Low carb diets for endurance exercise?

High carbohydrate, low fat (HC) diets with >60% carbs and <20% fat have long been favoured in the mainstream scientific literature for health and sports performance. However despite this advice alongside advances in nutritional science over recent decades, rates of obesity and type 2 diabetes have risen worldwide. During this time a growing number of researchers, the general public and athletes have been experimenting with low carb, high fat (LC) diets, with some clinical studies showing beneficial effects on metabolic factors and body composition compared to high carb. One ultra endurance runner, Timothy Ollsen switched to a low carb diet, and not only won, but broke the record of one of the world’s most prestigious 100 mile races.


So should we be rethinking current guidelines? The rationale for a LC, or ketogenic, diet is to become fat adapted, with ability to burn more fat at higher intensities, hence sparing limited muscle carbohydrate stores. Fat adaption is thought to take at least 21 days, and maybe several months (1), so the studies that originally promoted HC diets over LC were short duration (less than 2 weeks) with the latter leading to reduced muscle carbohydrate stores, fatigue and reduced performance may not have been long enough to show any benefits (2). A later study showed that a 5-6 day LC could improve fat burning during exercise, and that muscle glycogen could be restored with a 1-2 day HC diet, whilst maintaining the effects on fat burning (3).


However, despite the growing number of studies showing beneficial effects of LC diets on body composition, health, and ability to fat burn at higher intensities of exercise, the jury remains out on whether these factors translate into benefits for endurance performance (4). Decreases in performance at high intensity exercise have been reported with LC diets (1), which is not desirable for most athletes. Most studies are limited by their small scale and short duration, and it is also important to note that individuals may differ in their responses to LC diets, with some subjects improving performance and some decreasing. Maybe not surprisingly, LC diets show most promise for ultra-endurance athletes who may be able to maintain a higher level of aerobic exercise for longer, whilst sparing carb stores for later in the race (4).


With careful planning, it may be possible to get the benefits from both approaches. Carbohydrate periodisation, with intake varying between training phases and day to day exercise intensity is the subject of much current research and practice in endurance and team sports athletes (5, 6). For example, during a phase of off season low intensity, high volume training a LC diet could be followed to help optimise body composition and fat burning. Once training intensity steps up towards a competition, carb intake would increase. Similarly on a day to day basis, on rest days, low intensity or days of less than one hour training carbs would be limited, then eaten in high quantities to ensure the athlete optimises their higher intensity work outs. Again, research is in the early stages and has mixed results for endurance performance (5), however for individual cases carb periodisation may be a useful tool.


If you are interested in manipulating your diet to improve endurance performance I would advise working with a nutritionist to ensure you maximise the benefits.

  1. Zinn et al (2017) Ketogenic diet benefits body composition and well-being but not performance in a pilot case study of New Zealand endurance athletes. J Int Soc Sports Nutr. 12;14:22.
  2. Walker et al (2000) Dietary carbohydrate, muscle glycogen content, and endurance performance in well-trained women. J Appl Physiol (1985). 88(6):2151-8.
  3. Burke and Hawley (2002) Effects of short-term fat adaptation on metabolism and performance of prolonged exercise. Med Sci Sports Exerc. 34(9):1492-8.
  4. Chang et al (2017) Low-Carbohydrate-High-Fat Diet: Can it Help Exercise Performance? J Hum Kinet.12;56:81-92.
  5. Gelj et al (2017) No Superior Adaptations to Carbohydrate Periodization in Elite Endurance Athletes.
  6. Anderson et al (2016) Quantification of training load during one-, two- and three-game week schedules in professional soccer players from the English Premier League: implications for carbohydrate periodisation. J Sports Sci. 34(13):1250-9.

Charlie Ramsdale