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Are Low Carb High Fat diets for sports performance all hype?

Photo of Louise BurkeInterview with Louise Burke OAM, PhD, APD, FACSM

Louise is a sports dietitian with over 35 years of experience with elite athletes. She has been Head of Sports Nutrition at the Australian Institute of Sport since 1990 and the team dietitian for the Australian Olympic Teams for the 1996-2012 Summer Olympic Games. Louise has nearly 300 peer-reviewed research papers and book chapters, and written or edited several sports nutrition textbooks. She was awarded a Medal of the Order of Australia in 2009 for her contribution to sports nutrition.

You wrote a review in the journal Sports Medicine called ‘Re-examining high fat diets for sports performance: did we call the nail in the coffin too soon?’ What was your overall conclusion after reviewing the evidence?

The review concluded high fat diets do not improve performance across the range of sports in which athletes need to be able to work at high intensities/speeds/power outputs, even if it is only for a small part of the event. Although high fat diets, chronically or for shorter periods of “adaptation”, can enhance the athlete’s capacity to use fat as an exercise fuel, the downside is that they reduce the muscle’s capacity to use glycogen for fuel and reduce the capacity for high-intensity exercise. This is likely to be a disadvantage to competitive athletes in most sporting events.

What instigated this review?

The high degree of interest coupled with a lot of misinformation and misunderstanding about a specific type of low carb high fat (LCHF) diet for athletes: a chronic very low carbohydrate (<50 g/d), moderate protein, high fat (75-80% of energy) diet.  This differs from the previous version of the short-term high fat diet we studied for a decade, in which athletes switch to a reduced carbohydrate (~15-20% of energy), increased fat (60-65% of energy) diet for a short period before returning to diets with high carbohydrate availability for competition day.   This theory behind the chronic LCHF diet says the body adapts to fat as a fuel source for exercise, while gaining extra benefits from the chronic exposure to high levels of blood ketones. However, at the time of writing the review, there was only one investigation of such a diet on performance of competitive athletes. This study from the 1980s found that “keto-adaptation” dramatically increases fat oxidation during moderate intensity exercise, but even under conditions favouring fat utilisation, only extends exercise capacity to a similar level as found with a high carbohydrate diet.  I think it’s important to recognise that knowledge evolves, but in this case we found that there was little new scientific evidence to support the renewed interest in an old idea.

Was that “case closed” for you then?

No, I believe in respecting the enthusiasm for “new” ideas and testing them out under scientific rigour. So, since doing the review, we have embarked on some well controlled studies of the LCHF diet in highly competitive race walkers. I can’t reveal all the results from this large study yet, but I can say that carbohydrate has an advantage over fat as a muscle fuel in that it produces ATP at a lower oxygen cost. In other words, to produce the same speed/intensity/power output from fat oxidation, you need to use a slightly larger fraction of your VO2max. This mightn’t be a problem if your sport requires you to work at a moderate aerobic intensity – say 60-70% VO2max – as there’s plenty of capacity to absorb the need for extra oxygen use. However, athletes exercising at 80-90% VO2max need to have great “movement economy” to run/cycle/walk as fast as possible at any given rate of oxygen delivery to the muscle. In these zones, moving to increased fat burning reduces the economy of movement and reduces your highest sustainable speed.

Why do you think the Low Carb High Fat (LCHF) diet gained such traction in sporting circles?

People are generally intrigued by “new” diets, both for the opportunity to gain some of the promised performance advantages as well as the feeling of being ahead of the pack. Athletes might even be more likely to embrace extreme or different diets because they are already used to discipline in their lifestyle. Being able to make better use of body fat stores as an exercise fuel is tempting because it sounds true – it is an extension of some of the principles of endurance training. However, LCHF diets are not new: this is the third time it has gained traction in my career in sports nutrition. The difference this time, is that social media has escalated the hype.

Are there any particular sports for which a LCHF diet is warranted?

If you look critically at the requirements of competitive sport, the LCHF is applicable, (or less harmful), for only a small group of events or individual athletes. Sports involving moderate intensities, such as ultra-endurance events, are less likely to pay the price for impaired glycogen utilisation or reduced exercise economy. The advantage of being a better fat burner could mean you don’t have to take in so much carbohydrate during the event. And it may be an option for athletes or events where refuelling during exercise is problematic. But apart from fuelling muscle, the role of carbohydrate during exercise is to make the brain feel happy and able to choose a higher pace to work at.

How would you summarise the role of carbohydrates generally in physical performance?

Carbohydrate fuels the brain to work at its best around pace judgement, perception of effort, skill execution and decision making. It also provides a muscle fuel that can sustain exercise over a wide range of intensities, using both oxygen dependent and independent pathways. Many studies show that depletion of the various sources of carbohydrate in the muscle and brain are associated with fatigue and loss of performance. Having said that, the human body works best when it has “metabolic flexibility” – it has a beautiful system of integrating a variety of energy generating pathways. Athletes should train and eat to optimise all these pathways, with the focus on the ones that are critically involved with the performance of their event. It doesn’t make sense to try to cut out or impair any pathway – be it carbs or fat.

It shouldn’t be a war of ‘fats vs carbs’, but rather an army where all soldiers are ready to work together in the fight to win.


What is the role of sugar, if any, in the diet of an athlete?

Sugar is one form of carbohydrate that can contribute to an athlete’s total carbohydrate intake. Many dietary sources of sugars lack some of the overall nutritional qualities of other foods, so they shouldn’t make up the major carbohydrate source in an athlete’s diet. However, sugars often provide characteristics that make them useful for specific uses, particularly during exercise where they can provide a quickly digested and well-tolerated fuel source. Their compact nature and enjoyment value can also assist an athlete to meet very high energy requirements.

 

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