REVIEW
Open Access
A review of the ketogenic diet for
endurance athletes: performance enhancer
or placebo effect?
Caitlin P. Bailey*
and Erin Hennessy
Abstract
Background: The ketogenic diet has become popular among endurance athletes as a performance enhancer. This
paper systematically reviews the evidence regarding the effect of the endurance athlete’s ketogenic diet (EAKD) on
maximal oxygen consumption (VO2 max) and secondary performance outcomes.
Methods: PubMed and Web of Science searches were conducted through November 2019. Inclusion criteria were
documentation of EAKD (< 50 g daily carbohydrate consumed by endurance athletes), ketosis achieved (measured via
serum biomarker), VO2 max and/or secondary outcomes, English language, and peer reviewed-publication status. Articles
were excluded if they were not a primary source or hypotheses were not tested with endurance athletes (i.e., individuals
that compete at submaximal intensity for extended time periods). Study design, diet composition, adherence assessment,
serum biomarkers, training protocols, and VO2 max/secondary outcomes were extracted and summarized.
Results: Searches identified seven articles reporting on VO2 max and/or secondary outcomes; these comprised six
intervention trials and one case study. VO2 max outcomes (n = 5 trials, n = 1 case study) were mixed. Two of five trials
reported significant increases in VO2 max across all diets; while three trials and one case study reported no significant VO2
max findings. Secondary outcomes (n = 5 trials, n = 1 case study) were Time to Exhaustion (TTE; n = 3 articles), Race Time
(n = 3 articles), Rating of Perceived Exertion (RPE; n = 3 articles), and Peak Power (n = 2 articles). Of these, significant findings
for EAKD athletes included decreased TTE (n = 1 article), higher RPE (n = 1 article), and increased Peak Power (n = 1 article).
Conclusion: Limited and heterogeneous findings prohibit definitive conclusions regarding efficacy of the EAKD for performance benefit.
When compared to a high carbohydrate diet, there are mixed findings for the effect of EAKD consumption on VO2 max and other
performance outcomes. More randomized trials are needed to better understand the potentially nuanced effects of EAKD consumption on
endurance performance. Researchers may also consider exploring the impact of genetics, recovery, sport type, and sex in moderating the
influence of EAKD consumption on performance outcomes.
Keywords: Ketogenic diet, High fat diet, Ketosis, Endurance athlete, VO2 max
Background
The ketogenic diet prescribes a significant reduction in
carbohydrate intake, which facilitates physiological changes
that promote the utilization of ketones [1]. Recently this
diet has received attention from the endurance community
as a potential ergogenic aid because it minimizes the body’s
reliance on carbohydrates. Despite evidence-based guidance
for athletes to consume adequate carbohydrates [2], it has
been proposed that the biological constraints of carbohy-
drate storage may limit athletes who compete over ex-
tended time periods [3, 4]. Carbohydrates are stored in the
body predominately as glycogen in muscle tissue (300 g)
and liver tissue (90 g), in addition to glucose in the blood
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* Correspondence: caitlin.bailey@tufts.edu
The Gerald J. and Dorothy R. Friedman School of Nutrition Science and
Policy at Tufts University, 150 Harrison Avenue, Boston, MA 02111, USA
Bailey and Hennessy Journal of the International Society of Sports Nutrition
          (2020) 17:33 
https://doi.org/10.1186/s12970-020-00362-9
stream (30 g) [5]. This amounts to roughly 1680 kcal of
available energy from carbohydrate at any one time. As a
result, endurance athletes must replenish their glycogen
stores every one to three hours during activity [5]. This
continual consumption redirects nutrients from exercising
muscles to the gut to aide digestion, potentially leading to
reduced exercise economy and digestive disturbances,
which compromise the athlete’s ability to maximize training
and competition outcomes [3]. Additionally, research indi-
cates that training with low muscle glycogen availability
promotes molecular changes that enhance training-derived
endurance adaptations [6]. Furthermore, ketogenic diets
have been shown to reduce lactate accumulation after exer-
cise, contributing to enhanced recovery [7, 8]. Taken
together, this evidence suggests that reduced reliance on
carbohydrates via ketosis can produce beneficial results for
endurance athletes.
In contrast to the limitations of carbohydrate storage, the
body can reserve large amounts of energy in the form of
fat. One pound of fat yields approximately 3500 kcal, mak-
ing fat a vast source of energy, even among relatively lean
endurance athletes. In theory, if endurance athletes tolerate
the ketogenic diet, they could achieve longer training pe-
riods with sustained energy levels and reduced need for re-
fueling, allowing them to maximize the aerobic benefits
from training and competing. In fact, there is some evi-
dence that, among highly trained individuals, benefits of the
diet include a steady supply of energy for the body and
brain during prolonged exercise and accelerated recovery
time post-exercise [4]. While scientists continue to explore
potential benefits and drawbacks of the endurance athlete’s
ketogenic diet (EAKD), several public figures in the athletic
community have already embraced the diet as ergogenic [9,
10]. However, to the authors’ knowledge, there have been
no systematic reviews of EAKD consumption and endur-
ance outcomes (e.g., VO2 max, TTE, Race Time, RPE, Peak
Power) from which such conclusions may be drawn.
To fill this gap, the present review characterizes the na-
ture and extent of available scientific evidence regarding
the claim that EAKD consumption results in improved
endurance performance, as measured by maximal oxygen
uptake (VO2 max). VO2 max is considered the gold stand-
ard for measuring aerobic fitness. It is measured via a
graded exercise test on a treadmill or a cycle ergometer,
and quantified as the body’s maximum oxygen use in mil-
liliters per kilogram of body weight per minute [11].
Higher levels of VO2 max indicate greater endurance cap-
acity. It is important to note that while VO2 max is an
established measure of endurance capacity, relative VO2
max is confounded by changes in body weight and thus
not without limitations. For this reason, secondary per-
formance outcomes (i.e., time to exhaustion [TTE], race
time, rating of perceived exertion [RPE], peak power) were
also collected for analysis.
This manuscript is intended to enhance the athletic and
scientific communities’ knowledge of the potential benefits
and consequences of adopting the EAKD, and to identify
gaps in the current literature that may create opportun-
ities for future study. Specifically, this review focuses on
peer-reviewed articles examining endurance athletes (e.g.,
cyclists, runners, race walkers, triathletes) participating in
three or more weeks of EAKD consumption. The included
studies looked at a variety of outcomes; however, the pri-
mary outcome of interest to this review is VO2 max.
Main text
Methods
Articles were identified for inclusion via electronic data-
base literature searches. An initial search was conducted
using Web of Science and PubMed, on February 1, 2018.
Subsequent searches of Web of Science and PubMed
were conducted, using identical search criteria, in
order to capture the most recent publications avail-
able. The final search was conducted on November
17, 2019. The following key terms were used to
search the databases for articles by topic: ketogenic,
race, walker, cyclist, runner, marathon, endurance,
and athlete. The full search strategy used for both da-
tabases is as follows: ((ketogenic) AND (race[Title] OR
walker*[Title] OR cyclist*[Title] OR runner*[Title] OR
marathon*[Title] OR endurance[Title] OR athlet*[Ti-
tle])). Asterisks denote truncation. Additional inclu-
sion criteria were English language, peer reviewed-
publication status, ketosis achieved (as measured via
serum biomarkers), and documentation of VO2 max
and/or secondary outcomes. The following exclusions
were applied to the searches in order to narrow the
scope of the article lists generated: NOT (epilepsy or
child or mice or mouse or diabet* or rat* or seizure).
Articles were included for review if the title, abstract,
or key words indicated that the study focused on the ke-
togenic diet in the context of endurance sport training
and/or racing (i.e., the EAKD). Articles that met inclu-
sion criteria from each database were compiled using
Endnote software. Duplicates were removed, and ab-
stracts were pre-screened for source type. Articles were
excluded if they were not a primary source.
After identifying all eligible records, a data matrix was
developed and data were extracted on the following vari-
ables: study design, athlete type (i.e., sport, training level,
age range), diet type (i.e., EAKD, high carbohydrate,
periodised carbohydrate) and composition, recruitment
numbers, study length, dietary adherence assessment
method, serum biomarkers for ketosis, training proto-
cols, and VO2 max/secondary outcomes. Data from the
matrix are presented in Tables 1 and 2. Results were
synthesized qualitatively.
Bailey and Hennessy Journal of the International Society of Sports Nutrition           (2020) 17:33 
Page 2 of 11
Table 1 Descriptive results
Reference
Sample
size
Population, age
range
Study design
Study
length
Methods
Diet composition
Diet provision & assessment
Ketosis biomarker
Training protocol
VO2 Max
protocol
Prospective trials
Burke
et al. 2017
[12]
N = 29
Professional
male race
walkers with
international
race experience,
21–32 years
Self-selected diet
(non-random assignment)
3 weeks
Diets: EAKD (< 50 g CHO,
75–80% FAT, 15–20%
PRO [n = 10])a; HCD (60–
65% CHO, 20% FAT, 15–
20% PRO [n = 9])a; PCHO
(60–65% CHO, 20% FAT,
15–20% PRO [n = 10])a
Personalized menus
developed by professional
chef and RDs. All foods
provided/recorded by
research team.
Beta-hydroxybutyrate
levels post-EAKD:
0.8–2.0 mmol/liter
Olympic-level
training camp.
Included daily race
walking, resistance
training, and/or
cross training.
Treadmill test
Carr et al.
2018 [7]
N = 24
Male (n = 17)
and female (n =
7) elite race
walkers
Self-selected diet
(non-random assignment)
3 weeks
Diets: EAKD (< 50 g CHO,
75–80% FAT, 15–20%
PRO [n = 9]); HCD (60–
65% CHO, 20% FAT, 15–
20% PRO [n = 8]); PCHO
(60–65% CHO, 20% FAT,
15–20% PRO [n = 7])
Menus developed by
professional chef and RDs.
All foods provided/recorded
by research team.
Elevated serum
ketone bodies post-
EAKD: 1 mmol/liter
Supervised, sport-
specific, 3-week
training protocol.
Treadmill test
Heatherly
et al. 2018
[13]
N = 8
Middle-age,
recreationally
competitive
male runners,
39.5 ± 9.9 years
Pre-posttest
3 weeks
Diets: EAKD (< 50 g CHO,
target 70% FAT [ad
libitum]); HCD (habitual
pre-study diet, reported
as “moderate to high
CHO”)
Participants provided with
daily macronutrient targets
and instructed to self-track
diet using diet software.
Elevated serum
ketone bodies post-
dietary intervention
compared to pre-
EAKD levels: 0.7 ±
0.52 mmol/liter
(EAKD) vs. 0.25 ± 0.09
mmol/liter (CHO)
Participants
continued normal
recreational athletic
activity for study
duration.
Treadmill test
(pre-EAKD
only). %
baseline VO2
max at
various race
paces post-
EAKD
reported.
McSwiney
et al. 2018
[14]
N = 20
Male endurance
trained athletes
(e.g., triathlon,
cycling,
marathon, ultra-
marathon), 18–
40 years
Self-selected diet
(non-random assignment)
12 weeks
Diets: EAKD (< 50 g CHO,
> 75% FAT, 10–15% PRO
[n = 9]); HCD (65% CHO,
20% FAT, 14% PRO [n =
11])
Participants received detailed
handouts (e.g., example meal
plans, shopping lists),
nutrition counseling, and
weekly check-ins. Weekly
weighed food diary
submitted.
Beta-hydroxybutyrate
levels post-EAKD: 0.5
mmol/liter
≥ 7h hours of
endurance exercise
and 2 strength
training sessions
per week.
Cycle
ergometer
test
Phinney
et al. 1983
[15]
N = 5
Elite male
cyclists, 20–30
years
Pre-posttest
4 weeks
Diets: EAKD (< 20 g CHO,
85% FAT, 15% PRO); HCD
(1.75 g PRO/kg/day,
remainder as 66% CHO
and 33% FAT)a
Participants received three
meals per day. Portions were
weighed and intake
monitored.
Beta-hydroxybutyrate
levels post-EAKD:
1.28 ± 0.35 mmol/liter
Participants were
asked to continue
normal training,
monitored via daily
diary.
Cycle
ergometer
test
Shaw
et al. 2019
[16]
N = 8
Male endurance
trained athletes
(n = 2
marathoners,
n = 4 ultra-
marathoners,
n = 2 triathletes),
29.6 ± 5.1 years
Randomized repeated
measures crossover study
31-days
(4.5 weeks)
per
condition
with a 14-
to 21-day
washout
period
Diets: EAKD (< 50 g CHO,
75–80% FAT, 15–20%
PRO); HCD (43% CHO,
38% FAT, 19% PRO)a
Participants received
education session with RD,
info booklet, personalized
menu plan, meal/snack
examples, and lifestyle
advice. All had daily contact
with a registered dietitian for
monitoring.
Beta-hydroxybutyrate
levels post-EAKD:
≥0.3 mmol/liter by
day 3 and ≥ 0.5
mmol/liter by day 7
Participants
designed their own
28-day training plan
(running and cyc-
ling) and were
asked to replicate
this during each
dietary period.
Treadmill test
Bailey and Hennessy Journal of the International Society of Sports Nutrition           (2020) 17:33 
Page 3 of 11
Table 1 Descriptive results (Continued)
Reference
Sample
size
Population, age
range
Study design
Study
length
Methods
Diet composition
Diet provision & assessment
Ketosis biomarker
Training protocol
VO2 Max
protocol
Case studies
Zinn et al.
2017 [17]
N = 5
Recreational
athletes
involved in
competitive
endurance sport
for 5+ years, 49–
55 years
Pilot case study, mixed
methods research
10 weeks
Diet: EAKD (< 50 g CHO,
ad libitum FAT, 1.5 g/kg
PRO [n = 5])
Participants provided with
daily macronutrient
prescription and instructed
to self-track diet using diet
software.
Beta-hydroxybutyrate
levels: 0.5–4.2 mmol/
liter
Participants
continued normal
recreational athletic
activity for study
duration.
Cycle
ergometer
test
EAKD Endurance Athlete Ketogenic Diet, HCD High Carbohydrate Diet, PCHO Periodised carbohydrate diet: percentages based on weekly rather than daily diet, CHO Carbohydrate, PRO Protein, RD Registered dietitian
aIsocaloric diets
Bailey and Hennessy Journal of the International Society of Sports Nutrition           (2020) 17:33 
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Table 2 Study outcomes: VO2 max and secondary outcomes. Dashes indicate that studies did not assess the specified variable(s)
Reference
VO2 max outcomes (mL/kg/min)
Time to exhaustion
(TTE)
Race time/Time trial
Rating of perceived exertion (RPE)
Peak power
Prospective Trials
Burke
et al. 2017
[12]
Significant increase in VO2 max from baseline (p <
0.001) in all three groups. VO2 Max of the HCD
group was significantly lower than for the other
groups both pre- and post-diet (p ≤ 0.02).
Pre- vs. post-intervention
EAKD: 66.3 vs. 71.1
HCD: 61.6 vs. 66.2
PCHO: 64.9 vs. 67.0
_ _
EAKD group: Non-
significant increase in
10 km race time from
baseline.
HCD and PCHO
groups: Significant
decrease in race
time (p < 0.01).
Pre- vs. post-
intervention
EAKD: 23 s slower
HCD: 190 s faster
PCHO: 124 s faster
EAKD group: Significantly higher RPE values for
post-intervention graded economy test compared
with pre-intervention RPE values (p ≤ 0.01). Non-
significant trend for higher RPE values during 25 km
long walk for both pre- and post-testing.
_ _
Carr et al.
2018 [7]
Significant increase in VO2 max from baseline (p <
0.05) in all three groups. Between groups analysis
not reported.
Pre- v. post-intervention (M ± SD)
EAKD: 61.1 ± 5.3 vs. 63.4 ± 4.1
HCD: 57.6 ± 4.6 vs. 58.3 ± 4.1
PCHO: 58.1 ± 3.3 vs. 60.2 ± 3.8
_ _
_ _
_ _
_ _
Heatherly
et al. 2018
[13]
Post-EAKD VO2 max not measured. Study reported %
baseline VO2 max at various race paces. At 10 km, 21
km, 42 km and sub-42 km (but not 5 km) race
paces, % relative VO2 max was significantly
greater post-EAKD.
Example (10 km pace; p < 0.05):
EAKD: 98.7 ± 11.3
HCD: 92.8 ± 5.3
_ _
5 km time trial time
was not significantly
different pre- vs. post-
EAKD (p > 0.10).
Pre- vs. post-
intervention
EAKD: 23.45 ± 2.25
min.
HCD: 23.92 ± 2.57 min.
Overall RPE did not differ significantly pre- vs. post-
EAKD during 5 km time trial (P > 0.10).
Pre- vs. post-intervention
EAKD: 8.4 ± 1.2
HCD: 8.0 ± 1.0
_ _
McSwiney
et al. 2018
[14]
Increase in both groups post-diet. Non-significant dif-
ference between groups (p = 0.968).
Pre- vs. post-intervention
EAKD: 53.6 ± 6.8 vs. 57.3 ± 6.7
HCD: 52.6 ± 6.4 vs. 57.2 ± 6.1
_ _
100 km time trial time
was not significantly
different between
groups (p = 0.057).
Pre- vs. post-
intervention
EAKD: 4.07 min.sec
faster
HCD: 1.13 min.sec
faster
_ _
Post-intervention peak
power was significantly
different between
groups (p = 0.047).
Pre- vs. post-intervention
EAKD: 8.3 ± 2.2 vs. 9.7 ±
2.3; 1.4 watts/kg increase
HCD: 9.1 ± 2.6 vs. 8.4 ±
2.2; 0.7 watts/kg decrease
Phinney
et al. 1983
[15]
Non-significant decrease from baseline (HCD; p > 0.01).
Pre- vs. post-intervention
EAKD: 5.00 ± 0.20
HCD: 5.10 ± 0.18
Non-significant
increase in mean
exercise times from
baseline (HCD).
Pre- vs. post-
intervention
EAKD: 151 ± 25
_ _
_ _
_ _
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Page 5 of 11
Table 2 Study outcomes: VO2 max and secondary outcomes. Dashes indicate that studies did not assess the specified variable(s) (Continued)
Reference
VO2 max outcomes (mL/kg/min)
Time to exhaustion
(TTE)
Race time/Time trial
Rating of perceived exertion (RPE)
Peak power
min.
HCD: 147 ± 13 min.
Shaw
et al. 2019
[16]
No significant change from pre-intervention levels for
either dietary exposure (p > 0.05).
Pre-intervention (all athletes)
59.4 ± 5.2
No significant
difference between
dietary
interventions (p =
0.56).
Pre- vs. post-
intervention
EAKD: 239 ± 27 vs.
219 ±
53 min. (p = 0.36)
HCD: 237 ± 44 vs.
231 ± 35 min. (p =
0.44)
_ _
RPE values were similar for each dietary intervention
during run-to-exhaustion trials.
1-h, 2-h, at exhaustion
EAKD: 11.4 ± 0.9, 12.1 ± 1.4, 19.38 ± 0.52
HCD: 11.7 ± 0.8, 12.8 ± 0.9, 19.38 ± 0.52
_ _
Case studies
Zinn et al.
2017 [17]
Non-significant change from baseline (M ± SD): −
1.69 ± 3.4 (p = 0.63).
(with a decrease in four of the five athletes)
Significant
decrease in TTE
for all
participants (p =
0.004).
Mean change from
baseline
EAKD: − 2 ± 0.7
min.
_ _
_ _
Four out of five athletes
experienced a decrease
in peak power from
baseline (p = 0.07).
Mean change from
baseline
EAKD: − 18 ± 16.4 watts
EAKD Endurance Athlete Ketogenic Diet, HCD High Carbohydrate Diet, PCHO Periodised carbohydrate diet
Bailey and Hennessy Journal of the International Society of Sports Nutrition           (2020) 17:33 
Page 6 of 11
Results
Search results
Figure 1 illustrates the screening process and articles in-
cluded in this review. In brief, searches from Web of Sci-
ence and PubMed generated n = 60 articles (n = 33 and 27,
respectively). After removing duplicates and pre-screening,
28 articles remained. After further review, 21 additional re-
cords were excluded (see Fig. 1 for reasons for exclusion).
All exclusions were conducted to emphasize the effect of
ketogenic diet consumption on sport-specific performance
in endurance athletes. The screening process produced
seven eligible articles: six prospective trials (n = 1 random-
ized crossover study, n = 3 non-randomized trials, n = 2
pre-posttest), and one case study. See Fig. 1 for a flow
chart of the screening process.
Descriptive results
Among the seven studies included in this review, sex and
athlete type were inextricable variables. Five of seven stud-
ies examined VO2 max outcomes in only male athletes
Fig. 1 Flow chart depicting the literature search and review process to arrive at the final analytic sample (n = 7). Arrows pointing right indicate
the number of articles excluded and for what reason
Bailey and Hennessy Journal of the International Society of Sports Nutrition           (2020) 17:33 
Page 7 of 11
[12–16]. However, among those studies, athlete type var-
ied: one study recruited male runners [13], one recruited
male race walkers [12], one recruited male cyclists [15],
and two recruited a mixed sample of male endurance ath-
letes [14, 16]. Two of the seven studies recruited both
male and female athletes; one recruited a sample of race
walkers [7] and the other recruited a sample of mixed en-
durance athletes [17]. Ages for study participants ranged
from 18 to 55 years. All seven studies included an EAKD
(< 50 g daily carbohydrate). Of the six trial studies, all in-
cluded a standard, high carbohydrate comparison diet [7,
12–16], while the case study provided no comparison diet
[17]. Studies either provided participants with meals [7,
12, 15] or with dietary guidance, including sample meal
plans [13, 14, 16, 17]. Adherence to diet was tracked via
objective researcher observation and measurement [7, 12,
15] or participant self-report (e.g., weighed food diaries,
dietary analysis software) [13, 14, 16, 17]. All studies expli-
citly reported tracking serum ketone levels as a biomarker
for ketosis. All studies lasted between three and 12 weeks.
Performance outcome results
VO2 max outcomes (mL/kg/min; n = 6 studies) were
mixed: two studies reported significant increases in VO2
max across all diets [7, 12], and four reported no significant
VO2 max outcomes [14–17]. In a three-week nonrando-
mized trial, Carr et al. reported significant increases in VO2
max from baseline for all diet types (EAKD: 61.1 ± 5.3 vs.
63.4 ± 4.1; HCD: 57.6 ± 4.6 vs. 58.3 ± 4.1; PCHO: 58.1 ± 3.3
vs. 60.2 ± 3.8; p < 0.05) [7]. Using a similar design, Burke
et al. found a significant increase in VO2 max for all ath-
letes (EAKD: 66.3 vs. 71.1; HCD: 61.6 vs. 66.2; PCHO: 64.9
vs. 67.0; p < 0.001) [12]. McSwiney et al. showed a 3.7-unit
increase in relative VO2 max among the EAKD group after
12 weeks (53.6 ± 6.8 vs. 57.3 ± 6.7) [14]. This was a smaller
increase than the 4.6-unit increase observed in the com-
parison diet group (52.6 ± 6.4 vs. 57.2 ± 6.1); furthermore,
the increase in relative VO2 max during EAKD consump-
tion was inflated by a 6-kg mean reduction in body mass
among the participants. The difference in increase between
the two groups was not significant (p = 0.968) [14]. Shaw
et al., a randomized crossover study, found no significant
changes in VO2 max from baseline (59.4 ± 5.2) after either
31 days of EAKD or high carbohydrate comparison diet
(p > 0.05) [16]. Using a pre-posttest design, Phinney et al.
found no difference in VO2 max between a high carbohy-
drate comparison diet and EAKD (pre-intervention HCD:
5.10 ± 0.18; EAKD: 5.00 ± 0.20; p > 0.01) [15]. Heatherly
et al., also a pre-posttest design, measured VO2 max pre-
but not post-EAKD consumption [13]. Instead, this study
reported on the percent of baseline (pre-dietary interven-
tion) VO2 max achieved at various race paces tested post-
EAKD consumption. Researchers found that the percent of
baseline relative VO2 max achieved was significantly
greater post-EAKD at 10 km, 21 km, 42 km, and sub-42 km
(but not 5 km) race paces (see Table 2; p < 0.05) [13]. Fi-
nally, Zinn et al. showed a non-significant decrease from
baseline VO2 max in athletes consuming the EAKD after
10 weeks (− 1.69 ± 3.4; p = 0.63) [17]. Zinn et al. was a case
study with no reference comparison diet.
Secondary outcomes (n = 6 studies) were also mixed. Of
three studies that reported TTE, Shaw et al. and Phinney
et al. each found no significant difference in TTE by diet
type [15, 16], while Zinn et al. reported a significant
decrease from baseline (pre-dietary intervention) for all five
case study participants consuming the EAKD (− 2 ± 0.7
min.; p = 0.004) [17]. Differences in race times by dietary
intervention were reported by three studies [12–14] and
found to be significant in one [12]. Specifically, Burke et al.
reported a significant decrease in race time among high
carbohydrate and periodized carbohydrate groups (HCD:
− 190 s; PCHO: − 124 s; p < 0.01), while the EAKD group
had a non-significant increase in race time (EAKD: + 23 s;
p > 0.01) [12]. RPE was measured in three studies [12, 13,
16] and found to be significantly different from baseline in
one [12]. Burke et al. reported higher RPE values among
the EAKD group post-intervention compared with pre-
intervention (p ≤ 0.01) [12]. Finally, peak power was mea-
sured in two studies [14, 17]. McSwinney et al. reported
that post-intervention peak power was significantly differ-
ent between diets, with EAKD athletes improving their
peak power and comparison diet athletes decreasing their
peak power (EAKD: 8.3 ± 2.2 vs. 9.7 ± 2.3 watts/kilogram;
HCD: 9.1 ± 2.6 vs. 8.4 ± 2.2 watts/kilogram; p = 0.047). Zinn
et al. found a mean decrease in peak power from baseline
(− 18 ± 16.4 watts; p = 0.07) with a decrease in four out of
five athletes [17]. See Table 2 for a full list of results.
Discussion
It has been hypothesized that consuming a ketogenic diet
may enhance performance among endurance athletes by
promoting a shift in substrate utilization that enhances
physiological training benefits [3, 18]. The present review
explores
this
hypothesis
by
examining
associations
between EAKD consumption and VO2 max, a biomarker
for endurance capacity [11]. Two of the seven studies in-
cluded in this review found a significant increase in VO2
max post-EAKD consumption [7, 12]. However, both arti-
cles reported significant VO2 max increases across all di-
ets, and that outcomes were independent of dietary
intervention. Interestingly, both studies were conducted
among elite race walkers that self-selected their dietary
intervention, and the athletes that self-selected into the
EAKD had slightly higher average baseline and post-
treatment VO2 max values [7, 12]. Furthermore, Burke
et al., reported that VO2 max values for the high carbohy-
drate comparison group were significantly lower than
EAKD or periodised carbohydrate groups at baseline and
Bailey and Hennessy Journal of the International Society of Sports Nutrition           (2020) 17:33 
Page 8 of 11
follow-up (p ≤ 0.02) [12]. This suggests that other factors
may influence athletes’ choice of diet and aerobic capacity
concomitantly, such as genetic variation in trainability
and/or chronic substrate utilization [19, 20]. A review
conducted by Williams et al. revealed the potential for 97
genes to predict VO2 max trainability, suggesting that gen-
etics may account for differing training outcomes among
athletes [20]. Certain dietary preferences, which both
acutely and chronically influence substrate utilization,
have also been linked to gene variations, highlighting the
possibility for both dietary choices and training outcomes
to be mediated by genetics [19, 21]. Randomized con-
trolled trials and genome-wide association studies can be
leveraged to control for, and explore the impact of, such
factors in future studies of the EAKD.
Four of the seven studies reviewed reported non-
significant VO2 max outcomes [14–17]. In a non-
randomized trial, McSwiney et al. reported a VO2 max
increase in both groups of male endurance athletes post-
EAKD (EAKD: 53.6 ± 6.8 vs. 57.3 ± 6.7; HCD: 52.6 ± 6.4
vs. 57.2 ± 6.1) with a non-significant difference between
groups (p = 0.968) [14]. In a pre-posttest design, Phinney
et al. reported a non-significant decrease in VO2 max
from baseline among five elite male cyclists (pre- vs.
post-EAKD: 5.10 ± 0.18 vs. 5.00 ± 0.20; p > 0.01) [15]. In
a case study, Zinn et al. reported a non-significant de-
crease among five recreational endurance athletes con-
suming the EAKD (− 1.69 ± 3.4; p = 0.63) [17]. Finally, in
a randomized crossover study, Shaw et al. reported no
significant changes from baseline (59.4 ± 5.2) among
male endurance athletes during either dietary interven-
tion (p > 0.05) [16].
Heatherly et al. did not report VO2 max outcomes, in-
stead providing the percentage of baseline VO2 max
achieved at various race paces (i.e., 5 km, 10 km, 21 km, 42
km, sub-42 km) [13]. The significantly greater percentages
of baseline VO2 max achieved post-EAKD consumption at
10 km, 21 km, 42 km, and sub-42 km race paces demon-
strate that the EAKD was negatively correlated with the
athletes’ aerobic efficiency at these paces. This is corrobo-
rated by some of the secondary outcomes reported in Table
2, including reports of EAKD being associated with signifi-
cantly higher RPE [12], and decreased TTE [17]. Only one
study reported significant positive secondary findings: a
higher peak power in athletes post-EAKD compared to the
standard, high carbohydrate diet [14]. The authors of the
study hypothesized that this outcome was likely due to an
improved power to weight ratio among the EAKD athletes,
who lost an average of 6 kg of body mass.
Despite the popularity of the diet as an ergogenic aid,
this review provides evidence that EAKD consumption
produces mixed results, in terms of endurance perform-
ance, when compared to a high carbohydrate diet. Several
biological mechanisms may help to explain the potential
for mixed and/or detrimental effects, including changes in
fuel economy, production of certain metabolic byproducts,
and reduced energy intake. For example, the EAKD sig-
nificantly increases fat oxidation, requiring greater oxygen
consumption due to the increased oxygen demands dur-
ing fatty acid metabolism versus carbohydrate metabolism
[12, 22]. This increased demand for oxygen reduces the
beneficial impact of an increased VO2 max because a
greater percentage of maximal oxygen uptake is now re-
quired to maintain any given race pace [13]. Second,
EAKD metabolites such as tryptophan and ammonia may
promote fatigue by influencing the central nervous system
[23, 24]. Finally, it has been shown that the EAKD leads to
increased satiety and reduced energy intake [25]. Reduced
energy intake, and the accompanying weight loss, may be
beneficial for some individuals but could also present a
sustainability issue for highly active athletes. Substantial
reductions in body weight may negatively impact mental,
hormonal, and bone health, as well as recovery time and
general exercise performance [26, 27]. Illustrating these
mechanisms, Heatherly et al. reported that athletes exhib-
ited greater oxygen consumption at race pace on the
EAKD versus a high carbohydrate diet and that ad libitum
EAKD consumption resulted in decreased intake of
roughly 1000 kcal per day, leading to a 3 % loss of body
mass over the study period [13].
In multiple studies, participant self-reports (e.g., inter-
view data, training logs) suggested that the EAKD may
have promoted perceived fatigue and decreased ability to
train for certain athletes [17], particularly those training
in summer months [13]. This could be a combined re-
sult of the alterations in fuel economy, metabolism, and
energy intake described above, though not all athletes
reported experiencing negative side effects. Based on
focus group results, one study reported that athletes had
more positive than negative perceptions of the diet [17],
suggesting that there may be additional unknown vari-
ables influencing EAKD outcomes across individuals
and/or settings (e.g., temperature, humidity [13]).
One hypothesis for the variation in performance out-
comes among studies might stem from the heterogeneity
across the training/recovery protocols and fitness levels
of the athletes [28]. Both studies exhibiting a statistically
significant increase in VO2 max examined the effects of
EAKD consumption in professional race walkers with
high base levels of aerobic capacity, a factor that has
been associated with faster recovery times and greater
positive adaptations to training [29–31]. Both studies
also explicitly included a recovery protocol in their train-
ing prescription, which could impact the athletes’ train-
ing outcomes [28]. Due to limited information on
training/recovery protocols in many of these studies,
strong conclusions cannot be generated regarding the
impact of training versus diet on performance outcomes.
Bailey and Hennessy Journal of the International Society of Sports Nutrition           (2020) 17:33 
Page 9 of 11
However, based on previous evidence, it is reasonable to
hypothesize that these protocol differences may have con-
tributed to the diverse outcomes reported [6, 28, 32].
In examining the results, it is important to bear in mind
that this review consists of just seven studies, only one of
which was randomized [16]. Carr et al., Burke et al., and
McSwiney et al. were all prospective trials, however they
allowed participants to choose their dietary intervention
[7, 12, 14]. Although this self-selection method generally
improves rates of adherence to the diets, it also introduces
risk of bias in that those athletes who chose the EAKD
may have other lifestyle or dietary tendencies that could
affect their biological response to the diet. Heatherly et al.
and Phinney et al. were pre-posttest studies, which are
subject to threats to internal validity, such as the fact that
passage of time results in natural decreases in VO2 max
[13, 15]. Finally, Zinn et al. was a case study [17]. Al-
though the article provides a wealth of hypothesis generat-
ing observations, without a comparison group we cannot
conclude whether the EAKD was more or less effective
than the standard, high carbohydrate diet for athletes.
All studies had relatively small sample sizes, which re-
duced the statistical power of the analyses. It is possible
that, with a larger sample size, the seven studies might
have exhibited corroboratory results. The small sample
sizes also exacerbated the problem of drop-out rates,
which were considerable in one of the five studies.
McSwiney et al. lost 18 participants in the EAKD group
and nine in the comparison group, resulting in a partici-
pation rate of 33 and 55%, respectively [14].
At the review level, heterogeneity in dietary interven-
tions, adherence measurements, VO2 max testing proce-
dures, training protocols, and athlete types all introduced
variation that made comparisons across studies difficult.
For example, four studies measured VO2 max using a
treadmill test [7, 12, 13, 16], while the other three studies
used a cycle ergometer [14, 15, 17]. Previous reviews sug-
gest that these two testing procedures produce inconsist-
ent results, with higher VO2 max outcomes reported for
treadmill as compared to cycle ergometer tests [33].
Therefore, inter-article comparisons of the change in VO2
max by diet from baseline may be more reliable than
inter-article comparisons of the absolute outcome values
reported. Furthermore, research suggests that VO2 max
may be an inaccurate predictor of endurance performance
in runners, specifically due to variations in running econ-
omy and fatigue [34, 35]. Therefore, VO2 max may not be
a strong indicator of endurance capacity in some sports,
further complicating this measure as a comparison across
heterogeneous groups of athletes.
In addition to VO2 max outcomes, Table 2 provides a
matrix of secondary outcomes (i.e., TTE, race time, RPE,
peak power), which can be used to complement the VO2
max findings from this review. For example, although all
three diet groups in the study by Burke et al. experi-
enced a significant increase in VO2 max from baseline,
only the comparison groups (i.e., high carbohydrate, per-
iodised carbohydrate) experienced faster 10 km race
walk times. Furthermore, the EAKD group reported sig-
nificantly higher RPE values compared to baseline dur-
ing a graded economy test. Future research in this field
can benefit from utilizing a variety of performance met-
rics, such as the ones discussed in this review, to tri-
angulate overall effects of diet on athletic performance,
limiting biases introduced from relying on one marker
alone. Additionally, as this research area develops, it may
be prudent to conduct reviews among athletes of a single
type (e.g., runners only, cyclists only) to limit the hetero-
geneity among studies.
Because only two databases were used to identify arti-
cles for review, it is possible that other studies of EAKD
and endurance performance do exist in the literature.
However, exploratory investigations of other databases
retrieved no additional articles that met inclusion cri-
teria. It is noteworthy that six of seven studies included
in this review were published within the last 5 years,
suggesting that scientific attention to this topic is fairly
recent. Due to the contemporary nature of the research
question, it is also possible that yet-to-be-published re-
search exists on this topic. Therefore, future reviews
may eventually produce more conclusive evidence. Fi-
nally, the potential risk of reporting bias is always
present. Unfortunately, it is difficult to assess publication
bias because we cannot know the extent of the evidence
that has gone unpublished. However, due to the contro-
versial nature of this topic among scientists and lay
people alike, it seems likely that both significant and null
findings would be publishable.
Conclusions
Despite popular interest in the ketogenic diet as an ergo-
genic aid in endurance sport, there are few published
studies examining the effect of EAKD consumption on
VO2 max and other outcomes (i.e., TTE, race time, RPE,
peak power). When compared to a high carbohydrate
diet, there are mixed findings for the effect of EAKD
consumption on endurance performance. This may be
partially due to the heterogeneity across studies and/or
variability in athletes’ individual genetic factors, espe-
cially those that directly influence metabolism.
The limited number of published studies point to a need
for more research in this field. Specifically, randomized
studies performed in mixed sex samples are needed. Re-
searchers might also consider examining EAKD-like diets
that do not induce ketosis. Such research will expand our
understanding of the diet’s effects in diverse athlete popu-
lations, all of whom serve to benefit from further know-
ledge, be the findings supportive of the diet or not.
Bailey and Hennessy Journal of the International Society of Sports Nutrition           (2020) 17:33 
Page 10 of 11
Abbreviations
EAKD: Endurance athlete’s ketogenic diet; VO2 max: Maximal oxygen uptake;
TTE: Time to exhaustion; RPE: Rating of perceived exertion
Acknowledgments
The authors would like to thank Amy LaVertue, Research & Instruction
Librarian at the Tufts University Hirsch Health Sciences Library, for her time
and enthusiasm in the planning stages of this review.
Authors’ contributions
CB performed all background research, database searches, and wrote and
edited the final manuscript. EH provided guidance throughout the research,
writing, and submission processes, as well as editing of the final manuscript.
Author’s information
CB is a research scientist and recreational endurance athlete with a master’s
degree in Nutrition Interventions, Communication, and Behavior Change from
The Gerald J. and Dorothy R. Friedman School of Nutrition Science and
Policy at Tufts University.
EH is a Research Assistant Professor at The Gerald J. and Dorothy R.
Friedman School of Nutrition Science and Policy at Tufts University.
Funding
Not applicable.
Availability of data and materials
All data analyzed in this review are included in the following published
articles.
Burke, L.M., et al., Low carbohydrate, high fat diet impairs exercise economy
and negates the performance benefit from intensified training in elite race
walkers. J Physiol, 2017. 595(9): p. 2785-2807.Carr, A.J., et al., Chronic Ketogenic
Low Carbohydrate High Fat Diet Has Minimal Effects on Acid-Base Status in Elite
Athletes. Nutrients, 2018. 10(2).
Heatherly, A.J., et al., Effects of Ad libitum Low-Carbohydrate High-Fat Dieting
in Middle-Age Male Runners. Med Sci Sports Exerc, 2018. 50(3): p. 570–579.
McSwiney, F.T., et al., Keto-adaptation enhances exercise performance and body
composition responses to training in endurance athletes. Metabolism, 2018. 81:
p. 25–34.
Phinney, S.D., et al., The human metabolic response to chronic ketosis without
caloric restriction: Preservation of submaximal exercise capability with reduced
carbohydrate oxidation. Metabolism, 1983. 32(8): p. 769–776.
Shaw, D.M., et al., Effect of a Ketogenic Diet on Submaximal Exercise Capacity
and Efficiency in Runners. Med Sci Sports Exerc, 2019. 51(10): p. 2135–2146.
Zinn, C., et al., Ketogenic diet benefits body composition and well-being but not
performance in a pilot case study of New Zealand endurance athletes. J Int Soc
Sports Nutr, 2017. 14: p. 22.
Ethics approval and consent to participate
The studies examined in this review were approved by appropriate
governing bodies for ethical research.
Consent for publication
Not applicable.
Competing interests
The authors declare that they have no competing interests.
Received: 12 February 2019 Accepted: 4 June 2020
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A review of the ketogenic diet for endurance athletes: performance enhancer or placebo effect?
Bailey, Caitlin P,Hennessy, Erin