• Chris Webster // PhD Exercise Science – in

Low Carbohydrate High-Fat Diets and Sports Performance


Over the past 60 years, carbohydrates have earned a reputation as the most important energy substrate for exercise. The research was primarily focused on the role of glycogen stores in the muscle and liver and on the optimisation of ingested carbohydrate during exercise[1]. An outcome of this era of research is that a carbohydrate-rich diet is usually recommended for athletes and considered an essential component of good exercise performances. In athletes following such a diet, glucose is the primary substrate for oxidative energy production during exercise and fat plays a secondary role. However, it is possible for fat to become the primary substrate for oxidative metabolism, seemingly without compromising exercise performance. This is achieved by following a low carbohydrate high fat (LCHF) diet.

Fat Adaptation and Performance

Fat adaptation first appeared in the literature when Phinney et al (1983) found that five elite endurance-trained cyclists were able to maintain exercise performance after 28 days on a diet that contained virtually zero carbohydrate[2]. The study had many limitations but provided the first indication that a high-carbohydrate approach may not be the only available option for endurance athletes.

Over the following decades, many attempts were made to realise the potential advantage that an increased fat oxidation capacity could theoretically provide. A wide variety of trials have seen participants following LCHF diets for anything from 1 meal to 6 weeks; different performance measures were used such as time to exhaustion, time trials and sprint power; and in many cases, several days of LCHF eating was combined with high carbohydrate loading days. In the majority of these trials, there was very little difference in performance outcomes between LCHF and control athletes, despite the enormous differences in whole body metabolism. An advantage of increased fat oxidation rates could not be found. If anything, this area of research demonstrates the remarkable ability of metabolically healthy humans to effectively utilise whatever substrate is available. For a comprehensive review of fat adaptation and exercise performance see Burke (2015)[3].

The literature may be inconclusive but there are still many athletes (competitive and recreational) who follow a LCHF diet as their personal preference and claim that their exercise performance has improved or at least not changed. Two recent studies (unfortunately too recent to be included in Burke’s review) investigated and described competitive endurance athletes who had self-selected a LCHF diet[4,5]. They had been following the diet for 9 to 24 months, which makes them the most well-adapted athletes studied to date. Although the processes and mechanisms which occur in athletes after following a LCHF diet for the long-term are not well studied, it does seem that a longer adaptation period does confer benefits. The athletes in these studies had average fat oxidation rates three times greater than controls at exercise intensities of 65 to 75% VO2max (~1.2 g/min versus ~0.4 g/min). Some individuals were measured as being able to oxidise fat at ~1.8 g/min, which was previously unheard of in the literature[6]. Maximal fat oxidation rates also occurred at higher exercise intensities in the adapted LCHF athletes than the controls (70% VO2max compared to 55% VO2max). Fat is, therefore, a viable energy substrate for these athletes at meaningful exercise intensities.

Another key feature which appears to be important for successful fat adaptation is nutritionally induced ketosis, whereby blood ketone concentrations rise from 0 mmol/l up to between 0.5 and 4.0 mmol/l. Ketones are produced from fatty acids and they provide the majority of energy to the central nervous system after a strict LCHF diet. The central nervous system would otherwise rely entirely on glucose. Ketones are therefore important for successful fat adaptation since they facilitate a drastic reduction in the essential glucose requirements of the body.

Who Can Benefit?

Ultra-endurance athletes (at all levels) may be the most obvious group that could find fat adaptation useful. The longer the duration and lower the intensity the more likely it is that a clear benefit could be seen. The major concern for fat adaptation is that it compromises the athlete’s capacity for high-intensity exercise (glycolytic/lactate energy system). We know that LCHF athletes can sprint and many habitual LCHF athletes incorporate high-intensity intervals into their training. However, the prevailing perception is that high-intensity performance suffers to some extent. Performance trials in exercise modes that heavily utilise the glycolytic/lactate energy system are required to determine the extent to which high-intensity exercise suffers in well adapted LCHF athletes (if at all).

Since elite-level endurance racing often requires periods of very high intensities rather than a constant steady-state effort, high carbohydrate intake during training and racing may be very important at this level of the sport. For the majority of healthy amateur athletes (competitive and recreational), sporting performance would probably be similar on a wide variety of diets, including LCHF. Health considerations and personal preference are therefore important considerations when selecting a diet. However, there are indications that a LCHF diet may provide some benefits to athletes that: frequently hit the wall during exercise; experience reactive hypoglycaemia; suffer gastro-intestinal problems when ingesting high volumes of carbohydrate during exercise, and experience large fluctuations in weight depending on the stage of the season.

Additionally, there are populations whose sporting performances may benefit from fat adaptation primarily due to changes in health rather than due to improvements in fuelling strategies. For example, the LCHF diet seems particularly effective in managing diabetes and related conditions[7]. There are also many sporting codes other than endurance sport where LCHF diets could potentially be applied. Long duration, low-intensity sports such as golf and cricket seem like good candidates, as do sports where weight control is essential, such as gymnastics and horse racing (jockeys).

There is plenty of scope for continued research in this area and hopefully the full potential (or lack thereof) of fat adaptation as a tool for the elite as well as recreational athletes across many different sporting codes will be investigated.

  1. Jeukendrup, A. A step towards personalized sports nutrition: carbohydrate intake during exercise. Sports medicine (Auckland, N.Z.) 44 Suppl 1, S25-33, doi:10.1007/s40279-014-0148-z (2014).

  2. Phinney, S. D., Bistrian, B. R., Evans, W. J., Gervino, E. & Blackburn, G. L. The human metabolic response to chronic ketosis without caloric restriction: preservation of submaximal exercise capability with reduced carbohydrate oxidation. Metabolism: clinical and experimental 32, 769-776 (1983).

  3. Burke, L. M. Re-Examining High-Fat Diets for Sports Performance: Did We Call the 'Nail in the Coffin' Too Soon? Sports medicine (Auckland, N.Z.) 45 Suppl 1, 33-49, doi:10.1007/s40279-015-0393-9 (2015).

  4. Volek, J. S. et al. Metabolic characteristics of keto-adapted ultra-endurance runners. Metabolism: clinical and experimental 65, 100-110, doi:http://dx.doi.org/10.1016/j.metabol.2015.10.028 (2016).

  5. Webster, C. C. et al. Gluconeogenesis during endurance exercise in cyclists habituated to a long-term low carbohydrate high fat diet. J Physiol, doi:10.1113/jp271934 (2016).

  6. Achten, J. & Jeukendrup, A. E. Maximal fat oxidation during exercise in trained men. International journal of sports medicine 24, 603-608, doi:10.1055/s-2003-43265 (2003).

  7. Feinman, R. D. et al. Dietary carbohydrate restriction as the first approach in diabetes management: critical review and evidence base. Nutrition (Burbank, Los Angeles County, Calif.) 31, 1-13, doi:10.1016/j.nut.2014.06.011 (2015).

#LCHF #Performance #Nutrition

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