Keto or carbo? Low‐ <i>versus</i> high‐carbohydrate diets for fuelling elite endurance sport performance

The primary fuelling strategies for elite endurance athletes have historically involved diets that are rich in carbohydrates (CHO), which ensure adequate muscle/liver glycogen stores and exogenous CHO fuel supply are available to be utilized during training and competition. Indeed, diets and strategies promoting high carbohydrate availability (HCHO) have been the nucleus of nutritional guidelines for endurance sport, based on plentiful evidence that they sustain continuous high-intensity aerobic exercise. In recent years, investigators have challenged this status quo by promulgating the potential for low-carbohydrate–high-fat (LCHF) diets to fuel world-class endurance athletics. One diet, the ‘ketogenic diet’, is particularly popular within athletes’ social circles as an ostensible alternative to HCHO for sustaining aerobic exercise performance. Training while consuming a ketogenic LCHF diet has been purported to have the potential to enhance training-derived endurance adaptations and promote recovery, although convincing data are scarce (Bailey & Hennessy, 2020). It is generally acknowledged that LCHF diets correspond with lower endogenous CHO availability, which promotes the up- and downregulation of key regulatory enzymes involved in skeletal muscle fat and CHO metabolism, and subsequently increases the exercise intensity at which maximal fat oxidation occurs (Stellingwerff et al. 2006). Theoretically, those who have adapted to LCHF diets may be able to sustain exercise fuelled by the abundant endogenous lipid reserves for a longer duration compared to those who deplete their finite CHO stores. However, previous rigorously controlled studies have demonstrated impaired CHO metabolism contributing to subsequent performance detriments and increased perceived effort in elite endurance athletes after several weeks of adherence to an LCHF diet (Burke et al. 2017). Despite growing academic interest in LCHF, several topics have been left unaddressed including the length of time required for adaptation between LCHF and HCHO, and the effects of acute CHO restoration on performance in LCHF adapted athletes. In a recent article in The Journal of Physiology, Burke et al. (2021) attempted to bridge these gaps. Thirteen world-class male race walkers who consumed a standard HCHO diet underwent baseline exercise tests consisting of an exercise economy/ test, a 10 km sanctioned track race, and a 25 km walk approximated at their individual 50 km race pace. At various time points throughout these tests, various physiological variables were collected including blood metabolites, indirect calorimetry, heart rate and ratings of perceived exertion. Following the baseline testing, the athletes self-selected to follow either a HCHO (65% CHO, 15% protein, 20% fat) or a ketogenic LCHF (<50 g day−1 CHO; 2.2 g kg−1 day−1 protein; 80% fat) energy-matched diet for 5 days during the ‘adaptation’ block, followed by the same exercise tests they completed during the baseline testing. To determine if acute CHO restoration could rescue the previously observed performance decrement following adherence to a LCHF diet (Burke et al. 2017), both groups consumed the same 24 h HCHO glycogen loading diet and pre-race CHO-rich meal (2 g kg−1) prior to the ‘adaptation’ 10 km race. They then underwent a 5-day ‘restoration’ period in which both groups followed the same HCHO diet and completed a third 25 km walk. During the 4-week investigation, the athletes’ training and dietary habits were rigorously controlled and monitored while they lived in athlete residences. The results of the investigation provide an insightful view into the metabolic adaptability of elite endurance athletes to sudden shifts in dietary fuel provision. In just 5 days of adherence to a ketogenic LCHF diet, Burke et al. demonstrated marked increases (>200%) in fat oxidation and reduced CHO oxidation during the economy and 25 km walk tests. This robust shift in fat oxidation rates during exercise was accompanied by additional metabolic variables associated with ketogenic adaptation including reduced exercising plasma glucose and lactate levels, and elevated plasma free fatty acids and β-hydroxybutyrate in the LCHF condition compared to baseline. Consistent with their previous work (Burke et al. 2017), the authors found reduced walking economies and increased perceived effort following LCHF adaptation. They also demonstrated that, despite an acute 24 h HCHO diet and pre-race CHO-rich meal, rates of fat and CHO oxidation at speeds corresponding to the athletes’ 25 km and 50 km race pace were not sufficiently restored to baseline values. Accordingly, during the 10 km adaptation race, the athletes who were adapted to the LCHF diet demonstrated worsened real-life race performance compared to their baseline trials with the HCHO diet. Finally, after a 5–6 day CHO restoration, substrate utilization during the 25 km tests was restored to baseline values in the LCHF group. This study offers interesting observations surrounding the HCHO vs. LCHF debate, such as the flexible adaptation and restoration between diets. The authors showed that 5 days of a ketogenic LCHF diet achieved the stated benefits of the diet in relation to robust increases in fat oxidation rates and circulating ketones during exercise that were consistent with values in previous reports after long-term adaptation. This partly refutes their so-called ‘social media critics’ who have previously claimed that ketogenic diets require several weeks to months of adherence to accomplish their putative effects (Burke et al. 2021). However, it is important to consider the possibility that other adaptive responses to ketogenic adherence occur beyond this time frame which may have gone undiscovered in this study. The dramatic shift in fat oxidation and circulating ketones appears to come at the cost of CHO oxidation, which subsequently impaired race performance. Although mechanisms were not directly measured in this study, this is likely due to the downregulation of key regulatory enzymes in skeletal muscle CHO metabolism thereby impairing higher intensity exercise capacity (Stellingwerff et al. 2006). Additionally, the stoichiometry of fatty acid oxidation demonstrates that a greater amount of oxygen is required per unit of ATP generation than is the case for CHO oxidation. This study, along with a growing archive of previous investigations (Burke et al. 2017, 2020; Bailey & Hennessy, 2020), has once again shown impaired performance following adherence to a LCHF diet. Despite this, proponents of LCHF diets may still argue the utility of these diets for endurance sport. A commonly cited application for LCHF within athletic circles is the ‘train low–race high’ approach whereby athletes consume a LCHF diet during training and revert to HCHO on race day with the intent of restoring glycogen reserves and maximizing performance outcomes. Here, Burke et al. show that an acute 24 h CHO restoration only fractionally restored CHO oxidation rates relative to baseline values. This is consistent with previous studies that have shown the persistent impairment of CHO metabolism following 24 h HCHO restoration (Stellingwerff et al. 2006). However, a 5-day HCHO restoration restored fat and CHO oxidation rates to baseline levels. Naturally, LCHF advocates may speculate that this flexible adaptation and restoration between LCHF and HCHO presents the opportunity to strategically periodize between the two diets. Under this model, athletes could theoretically benefit from training under the conditions which maximize the alleged beneficial endurance adaptations of the ketogenic diet (Bailey & Hennessy, 2020), while permitting time to sufficiently restore glycogen reserves and enzyme activity when tapering for a competition. Another recent publication by Burke's group, which reproduced the detrimental LCHF performance effects of their previous model (Burke et al. 2017), found no additional performance benefit when combining training with LCHF and competing after a 2.5-week CHO restoration compared to training under HCHO alone (Burke et al. 2020). Thus, it seems unlikely that periodizing LCHF and HCHO diets provides any additional value to training under traditional HCHO diets; however, more long-term evidence will be welcome. This investigation expands on a growing framework for potential dietary strategies for elite male endurance athletes. However, the evidence on the metabolic and performance outcomes of ketogenic LCHF diets for female athletes is lacking. This may be partly a result of convenience sampling, as controlling for menstrual cycle fluctuations adds additional challenges to an already methodologically complex study design. Nevertheless, it is well known that the sexes differ in primary substrate use during aerobic exercise, with females oxidizing proportionally more lipids compared to males at the same relative intensities. It seems likely that CHO oxidation may be further impaired in females who follow LCHF diets, but convincing data are needed. It would therefore be unwise to draw universal recommendations for a one size fits all approach to dietary programming for elite endurance sport without further data on the matter. Burke et al. (2020) included female athletes in their sample, but they were unable to analyse a sex-specific response due to the low number of female participants in their study. Thus, an opportunity is present for future high-quality studies to investigate the effects of LCHF diets on similar responses in female endurance athletes. A more complete understanding of dietary strategies for both sexes will provide an improved framework for future nutritional guidelines. At present, Burke and colleagues’ findings lack a complete mechanistic explanation. Still, their insightful work provides a thorough overview of the short-term effects of a ketogenic LCHF diet on performance and metabolic variables in elite endurance trained male athletes. Although rapid adaptation to a LCHF diet is promising, the observed impairment to exercise economy and race performance does not bode well for the proponents of the LCHF lifestyle. Still, there may be room for both diets at the athlete's dinner table as more evidence on the structured periodization and sex-specific responses to these diets is required to draw proper conclusions. But for now, in the face of growing popularity of LCHF in endurance athletic circles, HCHO is the clear leader in the race for the optimal diet. No competing interests declared. Sole author. None.

Fetched live from OpenAlex and de-inverted. Abstracts are not stored in this database: the inverted indexes are 8.6 GB of the frame’s 9.3 GB of text, and the host has 13 GB free.