Our bodies have a large storage capacity for fat. This means that we have a large potential energy store to be used as fuel during exercise; bear in mind that the energy value of fat is 9 kcal/g. To compare, the energy value of carbohydrate is only 4 kcal/g. To put it into context, the average person has around 50 000 to 100 000 kcal stored as fat, which could equate to around 120 hours of low-intensity endurance exercise, whereas our average carbohydrate energy stores equate to around only 90 minutes of low-intensity endurance exercise.
As our muscles prefer burning carbohydrate (glucose) for fuel during exercise, sports nutrition advice generally details ways to increase our body’s glucose store and how to eat enough carbohydrate during events to improve and prolong endurance exercise. Our muscles are however capable of burning (oxidising) both glucose and fat (fatty acids) as a fuel during exercise.
The fat adaptation theory hypothesises that if we can ‘retool’ our metabolism to rather burn fat instead of glucose for energy during endurance exercise, then our need for a high carbohydrate diet is reduced. So the question becomes, “Are we able to promote muscular fatty acid oxidation over glucose oxidation and thereby improve exercise capacity?”
The answer might be yes, but not necessarily when we need it the most.
Typically, muscular oxidation of fat is limited, especially during high power outputs and high exercise intensities. Lipolysis (the breakdown of fat to fatty acids) is not as closely matched to our energy demands as glucose oxidation is. During low-intensity exercise (25% of VO2max), walking pace, most of our energy needs can be met by burning fat. The higher the exercise intensity (up to 85% of VO2max), the more we depend on burning glucose to meet our energy needs.
So it is unfortunately not as straightforward as one would hope.
Various ways to increase muscular fatty acid oxidation have been explored. Fat adaptation is one of these. On this ‘diet’, athletes eat meals high in fat and very low in carbohydrate. In the first few days, fat adaptation has a negative effect on exercise performance due to having less glucose around to be burnt, with no replacement fuel. Thereafter fat adaptation does increase the amount of fat we can burn during low-intensity exercise, and it reduces how much glucose we need to burn.
Remember though that the higher the intensity of the exercise the more our muscles depend on glucose as a fuel source. Your exercise performance can suffer if you are fat adapted, and the majority of your race time occurs at high intensities. Often the deciding factor in athletic performance is the sprint to the finish, or the surge during an uphill stage, which can mean the difference between a gold or a silver medal – all of these strategic events happen at high exercise intensities. There has also been no scientific evidence to suggest that the increase in fat oxidation seen in fat adaptation translates to measurable performance benefits.
So what is the solution if I am exercising at high intensities as well as at an endurance pace?
If a strict high fat, low carbohydrate diet is followed, fat adaptation can occur in as little as one to four weeks. There is not necessarily a need for long-term exposure to the diet, which can be impractical, impose health risks and be low in certain nutrients.
Athletes should limit their exposure to long-term high-fat diets. Instead ‘nutritional periodisation’ could be looked at and potentially offers the athlete the best of both scenarios. This would involve an athlete altering the composition of their diet over the course of a year or race season, varying the ratios of carbohydrate to protein to fat that they eat. This could help to ensure maximum training outcomes, body composition gains and health, ensure that fat is burnt during endurance exercise, that less carbohydrate would need to be eaten during exercise and that the body can still burn glucose during any high-intensity bursts.
Ultimately each athlete is unique and a one size fits all approach should never be adopted. Tailor-made techniques are the preferred dietary approach to enhance endurance performance.
Kerryn is a registered dietitian in private practice in working Durban and Ballito. She has a special interest in sports nutrition and using this to help athletes improve their performance. Read more about Kerryn Wuth