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By: Paul Hough  (Senior Lecturer - Health and Exercise Science)


Exercise scientists and sports nutritionists have historically recommended for athletes involved in endurance and team sports to base their diet around carbohydrates and, in some cases, consume carbohydrates during competition.

This recommendation is based on the premise that muscles predominantly use glycogen (stored carbohydrate) as fuel during moderate to high intensity exercise and performance declines when liver/muscle glycogen stores become depleted. This is akin to a car running out of petrol. 

The importance of carbohydrates for athletes has recently been challenged by proponents of low carbohydrate, high fat (LCHF) diets, who suggest that high carb intakes are unnecessary for endurance athletes. The central theory surrounding LCHF diets for athletes is that there is almost an unlimited supply of fat reserves (around 50,000 to 60,000 kcal of energy) within the human body, which can be used as an energy source during exercise if an athlete adopts a LCHF diet.

Burning fat could potentially improve exercise performance through ‘sparing’ carbohydrate stores within the muscle/liver, which would avoid glycogen depletion and a subsequent decrease in exercise performance. This shift in the muscles using fat instead of carbohydrate, following a LCHF diet, is known as becoming ‘fat adapted.’

Chris Froome’s ‘low carb’ diet 

The subject of LCHF for athletes became a hot topic in 2016 when Chris Froome posted a photo of his low carb breakfast on social media, leading many to speculate Froome was racing the Tour de France on a low carb diet. This claim has since been discredited by Team Sky, who published Froome’s performance and diet records. Nevertheless, the LCHF method has become popular amongst amateur endurance athletes, with many reporting (via social media) that a LCHF diet has improved their endurance performance.

Following the Froome story, I was contacted by a cycling journalist who asked me an interesting question... Could a LCHF diet improve cycling performance? 

A low carb diet does not improve performance, was the obvious answer to the journalists original question. However, as with most questions in sport science, the answer was not that simple!

During my fist discussion with the journalist I mentioned the seminal 1983 study by Phinney and colleagues, which is often presented as evidence that a LCHF diet can improve endurance performance. In this study, five trained cyclists consumed a ‘balanced diet’ for 1 week (1.75 g protein/kg/day, 66% CHO and 33% fat). This was followed by a 28 day ketogenic diet (<20 g/carbohydrate/day). Exercise capacity was assessed by the time it took the cyclists to reach fatigue when cycling at 63 % of their VO2max. The results showedthe time to fatigue was not statistically different between the balanced and ketogenic diet conditions (147 ± 13 vs. 151 ± 25 min). To my surprise, I found that very few studies had been conducted since this 1983 study.

A more recent study (Volek et al. 2016) reported high rates of fat oxidation amongst elite ultra-endurance athletes following a LCHF diet (10% carbohydrate, 19% protein, 70% fat) diet for an average of 20 months. In this observational study, fat oxidation (burning) rates of 1.54 g/min were recorded at an exercise intensity corresponding to 70% of VO2max compared to 0.67 g/min at ~55% VO2max in the high carb diet group.  Furthermore, the LCHF athletes achieved much higher fat oxidation rates (1.21 vs 0.76 g/min) during a 3-hour run.

Supporters of LCHF for athletes often cite the (1983 and 2016) studies above to support the theory that a LCHF diet can improve endurance performance. However, there are a few issues with this:

  • The studies did not actually measure performance which is relevant to athletes – there are no gold medals for the highest rates of fat burning!  
  • The participants’ diets were not closely monitored throughout the studies, so It is uncertain whether the participants stringently adhered to the ketogenic diet, as short-term (3 day) dietary reporting has been shown to be unreliable.   
  • A period of endurance training itself can increase the contribution of fat to total energy expenditure during exercise, irrespective of diet. Therefore, it is unclear to what extent the changes in metabolism and fitness reported during previous studies were related to the LCHF diet, as exercise habits were not closely monitored and standardised.

To address these issues, Burke et al. (2017) conducted a meticulous study which investigated the effects of a three-week ketogenic diet on metabolism and performance of elite race walkers. At the end of the study all dietary groups (1. high carb, 2. periodised carb and 3. ketogenic) improved aerobic capacity by 3-7%, indicating physiological markers of performance can improve following a LCHF diet. However, in contrast to the other groups, the ketogenic group’s race performance did not improve (-1.6%) and they were less economical (using more oxygen).

A low carb diet does not improve performance, was the obvious answer to the journalists original question. However, as with most questions in sport science, the answer was not that simple!

Just as the LCHF for athletes idea was being discredited, a new study reported a 12 week LCHF diet improved measures of maximal cycling performance.

Much to the dismay of the journalist who asked the original question ‘Could a LCHF diet improve cycling performance?’ my answer was, ‘we don’t know for sure’. To attempt to shed some light on the subject, I agreed to conduct a LCHF diet study (similar to Phinney, 1983) for the low carb feature the journalist was working on within Cycling Plus magazine. The purpose of this case study was to investigate the effect of a three week LCHF diet on exercise metabolism and performance of amateur cyclists.


Fat burning

The participants recorded all their food intake, using an app, throughout the study. The goal was to limit carb intake to 10% of total energy intake. However, despite their best efforts, the participants could not achieve this - the actual carb intake was 23-26%. Therefore, this was not a ketogenic diet. Despite this (some would argue) high carb intake, the data suggest a ketogenic diet may not be necessary to markedly increase rates of fat oxidation during sub-maximal cycling.

As with previous low carb studies, the (3 week) low carb diet noticeably increased fat oxidation during sub-maximal cycling. For example, two of the participants’ peak rates of fat oxidation more than doubled, similar to the values in the previously mentioned 1983 study

Diet and weight (fat) loss

The low carb diet promoted positive dietary habits. For example, the participants avoided energy-dense snacks, such as crisps and chocolate, and alcohol intake was minimal. This change in eating habits meant the cyclists consumed less calories during the low carb diet, which resulted in two participants reducing their body fat.   


Every week the participants performed a 20-minute cycling time-trial (TT) to assess performance. The increased fat burning capacity, and lower body fat outcomes did not translate into improved TT performance. In fact, the data indicated two participants performance decreased* during the low-carb diet.

*This was a case study (3 participants), as such there is not enough data to establish if the changes in TT performance were statistically significant. 

So... what diet is best for athletes?

The athlete’s dietary choice should account for several factors such as:

  • The type of event (e.g. long endurance, multiple sprint etc.)
  • Training frequency (how much training is done on a weekly basis)
  • Volume and intensity of training
  • Time of season (e.g. preparatory or competitive training stages)
  • Dietary preferences

All of the above factors are important when considering a dietary strategy for athletes. Therefore, there is there is no ‘one size fits all’ diet!

A LCHF diet could be suited to athletes wishing to decrease body fat and for individuals who perform most of their training at low-moderate intensities. For example, cyclists who mainly go out for 3-4-hour rides (avoiding the cake at the café!).

The energy for ultra-endurance events, such as an Ironman Triathlon, could theoretically be achieved by an athlete who has trained their body to use fat as a fuel (fat adapted). This could be advantageous to athletes who are unable to tolerate ingesting carbohydrate during exercise.   

In terms of endurance athletes, a LCHF diet could theoretically improve performance by decreasing body fat. For example, if cardiovascular fitness remains the same, losing body fat would be beneficial to a marathon runner due to an improvement in power-weight ratio. However, competitive endurance athletes should apply LCHF with caution if performance (winning a race, achieving a personal best) is the focus, as endurance sports often involve brief bouts of high and near maximal intensity exercise (e.g. breakaways and sprint finishes). Although short, these periods can determine the outcomes of races .

A LCHF diet could be counterproductive to performance in sports that involve frequent periods of high-intensity exercise (e.g. football, tennis, rugby), as muscle glycogen is an important fuel during high intensity exercise. To date, there is no experimental evidence that demonstrates a LCHF diet improves performance within team sports. 

The health benefits of low carb diets, such as weight loss, have been well documented, predominantly amongst overweight/obese individuals. This type of research is often used to justify a LCHF diet for athletes. However, it is unclear if the health benefits reported within clinical populations (obese, diabetic etc.) are experienced to the same degree within athletic populations, particularly amongst well-trained athletes.

Recent research indicates that a short-term low carbohydrate diet, or even training in a fasted state, can increase the rate of fat burning and improve the volume/function of the mitochondria (energy producing cells). Therefore, in recent times, some endurance athletes have adopted a strategy where training sessions are deliberately performed with low carbohydrate stores and carbohydrates are consumed prior to competition. This strategy has been termed ‘train-low, compete-high’. However, it has been suggested that frequent training in a low carbohydrate state could reduce the capacity to burn carbohydrate fuels during high intensity exercise. Therefore, athletes adopting a ‘train-low, compete high’ approach are typically advised to include training sessions with normal or high carbohydrate availability so that their capacity to burn carbohydrates (during high intensity exercise) is not reduced.  

Find out more

To find out more about St Mary’s sport and exercise science services please check our Sport and Health Services pages or contact

For information on low carbohydrate diets for athletes contact Paul on Twitter (@the_hough) or email