Type: Research Essays
Sample donated: Hugh Poole
Last updated: September 24, 2019
Athletes are challenged to succeed andperform at a wide variety of sporting events, ranging from short duration ofsprints to ultra-endurance events. Access to carbohydrate (CHO) levels is keyin order to fuel athletic performance however within the body there are limitedCHO stores that are only sufficient to fuel up to 3 hours of sub-maximal exercise(70-80% maximal oxygen uptake VO2max). Muscle fatigue and impairment ofperformance become evident when CHO stores are not readily available (Burke andHawley, 2002). The idealisation oftraining is to accumulate physiological adaptations in order to improveathletic performance and success through the combination of nutrition andexercise (Burke, 2015). It is key that enduranceathletes implement strategies to optimise rate of glycogen use during not onlytraining but also endurance events.
Nutritional schemes areemployed to optimise athletic performance including CHO-loading (Hawley et al.,1997), consuming a CHO-rich meal before exercise (Hawley and Burke, 1997) andconsuming CHO throughout an event (Coyle et al., 1986), all of which have beenshown to improve endurance performance. An alternative method toenhance exercise capacity involves utilisation of a different fuel source, fat.
Evident within even lean athletes, fat stores are abundant and provide a fuelsource for exercise lasting up to several days (Burke and Hawley, 2002). A classicresponse to exercise includes physiological changes within the body that allowfor fat oxidation to fuel exercise, therefore a high fat diet has beensuggested as a strategy to utilise fat and augment fat oxidation (Burke andHawley, 2002). Over recent yearspublicity for fat adaptation amongst athletes has continued to be prevalent,however when interpreting data from studies evaluating the effect of fatadaption results often do not support performance being improved whilst thereare 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 performance due to muscle glycogen levelsbecoming reduced without any compensation for reduced CHO availability (Burkeet al.
, 2001). Appendix one highlights studies conducted that evaluate 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 in oxygen consumption, perceived exertion and heart rate duringexercise, however, respiratory exchange ratio in HCM group was significantly higher during the first 40 minutesof exercise alongside a significantly higher serum insulin level at the startof exercise. These results are suggestive that a single HCM and HFM given 4 hoursbefore exercise has an effect on utilisation of fuel in the initial stages ofprolonged cycling, but these meals may have little effect on endurance capacity. Similarly, Whitley et al., (1998) foundincreased plasma insulin levels during exercise alongside increased growth hormone levels and plasmaepinephrine, however, despite these differences in substrate and hormoneconcentrations, a HFM prior to training failed to alter fuel utilisation during90 minutes moderate intensity exercise as substrate oxidation during enduranceexercise is notably 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 twohighlights studies evaluating the effects of fat adaptation for a period of upto 3 days.Lima-Silva et al., (2013) foundthat time to exhaustion was reduced alongside a lower total aerobic energycontribution in participants who consumed a low-CHO diet. Despite no evidenteffect on plasma levels of insulin, peak potassium (K+) and glucose levels, itis questionable whether due to insufficient resources to replenish ATP lead toa faster rate of reduction in K+ from the muscle which as a result would accountfor the reduction in 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 the period of timeexamined is long enough in duration to see physiological changes within thebody, and if not, what is the optimal period of time for fat adaptation tooccur? Evidence is suggestive that a high-fat, low-CHOdiet (<72hour period) is detrimental to exercise and endurance performance.This is likely as a result from the premature depletion of muscle glycogenstores and there being no valuable increase in capacity for fat utilisation inorder 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 andwith the mentioned study consisting of only males it is un-reliable togeneralise 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)may allow for metabolic adaptations to enhance rates of fat oxidation andcompensate for reduced CHO availability (Burke and Hawley, 2002). Appendixthree highlights 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 in order to improve performancein elite athletes. Results demonstrated that despite peak aerobic capacityimproving, performance was negatively effected in elite endurance athletes’ dueto reduced exercise economy. However, there was a limited duration of the studyto only 3 weeks alongside the application of slightly hypocaloric diets. Similarly, there were no significantdifferences in a study conducted by Paoli et al., (2012) when elite artisticgymnasts were requested to consume a modified ketogenic, however there areseveral methodological flaws evident within the study.
It is questionablewhether the strength tests used were hard enough to challenge the gymnasts andcreate physiological changes. Physiological changes to ketogenic diet isthought to take 4-6 weeks therefore a longer period to study the physiologicaleffects is recommended. Skin fold measurements were taken to measure body fat howeverDEXA would have produced greater reliability in results due to the low-CHO dietconsisting of diuretics and the fact that diuretics reduce skin-folds. Hulston et al., (2010) report that we physiologicallydo not need to solely rely on CHO consumption to fuel performance as fatoxidation occurs under low glycogen training. However, this way of training maybe counter-productive for endurance athletes as no improvement in performancewas seen, despite the physiological changes. It is important to note that resultsfrom this study do not apply to athletes other than cyclists.
In a study consisting of Taekwondo athletes, Rhyuand Cho (2014) found that a ketogenic diet (high-fat, low CHO) had no effect onperformance however, it is useful for weight category athletes as weight,percentage body fat, BMI and lean muscle decreased after 3 weeks consumption.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 relatable to high-levelathletes. Over 4 weeks, Phinney et al., (1983)evaluated 5 well-trained cyclists who consumed a fat adaption diet followed bycompletion of a ride to exhaustion..Results demonstrated that four subjects showedminimal changes and impairment in exercise capacity post high-fat diet, howeverone cyclist highlighted an abnormally large improvement in performance, thereforeeffecting overall results.
It is difficult to apply results to high intensityendurance events or sprints due to the protocol being undertaken at fixedsubmaximal workload, only equivalent to ultra-endurance events. Across-over design study by Lambert et al., (1994) evaluated the effects of a2-week fat adaption diet on cyclists who completed 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 followingconsumption of a high-fat diet over only 5 days (Goedecke et al.
, 1999). This isprudent as it is suggestive that only after a relatively small period ofdietary adaptation to a high-fat diet, there is a metabolic change where utilisationof fat is increased, which would be far better tolerated by athletes than aprolonged period of fat adaption. Fat-Adaptionin combination 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 access both glycolytic and lipolyticpathways during exercise and therefore enhance fuel provision (Hawley andHopkins, 1995). Appendix four highlights studies examining the effects of ahigh-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, itcompromised high-intensity 1-km sprint performance in cyclists despite fatoxidation being evident. It is questionable whether this was due to sympatheticactivation being increased, contractile function being altered and/or the bodybeing unable to oxidise already available carbohydrates during the highintensity sprints (Havemann et al., 2006). Consumption of high levels of fat createda greater reliance on fat and reduced CHO oxidation which persisted despite 1day of CHO loading (Havemann et al.
, 2006). An advantage to this study is howthe authors included high-intensity sprints alongside endurance exercise (meanpower output during 1-km sprints >90% of Wpeak) which allowed for the simulationof race conditions and thus becoming representative to real-life sportingevents. 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 morepredominantly, under-reporting which was the method of data collection (Toozeet al., 2012).
Through measurement ofmuscle glycogen levels, Burke et al., (2000) concluded that 1 day of rest andCHO loading was sufficient to restore muscle glycogen levels to above baselinewhilst resulting in a significant reduction in muscle glycogen utilisation duringa 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: Thecurrent paper has discussed the use of fat adaption over varying periods oftime and despite the enhanced ability to utilise an abundant fuel source, fatadaptation does not appear to improve exercise capacity or performance. This maybe allocated to significant methodological flaws conducted within studies withtype II statistical error and a failure to detect small change being a commonflaw evident. A critic highlighted from all studies discussed within this paperis the small sample size used, making it hard to detect small changes and difficultymaking results representative to the general population. Type II statisticalerror may also occur due to poor reliability of the performance protocolswhilst findings are limited to the specific protocols used within studies.
Another issue is that benefits are limited to specific individuals.