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Large variation in the way athletes use fat during exercise

Athletes use a mix of fat and carbohydrate for energy during exercise, but the relative proportion of the two varies significantly even in athletes within the same sport. A recent study aimed to profile the rates of fat oxidation in 1121 athletes from a variety of different sports during treadmill running in the laboratory. The average rate at which an athlete can oxidise fat most rapidly was found to be 0.59 grams per minute. However, this varied significantly among individuals with the highest rate of fat oxidation being recorded at 1.27 grams per minute and the lowest at 0.17 grams per minute. This means that during an hour of exercise, it is possible that some athletes might burn more than 75 grams of fat, but other athletes might only use 10 grams of fat. Men had higher absolute values for maximal fat oxidation than women, but when rates of fat oxidation were expressed per kg of fat free mass, women had higher maximal fat oxidation rates than men. The rate of maximal fat oxidation occurred at an average exercise intensity of 49% of maximal oxygen consumption, but again there was high individual variability in this measure. Interestingly, there was a positive correlation between the maximal rate of fat oxidation and the intensity of exercise at which this occurred. This means that the athletes capable of high rates of fat oxidation were also doing this at higher exercise intensities than the athletes less capable of high rates of fat oxidation. There was also a correlation between the maximal rate of fat oxidation and VO2max. This means that the most aerobically fit athletes were capable of higher rates of fat oxidation. Although these correlations were statistically significant, they were not strong relationships and the authors suggested that there are likely to be many other factors that explain the variation in maximal fat oxidation.

The study findings replicate what has been seen in other, smaller studies conducted on healthy rather than athletic populations. However, this study is the first of its kind to be done on such a large number of athletes. This gave tremendous insight into the huge individual variability in macronutrient metabolism during exercise. The actual implications of this are still yet to be determined, but it is logical to assume that the high individual variability of macronutrient metabolism during exercise might also mean a high variability in dietary macronutrient requirements for different athletes, even if they are involved in similar sports with apparently similar physiological demands. Many sports nutrition researchers and practitioners think that there is no ‘one-size-fits-all’ approach to using nutrition to support athletes to perform at their best and this study provides another bit of evidence to support that contention. Measuring the capacity of an athlete to oxidise fat during exercise may be one of the many pieces in the puzzle that provide some insight into the mix of dietary macronutrients that best suit an individual athlete.

Reference:

Randell, R. K., Rollo, I., Roberts, T. J., Dalrymple, K., Jeukendrup, A. E., & Carter, J. M. (2016). Maximal Fat Oxidation Rates in an Athletic Population. Medicine and Science in Sports Exercise, 49(1), 133-140.