The human body has evolved to be less than fussy about what foods it can utilize for energy. Carbohydrates, fat, protein -- all of the major or macronutrients get used as fuel in some way depending on a variety of circumstances including which individual macronutrient is in abundant supply. For example, when carbohydrate (glucose) is low, the body burns more fat and that is pretty much the idea behind low-carb diets. Not that it makes any difference to weight control because fat can be stored and retrieved, it's not fixed in storage, and you eat more fat on low-carb diets anyway. The only real determinant of how many pounds you pack on or lose is in the calorific value of the food you eat and the physical activity that uses it up.

Now here's the thing. With low carb, certain enzymes are emphasized -- those to do with fat burning -- in preference to those that facilitate carbohydrate or glucose burning. Fat substitutes quite well for glucose for energy purposes in exercise of low to moderate intensity. However, in high-intensity exercise for longer than about 10 seconds, for example when you run a 400 meter race or thrust a barbell overhead several times in a set, that's when the carbs and glucose do their best work. That's basic exercise physiology.

Exercise scientists have tried for many years to develop diets that make use of the vast fat stores of energy we all have -- it's much more than glucose -- yet still maintain the use of glucose for the more intense aspects of sports competition. Marathon running is a good example. Fat will fuel the endurance part of the race, especially in the later stages, but when it comes to a sprint up a hill or to the finish line, the research says that unless those enzymes that get glucose from storage are operating at peak efficiency, you will be slower in the sprint, or perhaps you will lift less at the gym. Eating to emphasize fat burning does just this: it degrades the response of these glucose-providing enzymes -- and that makes you slower at shorter, high-intensity effort. And this process doesn't get reversed quickly with more carbohydrate consumption. It takes time to favor glucose burning again.

Next time you feel a bit tired at the gym when trying to pull a 5 X 5 deadlift session, think about the carbs. That's what you need, not low carb and fat burning.

Sources: Burke LM, Hawley JA, Angus DJ, Cox GR, Clark SA, Cummings NK, Desbrow B, Hargreaves M. Adaptations to short-term high-fat diet persist during exercise despite high carbohydrate availability. Med Sci Sports Exerc. 2002 Jan;34(1):83-91.
Stellingwerff T, Spriet LL, Watt MJ, Kimber NE, Hargreaves M, Hawley JA, Burke LM. Decreased PDH activation and glycogenolysis during exercise following fat adaptation with carbohydrate restoration. Am J Physiol Endocrinol Metab. 2006 Feb;290(2):E380-8.