When it comes to athletic performance,individuals are highly dependent on carbohydrates (CHO) for fuel. Within thehuman body, CHO stores are finite and only sufficient to fuel ~3h ofcontinuous, sub-maximal exercise (70-80% maximal oxygen uptake VO2max), withfatigue and impairment of exercise capacity being evident with depletion of CHOreserves (Burke and Hawley, 2002). Athletes are challengedwith the requirement for a wide variety of complex characteristics that underpin success in sporting events, varying from short duration (e.g. sprints) toprolonged duration (e.
g. Ultra-marathons or Tour de France). The role oftraining is to accumulate physiological adaptations to improve athleticcharacteristics that underpin success through nutrition and exercise (Burke, 2015). More specifically, endurance athletes areinterested in strategies to reduce their rate of glycogen use during exercise. Nutritional strategies areemployed to optimise athletic performance including CHO-loading (Hawley et al.,1997), consuming a CHO-rich meal prior to exercise (Hawley and Burke, 1997) andconsuming CHO throughout an event (Coyle et al., 1986), all of which have beenshown to enhance endurance performance.
An alternative method toenhance exercise capacity involves utilisation of a different fuel source, fat.Even found within the leanest athlete, fat stores are abundant and provide theability to fuel exercise lasting at least several days (Burke and Hawley, 2002).Evidence suggests that a classic response to exercise in athletes is theability to oxidise fat, therefore a high fat diet has been suggested as astrategy to utilise fat and augment fat oxidation (Burke and Hawley, 2002). There is widespread publicityfor fat adaptation amongst athletes however when interpreting data from studiesevaluating the effect of fat adaption results often do not support a perceived performancebenefit whilst there are frequent methodological flaws. This paper criticallyevaluates the literature on this paradigm for training adaptation and theeffects of fat adaptation strategies. Fat-Adaption pre-exercise:There is evidence to suggest that short termfat adaptation is detrimental to athletic performance due to the reduction inresting muscle glycogen without compensation for reduced CHO availability (Burkeet al., 2001).
Appendix one outlines the results from studies evaluating theeffects of short term fat adaptation prior to exercise. Okano et al., (1996) studied the effect of a high-CHOmeal (HCM) and high-fat meal (HFM) given 4 hours before a cycling protocol concluding that there was no significantdifference between diet plans when measuring heart rate, oxygen consumption andperceived exertion during exercise however respiratory exchange ratio in HCMgroup was significantly higher during the first40 minutes of exercise alongside a significantly higher serum insulin level atthe start of exercise. These results are suggestive that a single HCM and HFMgiven 4 h before exercise influences fuel utilization in the initial stages ofprolonged cycling, but these meals may have little effect on endurance capacity. Similarly, Whitley et al., (1998) found increasedplasma insulin levels during exercise alongside increased plasma epinephrine andgrowth hormone concentrations however despite these differences in substrateand hormone concentrations, a high-fat meal prior to exercise failed to alterfuel utilisation during 90 minutes moderate intensity exercise as substrateoxidation during endurance exercise is remarkably resistant to alteration. Fat-Adaptation <3 days:The idealisation of ahigh-fat, low-CHO diet for <3 days is to reduce glycogen stores within themuscle and liver (Bergstrom, Hermansen, Hultman and Saltin, 1967).
Appendix twooutlines results from studies evaluating the effects of short term fat adaptation.Lima-Silva et al., (2013) foundthat a low-CHOdiet reduced time to exhaustion accompanied by a lower total aerobic energycontribution (-39%). Despite there being no evident effect on the plasmaconcentration of insulin, glucose and peak potassium (K+) levels, it isquestionable whether the lack of readily available resources to replenish ATPlead to an increased speed of loss of K+ from the muscle which as a resultwould explain the reduced time to exhaustion in the low-CHO group. Dietary records were the method ofchoice for data collection which leaves room for bias and systematical error ofresults (Tooze et al., 2012). It is questionable whether this study allowsparticipants long enough to see physiological changes within the body, and ifnot, 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 enduranceperformance.
This is likely as a result from the premature depletion of muscleglycogen stores and there being no valuable increase in capacity for fatutilisation in order to compensate for the lack of availability of CHO fuel(Burke and Hawley, 2002). However, due to current research only using suchsmall sample sizes and with the mentioned study consisting of only males it isun-reliable to generalise these results to the general population. Fat-Adaptation >5 days:It is thought that a longer period of fatadaptation through the implementation of a high-fat, low-CHO diet (>6 days) mayallow for metabolic adaptations to enhance rates of fat oxidation andcompensate for reduced CHO availability (Burke and Hawley, 2002).
Appendixthree 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 performancebenefits in elite athletes. Results demonstrated that despite improvementsbeing evident in peak aerobic capacity, performance was impaired in eliteendurance athletes’ due to reduced exercise economy. However, there was alimited duration of the study to only 3 weeks alongside the application ofslightly hypocaloric diets. Similarly, Paoli et al.
, (2012) also found nosignificant differences when comparing a modified ketogenic diet in eliteartistic gymnasts, however there are several methodological flaws evidentwithin the study. It is questionable whether the strength tests used were hardenough to challenge the gymnasts in order to see physiological changes. Adaptationto ketogenic diet is thought to take 4-6 weeks therefore a longer period tostudy the physiological effects is recommended. Body fat was measured throughuse of skin folds however DEXA would have produced more reliable results due tothe low-CHO diet consisting of diuretics and the fact that diuretics reduceskin-folds. Hulston et al., (2010) found that fatoxidation was utilised under low glycogen training which demonstrates that thebody does not need to rely on CHO consumption to fuel performance. However,this way of training may be counter-productive for anaerobic athletes as noimprovement in performance was seen, despite the physiological changes. Whilstresults from this study do not apply to athletes other than cyclists.
In a study conducted looking at fat adaptionin Taekwondo athletes, despite there being no improvement in performance, Rhyuand Cho (2014) found that a ketogenic diet (high-fat, low CHO) is helpful forweight category athletes as after 3 weeks, weight, % body fat, BMI and leanmuscle decreased. In contrast to these findings, Cochran etal., (2015) saw an improvement in performance, however further research needsto be conducted to see if these results can be carried over into highly-trainedindividuals. A well-publicised study conducted by Phinneyet al., (1983) took 5 well-trained cyclists who consumed a fat adaption diet overa 4-week period followed by completion of a ride to exhaustion.
.Resultsdemonstrated that four subjects showed minimal changes and impairment inexercise capacity post high-fat diet, however one cyclist demonstrated anabnormally large improvement in performance, thus altering overall results. Itis difficult to apply results to high intensity endurance events or sprints dueto the protocol being undertaken at fixed submaximal workload, only equivalent toultra-endurance events. Across-over design study by Lambert et al., (1994) evaluated the effects of a 2-weekfat adaption diet on trained cyclists during multiple cycling protocols howeverit is difficult to isolate the effects of the different dietary protocols onperformance due to two different cycling protocols having been applied. Furthermore, the cycling protocol usedwithin the study makes it hard to relate to real-life sporting events.
Goedecke et al., (1999) also failed todemonstrate improved performance, however, despitelack of improvement in performance, a major finding from the study demonstratedthat rates of fat oxidation during submaximal exercise were increased followingonly 5 days of a high-fat diet (Goedecke et al., 1999). This finding is key asit is suggestive that only after a relatively small period of dietaryadaptation to a high-fat diet, there is a metabolic shift, increasing fat utilisationwhich would be far better tolerated by athletes than a prolonged period of fatadaption. Fat-Adaptioncombined with CHO restoration:Despite the current lack of evidence forshort term fat adaption improving performance, it is thought that therestoration of glycogen following a period of fat-adaption could theoreticallyprovide athletes with the opportunity to tap into both glycolytic and lipolyticpathways 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 LCHFstrategy would create a glycogen-sparing effect when in contrast, it compromisedhigh-intensity 1-km sprint performance in 8 well-trained cyclists despite fatoxidation being evident. It is questionable whether this was due to increased sympatheticactivation, altered contractile function and/or the inability to oxidise theavailable carbohydrate during the high intensity sprints (Havemann et al., 2006).
Consuming high levels of fat created a greater reliance on fat and reduced CHOoxidation which persisted despite 1 day of CHO loading (Havemann et al., 2006).An advantage to this study is how the authors included high-intensity sprintsalongside endurance exercise (mean power output during 1-km sprints >90% ofWpeak) whichallowed for the simulation of race conditions and thus becoming representativeto real-life sporting events. Findings are consistent with Burke et al.,(2000) and Carey et al. (2001), who also demonstrated an increase in fatoxidation with a short-term high-fat diet that persisted even after restorationof CHO levels (Havemann et al., 2006).
Carey et al., (2001) demonstratedthat fat oxidation increased during prolonged submaximal exercise, however,despite the sparing of CHO, this study failed to detect a significant benefitto performance of a 1-h TT. A logical conclusion for this could be contributedto there being too small sample size creating an ergogenic effect and as aresult of small sample size being unable to exclude type II error. Whilst theuse 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) measuredmuscle glycogen levels over a 5 day-period of fat-adaptation concluding that 1day of rest and CHO loading was sufficient to restore muscle glycogen levels toabove baseline whilst resulting in a significant reduction in muscle glycogen utilisationduring a 120-min cycle at ~70% max O2 consumption. In contrast to Havemann etal., (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 intensitysprint bouts (>90% of Wpeak) are integral to performance. Conclusion: The current paper has discussed the use of fatadaption over varying periods of time and despite the enhanced capacity ofutilisation of an abundant fuel source, fat adaptation does not appear toimprove exercise capacity or performance. This may be allocated to significantmethodological flaws conducted within studies with type II statistical errorand a failure to detect small change being a common flaw evident.
A critic highlightedfrom all studies discussed within this paper is the small sample size used,making it hard to detect small changes and difficulty making resultsrepresentative to the general population. Type II statistical error may alsooccur due to poor reliability of the performance protocols whilst findings arelimited to the specific protocols used within studies. Another issue is thatbenefits are limited to specific individuals.