When More specifically, endurance athletes are interested in

When it comes to athletic performance,
individuals are highly dependent on carbohydrates (CHO) for fuel. Within the
human body, CHO stores are finite and only sufficient to fuel ~3h of
continuous, sub-maximal exercise (70-80% maximal oxygen uptake VO2max), with
fatigue and impairment of exercise capacity being evident with depletion of CHO
reserves (Burke and Hawley, 2002).


Athletes are challenged
with the requirement for a wide variety of complex characteristics that under
pin success in sporting events, varying from short duration (e.g. sprints) to
prolonged duration (e.g. Ultra-marathons or Tour de France). The role of
training is to accumulate physiological adaptations to improve athletic
characteristics that underpin success through nutrition and exercise (Burke, 2015).
 More specifically, endurance athletes are
interested in strategies to reduce their rate of glycogen use during exercise.

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Nutritional strategies are
employed to optimise athletic performance including CHO-loading (Hawley et al.,
1997), consuming a CHO-rich meal prior to exercise (Hawley and Burke, 1997) and
consuming CHO throughout an event (Coyle et al., 1986), all of which have been
shown to enhance endurance performance.

An alternative method to
enhance exercise capacity involves utilisation of a different fuel source, fat.
Even found within the leanest athlete, fat stores are abundant and provide the
ability to fuel exercise lasting at least several days (Burke and Hawley, 2002).
Evidence suggests that a classic response to exercise in athletes is the
ability to oxidise fat, therefore a high fat diet has been suggested as a
strategy to utilise fat and augment fat oxidation (Burke and Hawley, 2002).

There is widespread publicity
for fat adaptation amongst athletes however when interpreting data from studies
evaluating the effect of fat adaption results often do not support a perceived performance
benefit whilst there are frequent methodological flaws. This paper critically
evaluates the literature on this paradigm for training adaptation and the
effects of fat adaptation strategies.

Fat-Adaption pre-exercise:

There is evidence to suggest that short term
fat adaptation is detrimental to athletic performance due to the reduction in
resting muscle glycogen without compensation for reduced CHO availability (Burke
et al., 2001). Appendix one outlines the results from studies evaluating the
effects of short term fat adaptation prior to exercise.


Okano et al., (1996) studied the effect of a high-CHO
meal (HCM) and high-fat meal (HFM) given 4 hours before a cycling protocol concluding that there was no significant
difference between diet plans when measuring heart rate, oxygen consumption and
perceived exertion during exercise however respiratory exchange ratio in HCM
group was significantly higher during the first
40 minutes of exercise alongside a significantly higher serum insulin level at
the start of exercise. These results are suggestive that a single HCM and HFM
given 4 h before exercise influences fuel utilization in the initial stages of
prolonged cycling, but these meals may have little effect on endurance capacity.


Similarly, Whitley et al., (1998) found increased
plasma insulin levels during exercise alongside increased plasma epinephrine and
growth hormone concentrations however despite these differences in substrate
and hormone concentrations, a high-fat meal prior to exercise failed to alter
fuel utilisation during 90 minutes moderate intensity exercise as substrate
oxidation during endurance exercise is remarkably resistant to alteration.


Fat-Adaptation <3 days: The idealisation of a high-fat, low-CHO diet for <3 days is to reduce glycogen stores within the muscle and liver (Bergstrom, Hermansen, Hultman and Saltin, 1967). Appendix two outlines results from studies evaluating the effects of short term fat adaptation. Lima-Silva et al., (2013) found that a low-CHO diet reduced time to exhaustion accompanied by a lower total aerobic energy contribution (-39%). Despite there being no evident effect on the plasma concentration of insulin, glucose and peak potassium (K+) levels, it is questionable whether the lack of readily available resources to replenish ATP lead to an increased speed of loss of K+ from the muscle which as a result would explain the reduced time to exhaustion in the low-CHO group. Dietary records were the method of choice for data collection which leaves room for bias and systematical error of results (Tooze et al., 2012). It is questionable whether this study allows participants long enough to see physiological changes within the body, and if not, what is the optimal period of time for fat adaptation to occur? Evidence is suggestive that a high-fat, low-CHO diet (<72hour period) is detrimental to exercise and endurance performance. This is likely as a result from the premature depletion of muscle glycogen stores and there being no valuable increase in capacity for fat utilisation in order to compensate for the lack of availability of CHO fuel (Burke and Hawley, 2002). However, due to current research only using such small sample sizes and with the mentioned study consisting of only males it is un-reliable to generalise these results to the general population. Fat-Adaptation >5 days:

It is thought that a longer period of fat
adaptation through the implementation of a high-fat, low-CHO diet (>6 days) may
allow for metabolic adaptations to enhance rates of fat oxidation and
compensate for reduced CHO availability (Burke and Hawley, 2002). Appendix
three outlines the effects of fat adaptation studies for >5 days.

Despite this idealisation, Burke et al.,
(2017) concluded that a fat adaptation diet is not optimal for performance
benefits in elite athletes. Results demonstrated that despite improvements
being evident in peak aerobic capacity, performance was impaired in elite
endurance athletes’ due to reduced exercise economy. However, there was a
limited duration of the study to only 3 weeks alongside the application of
slightly hypocaloric diets.

Similarly, Paoli et al., (2012) also found no
significant differences when comparing a modified ketogenic diet in elite
artistic gymnasts, however there are several methodological flaws evident
within the study. It is questionable whether the strength tests used were hard
enough to challenge the gymnasts in order to see physiological changes. Adaptation
to ketogenic diet is thought to take 4-6 weeks therefore a longer period to
study the physiological effects is recommended. Body fat was measured through
use of skin folds however DEXA would have produced more reliable results due to
the low-CHO diet consisting of diuretics and the fact that diuretics reduce

Hulston et al., (2010) found that fat
oxidation was utilised under low glycogen training which demonstrates that the
body does not need to rely on CHO consumption to fuel performance. However,
this way of training may be counter-productive for anaerobic athletes as no
improvement in performance was seen, despite the physiological changes. Whilst
results from this study do not apply to athletes other than cyclists.

In a study conducted looking at fat adaption
in Taekwondo athletes, despite there being no improvement in performance, Rhyu
and Cho (2014) found that a ketogenic diet (high-fat, low CHO) is helpful for
weight category athletes as after 3 weeks, weight, % body fat, BMI and lean
muscle decreased.

In contrast to these findings, Cochran et
al., (2015) saw an improvement in performance, however further research needs
to be conducted to see if these results can be carried over into highly-trained

A well-publicised study conducted by Phinney
et al., (1983) took 5 well-trained cyclists who consumed a fat adaption diet over
a 4-week period followed by completion of a ride to exhaustion..Results
demonstrated that four subjects showed minimal changes and impairment in
exercise capacity post high-fat diet, however one cyclist demonstrated an
abnormally large improvement in performance, thus altering overall results. It
is difficult to apply results to high intensity endurance events or sprints due
to the protocol being undertaken at fixed submaximal workload, only equivalent to
ultra-endurance events.

cross-over design study by Lambert et al., (1994) evaluated the effects of a 2-week
fat adaption diet on trained cyclists during multiple cycling protocols however
it is difficult to isolate the effects of the different dietary protocols on
performance due to two different cycling protocols having been applied. Furthermore, the cycling protocol used
within the study makes it hard to relate to real-life sporting events.


Goedecke et al., (1999) also failed to
demonstrate improved performance, however, despite
lack of improvement in performance, a major finding from the study demonstrated
that rates of fat oxidation during submaximal exercise were increased following
only 5 days of a high-fat diet (Goedecke et al., 1999). This finding is key as
it is suggestive that only after a relatively small period of dietary
adaptation to a high-fat diet, there is a metabolic shift, increasing fat utilisation
which would be far better tolerated by athletes than a prolonged period of fat

combined with CHO restoration:

Despite the current lack of evidence for
short term fat adaption improving performance, it is thought that the
restoration of glycogen following a period of fat-adaption could theoretically
provide athletes with the opportunity to tap into both glycolytic and lipolytic
pathways during exercise and thus enhance fuel provision (Hawley and Hopkins,
1995). Appendix four outlines studies examining the effects of a high-fat,
low-CHO in combination with CHO restoration.

Havemann et al, (2006) hypothesised that a LCHF
strategy would create a glycogen-sparing effect when in contrast, it compromised
high-intensity 1-km sprint performance in 8 well-trained cyclists despite fat
oxidation being evident. It is questionable whether this was due to increased sympathetic
activation, altered contractile function and/or the inability to oxidise the
available carbohydrate during the high intensity sprints (Havemann et al., 2006).
Consuming high levels of fat created a greater reliance on fat and reduced CHO
oxidation which persisted despite 1 day of CHO loading (Havemann et al., 2006).
An advantage to this study is how the authors included high-intensity sprints
alongside endurance exercise (mean power output during 1-km sprints >90% of
Wpeak) which
allowed for the simulation of race conditions and thus becoming representative
to real-life sporting events.

Findings are consistent with Burke et al.,
(2000) and Carey et al. (2001), who also demonstrated an increase in fat
oxidation with a short-term high-fat diet that persisted even after restoration
of CHO levels (Havemann et al., 2006).

Carey et al., (2001) demonstrated
that fat oxidation increased during prolonged submaximal exercise, however,
despite the sparing of CHO, this study failed to detect a significant benefit
to performance of a 1-h TT. A logical conclusion for this could be contributed
to there being too small sample size creating an ergogenic effect and as a
result of small sample size being unable to exclude type II error. Whilst the
use of dietary records has the potential for bias and miss-reporting, and more predominantly,
under-reporting which was the method of data collection (Tooze et al., 2012)

Burke et al., (2000) measured
muscle glycogen levels over a 5 day-period of fat-adaptation concluding that 1
day of rest and CHO loading was sufficient to restore muscle glycogen levels to
above baseline whilst resulting in a significant reduction in muscle glycogen utilisation
during a 120-min cycle at ~70% max O2 consumption.

In contrast to Havemann et
al., (2006) both studies conducted by Carey et al., (2001) and Burke et al.,
(2001) did not simulate race conditions as in order to do so, high intensity
sprint bouts (>90% of Wpeak) are integral to performance.




The current paper has discussed the use of fat
adaption over varying periods of time and despite the enhanced capacity of
utilisation of an abundant fuel source, fat adaptation does not appear to
improve exercise capacity or performance. This may be allocated to significant
methodological flaws conducted within studies with type II statistical error
and a failure to detect small change being a common flaw evident. A critic highlighted
from all studies discussed within this paper is the small sample size used,
making it hard to detect small changes and difficulty making results
representative to the general population. Type II statistical error may also
occur due to poor reliability of the performance protocols whilst findings are
limited to the specific protocols used within studies. Another issue is that
benefits are limited to specific individuals. 


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