carbs in endurance sportsAs a nutritionist who specialises in working with triathletes, cyclists and runners to improve their endurance sports performance, while at the same time needing to ensure that they remain in good health, I spend a lot of time discussing carbohydrate intake. Traditionally, a high carb diet has been seen as the obvious choice for endurance athletes, given that carbohydrate provides the main source of energy when training hard or racing. But recent years have seen the emergence of the alternative low carb high fat (LCHF) or ketogenic diet, which seems to improve the body’s ability to use fat as fuel at higher intensities, reducing the need for carbohydrate. This approach has been gaining popularity in the endurance sports community, particularly among those involved in longer distance events such as ultra-marathons or Ironman, where taking on large amounts of carbohydrate may cause gastrointestinal distress or be impractical.

Until recently, there was very little research available on the impact of LCHF diets on endurance performance, one of the limitations being that it appears to take weeks/months to adapt metabolically to this type of diet. Initial studies into the use of short term high fat diets to improve race performance were generally unsuccessful, which may well have been linked to a lack of adaptation time. However, the publication of results in March 2016 from the FASTER study 1 has shed new light on the topic. This compared elite ultra-marathon and iron-distance triathletes during 3 hours of running on a treadmill at 64% of VO2max (ie an easy to moderate pace). One group followed a traditional high carbohydrate diet (60% carbs, 14% protein, 25% fat). The other group had adapted to the LCHF diet approach (10% carbs, 19% protein, 70% fat) over an average of 20 months. During the study, the LCHF athletes demonstrated average fat oxidation rates that were 59% higher than the high carb athletes. This means that they clearly used far more fat as fuel and were still burning fat at higher intensities than previously seen in research. If you want to learn more about the FASTER study, I recommend listening to a discussion on the Endurance Planet podcast with the study author Dr Jeff Volek, which can be found here.

The results of this study are certainly interesting and tell us more about LCHF and endurance performance in elite athletes. However, the results might well not be as positive in less highly trained athletes and may be different in men and women. They are also very unlikely to apply to shorter duration events performed at greater than 64% of VO2max. The research literature is clear that carbohydrate is the preferred fuel source for events lasting less than three hours 2 The standard recommendation is that athletes should consume 30-60g of carbohydrate per hour in events and training sessions lasting over 90 minutes. 3

There have been shifts in the research over the last few years, however, on the role of carbohydrates in training for endurance sport. It has become clear that doing some of your lower intensity training in a carbohydrate depleted state can help to facilitate adaptations to endurance training, such as increasing the number of energy-producing mitochondria in the muscle cells. 4 This approach is very different, however, from following an LCHF approach to your daily diet or using fat as the predominant fuel source for racing, and I have written more about it in my blog post Low Glycogen Training for the Endurance Athlete – see here.

So, as an endurance athlete, how should you best use the research to help you manage your carbohydrate intake?

  • If you are an ultra-runner, long distance cyclist or long course triathlete who struggles with carbohydrate intake during events, you may see benefits from trialling a LCHF dietary approach. Bear in mind that it will take some months to see benefits and that it is likely that your performance will decline in the short term, so it’s best to start this process in the off season and certainly not in the middle of your competitive season. It can be a dietary approach that’s hard to implement – a good resource is this e-book by Jeff Volek and Stephen Phinney, The Art and Science of Low Carbohydrate Performance. The Endurance Planet podcast mentioned before is also helpful, also the book “What The Fat? Sports Performance: Leaner, Fitter, Faster on Low-Carb Healthy Fat.” is useful. You may also find that you still need to use a certain amount of carbohydrate during races, particularly those with short periods of high intensity. Some people, particularly women, may find a very low carbohydrate diet adversely affects their hormone balance, particularly if care if not taken to replace carbohydrate calories with calories from fat.
  • If you are looking to reduce your dependence on carbohydrates during endurance events, perhaps due to gastrointestinal issues, but are reluctant to commit to a full LCHF or ketogenic approach, you might like to try the Metabolic Efficiency approach which teaches your body to burn more fat for fuel. It can also be a helpful approach for improving body composition. See this e-book by Bob Seebohar, (scroll down to the second book on the page) for more information.
  • If you train for endurance events lasting over three hours, but have no gastrointestinal issues with consuming 30-60g carbohydrate during training or events, you may benefit from a periodized approach to carbohydrate intake. While you are building your endurance base with low intensity sessions, some months out from competition, aim to reduce your overall carbohydrate intake (perhaps to 40% of calories) and replace it with an increased protein intake (up to 2.0g/kg of body weight per day) and more healthy fats such as oily fish, nuts and seeds, olive oil and avocado. Train once or twice a week with depleted carbohydrate stores to enhance adaptations to endurance training. Strategies might include training before breakfast 1-2 times a week, performing your second workout of the day with depleted muscle glycogen stores by not refuelling with carbohydrate after your first workout, or heading out for up to 4 hours for a weekend ride on a coffee and protein breakfast (eg omelette) and not consuming any sports nutrition products during it. The carbohydrates in your daily diet should come largely from vegetables, fruit and a limited amount of wholegrains, with sugary foods kept to a minimum. This all helps to improve your fat burning capacity and enhance endurance. Then, as you enter your build period and subsequently the competitive season, gradually increase your carbohydrate intake and use of sports nutrition products to enable you to perform your longer sessions at a higher intensity. Reduce your protein and fat intake a little, but ensure that you are still consuming enough calories to fuel your increased training volume and intensity. This approach also enables you to practise your race day nutrition.
  • If your events are generally under three hours and performed at an intensity close to threshold, which requires carbohydrate as the predominant fuel, a good approach is to ‘fuel for the work required’. This means that your carbohydrate intake varies on a day to day basis throughout the year, depending on the type of sessions that you are undertaking. You need to start your short, high intensity workouts with full muscle glycogen stores, which means eating or drinking  carbohydrate in the hour or two beforehand (1g/kg of body weight is a good guide) and then replenishing your depleted glycogen stores afterwards with a small amount of carbohydrate. But if you are not training again for 24 hours, you might have only a small amount of carbohydrate at your other meals. If you are training twice in a day, you increase your carbohydrate intake accordingly. An easy day might see you use one of the ‘glycogen depleting’ strategies mentioned earlier, eg training in a fasted state before breakfast, or generally consuming fewer carbs (and calories) than on harder days. If you follow a relatively high carbohydrate diet overall (eg 60% of calories), aim for most of your intake to come from minimally processed carbohydrates such as oats, sweet potato, rice, whole grain bread, wholemeal pasta, vegetables and fruit, rather than sports nutrition products or refined products like white bread and pasta, cakes, biscuits and pastries.

To sum up, there’s no one approach to carbohydrate intake that is going to suit all endurance athletes. The type of event you participate in is a major factor to take into consideration. Your ability to digest carbohydrates during long training sessions and races also need to be thought about. Some people feel better on a high fat diet than others; this is likely to be linked to genetic factors around fat metabolism which are still being researched. Women in particular need to take care with low carbohydrate diets, as insufficient carbohydrate (and calories) may lead to hormonal deficiencies, which in turn adversely impact on both performance and health. In this blog post, I’ve tried to outline some of the different approaches you can take. I hope you will find it helpful.

Jo Scott-Dalgleish BSc (Hons) is a BANT Registered Nutritionist, writing and giving talks about nutrition for endurance sportBased in London, she also works as a Registered Nutritional Therapist, conducting onetoone consultations with triathletes, distance runners and cyclists to help them eat well, be healthy and perform better through the creation of an individual nutritional plan. To learn more about these consultations, please visit



1 Volek et al. Metabolic characteristics of keto-adapted ultra-endurance runners. Metabolism. 2016. March; 65(3): 100-110.

2 Hawley JD and Leckey JJ. Carbohydrate dependence during prolonged intense endurance exercise. Sports Med. 2015; 45(Suppl 1): 5–12.

3 Position of the Academy of the Nutrition & Dietetics, Dieticians of Canada and the American College of Sports Medicine. Nutrition and Athletic Performance. Medicine & Science in Sports & Exercise: March 2016, 48(3): 543-568

4 Bartlett JD, Hawley JA and Morton JP. Carbohydrate availability and exercise training adaptation: too much of a good thing? Eur J Sport Sci. 2015; 15(1): 3-12