & Hargreaves M (2004). Exercise and myocyte
enhancer factor 2 regulation in human skeletal muscle.
Diabetes 53, 1208–1214.
McKinsey TA, Zhang CL & Olson EN (2001). Control of
muscle development by dueling HATs and HDACs. Curr
Opin Genet Dev 11, 497–504.
Maizels EZ, Ruderman NB, Goodman MN & Lau D (1977).
Effect of acetoacetate on glucose metabolism in the soleus
and extensor digitorum longus muscles of the rat. Biochem J
162, 557–568.
Mazzeo RS, Brooks GA, Schoeller DA & Budinger TF
(1986). Disposal of blood [1-13C]lactate in humans during
rest and exercise. J Appl Physiol (1985) 60, 232–241.
Mikkelsen KH, Seifert T, Secher NH, Grondal T & van Hall G
(2015). Systemic, cerebral and skeletal muscle ketone body
and energy metabolism during acute
hyper-D-beta-hydroxybutyratemia in post-absorptive
healthy males. J Clin Endocrinol Metab 100,
636–643.
Morton JP, Croft L, Bartlett JD, Maclaren DP, Reilly T, Evans L,
McArdle A & Drust B (2009). Reduced carbohydrate
availability does not modulate training-induced heat shock
protein adaptations but does upregulate oxidative enzyme
activity in human skeletal muscle. JApplPhysiol106,
1513–1521.
Nair KS, Welle SL, Halliday D & Campbell RG (1988). Effect of
beta-hydroxybutyrate on whole-body leucine kinetics and
fractional mixed skeletal muscle protein synthesis in
humans. JClinInvest82, 198–205.
Noakes TD (2011). Time to move beyond a brainless exercise
physiology: the evidence for complex regulation of human
exercise performance. Appl Physiol Nutr Metab 36,
23–35.
Ohmori H, Kawai K & Yamashita K (1990). Enhanced ketone
body uptake by perfused skeletal muscle in trained rats.
Endocrinol Jpn 37, 421–429.
Owen OE & Reichard GA Jr (1971). Human forearm
metabolism during progressive starvation. JClinInvest50,
1536–1545.
Paoli A, Rubini A, Volek JS & Grimaldi KA (2013). Beyond
weight loss: a review of the therapeutic uses of very-
low-carbohydrate (ketogenic) diets. Eur J Clin Nutr 67,
789–796.
Pinckaers PJ, Churchward-Venne TA, Bailey D & van Loon LJ
(2016). Ketone bodies and exercise performance: the next
magic bullet or merely hype? Sports Med. (in press; DOI:
10.1007/s40279-016-0577-y).
Randle PJ, Newsholme EA & Garland PB (1964).
Regulation of glucose uptake by muscle. 8. Effects of
fatty acids, ketone bodies and pyruvate, and of alloxan-
diabetes and starvation, on the uptake and metabolic fate of
glucose in rat heart and diaphragm muscles. Biochem J 93,
652–665.
Rennie MJ, Jennett S & Johnson RH (1974). The metabolic
effects of strenuous exercise: a comparison between
untrained subjects and racing cyclists. QJExpPhysiolCogn
Med Sci 59, 201–212.
Rennie MJ & Johnson RH (1974a). Alteration of metabolic and
hormonal responses to exercise by physical training. Eur J
Appl Physiol Occup Physiol 33, 215–226.
Rennie MJ & Johnson RH (1974b). Effects of an exercise-diet
program on metabolic changes with exercise in runners.
JApplPhysiol37, 821–825.
Roberts LD, Bostrom P, O’Sullivan JF, Schinzel RT,
Lewis GD, Dejam A, Lee YK, Palma MJ, Calhoun S,
Georgiadi A, Chen MH, Ramachandran VS, Larson MG,
Bouchard C, Rankinen T, Souza AL, Clish CB, Wang TJ,
Estall JL, Soukas AA, Cowan CA, Spiegelman BM & Gerszten
RE (2014). β-Aminoisobutyric acid induces browning of
white fat and hepatic β-oxidation and is inversely
correlated with cardiometabolic risk factors. Cell Metab 19,
96–108.
Robinson AM & Williamson DH (1980). Physiological roles of
ketone bodies as substrates and signals in mammalian
tissues. Physiol Rev 60, 143–187.
Sato K, Kashiwaya Y, Keon CA, Tsuchiya N, King MT, Radda
GK, Chance B, Clarke K & Veech RL (1995). Insulin, ketone
bodies, and mitochondrial energy transduction. FASEB J 9,
651–658.
Sherwin RS, Hendler RG & Felig P (1975). Effect of ketone
infusions on amino acid and nitrogen metabolism in man.
JClinInvest55, 1382–1390.
Shimazu T, Hirschey MD, Newman J, He W, Shirakawa K, Le
Moan N, Grueter CA, Lim H, Saunders LR, Stevens RD,
Newgard CB, Farese RV Jr, de Cabo R, Ulrich S, Akassoglou
K & Verdin E (2013). Suppression of oxidative stress by
β-hydroxybutyrate, an endogenous histone deacetylase
inhibitor. Science 339, 211–214.
Stellingwerff T, Spriet LL, Watt MJ, Kimber NE, Hargreaves M,
Hawley JA & Burke LM (2006). Decreased PDH activation
and glycogenolysis during exercise following fat adaptation
with carbohydrate restoration. Am J Physiol Endocrinol
Metab 290, E380–E388.
Svensson K, Albert V, Cardel B, Salatino S & Handschin C
(2016). Skeletal muscle PGC-1αmodulates systemic ketone
body homeostasis and ameliorates diabetic hyperketonemia
in mice. FASEB J 30, 1976–1986.
Thomas C, Bishop DJ, Lambert K, Mercier J & Brooks GA
(2012). Effects of acute and chronic exercise on sarcolemmal
MCT1 and MCT4 contents in human skeletal muscles:
current status. Am J Physiol Regul Integr Comp Physiol 302,
R1–R14.
Thomas LK, Ittmann M & Cooper C (1982). The role of leucine
in ketogenesis in starved rats. Biochem J 204, 399–403.
Torrens SL, Areta JL, Parr EB & Hawley JA (2016).
Carbohydrate dependence during prolonged simulated
cycling time trials. Eur J Appl Physiol 116, 781–790.
vanLoonLJ,GreenhaffPL,Constantin-TeodosiuD,SarisWH
& Wagenmakers AJ (2001). The effects of increasing exercise
intensity on muscle fuel utilisation in humans. JPhysiol536,
295–304.
van Loon LJ, Thomason-Hughes M, Constantin-Teodosiu D,
Koopman R, Greenhaff PL, Hardie DG, Keizer HA, Saris
WH & Wagenmakers AJ (2005). Inhibition of adipose tissue
lipolysis increases intramuscular lipid and glycogen use in
vivo in humans. Am J Physiol Endocrinol Metab 289,
E482–E493.
C
2016 The Authors. The Journal of Physiology C
2016 The Physiological Society
J Physiol 000.00 Ketone bodies and exercise 15
Veech RL (2004). The therapeutic implications of ketone
bodies: the effects of ketone bodies in pathological
conditions: ketosis, ketogenic diet, redox states, insulin
resistance, and mitochondrial metabolism. Prostaglandins
Leukot Essent Fatty Acids 70, 309–319.
Volek JS, Freidenreich DJ, Saenz C, Kunces LJ, Creighton BC,
Bartley JM, Davitt PM, Munoz CX, Anderson JM, Maresh
CM, Lee EC, Schuenke MD, Aerni G, Kraemer WJ & Phinney
SD (2016). Metabolic characteristics of keto-adapted
ultra-endurance runners. Metabolism 65, 100–110.
Wagenmakers AJ, Beckers EJ, Brouns F, Kuipers H, Soeters PB,
van der Vusse GJ & Saris WH (1991). Carbohydrate
supplementation, glycogen depletion, and amino acid
metabolism during exercise. Am J Physiol Endocrinol Metab
260, E883–E890.
Wahren J, Sato Y, Ostman J, Hagenfeldt L & Felig P (1984).
Turnover and splanchnic metabolism of free fatty
acids and ketones in insulin-dependent diabetics at
rest and in response to exercise. JClinInvest73,
1367–1376.
Winder WW, Baldwin KM & Holloszy JO (1973).
Exercise-induced adaptive increase in rate of oxidation of
β-hydroxybutyrate by skeletal muscle. Proc Soc Exp Biol Med
143, 753–755.
Winder WW, Baldwin KM & Holloszy JO (1974). Enzymes
involved in ketone utilization in different types of muscle:
adaptation to exercise. Eur J Biochem 47, 461–467.
Winder WW, Baldwin KM & Holloszy JO (1975).
Exercise-induced increase in the capacity of rat skeletal
muscle to oxidize ketones. Can J Physiol Pharmacol 53,
86–91.
Youm YH, Nguyen KY, Grant RW, Goldberg EL, Bodogai M,
Kim D, D’Agostino D, Planavsky N, Lupfer C, Kanneganti
TD, Kang S, Horvath TL, Fahmy TM, Crawford PA, Biragyn
A, Alnemri E & Dixit VD (2015). The ketone metabolite
β-hydroxybutyrate blocks NLRP3 inflammasome-mediated
inflammatory disease. Nat Med 21, 263–269.
ZouX,MengJ,LiL,HanW,LiC,ZhongR,MiaoX,CaiJ,
Zhang Y & Zhu D (2016). Acetoacetate accelerates muscle
regeneration and ameliorates muscular dystrophy in mice.
J Biol Chem 291, 2181–2195.
Additional information
Competing interests
The authors declare no conflict of interest.
Author contributions
B.E. conceived the review and drafted the outline. M.E., K.E.C.
and B.E. drafted the initial manuscript, revised and finalised
the content. All authors have approved the final version of the
manuscript and agree to be accountable for all aspects of the
work. All persons designated as authors qualify for authorship,
and all those who qualify for authorship are listed.
Funding
M.E. is supported by funding through a UCD Institute for Sport
and Health Student Bursary. K.E.C. is supported by funding
from Food for Health Ireland (FHI).
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