xercise is considered by many to cause weight loss. This is appears so self-evident that questioning such a belief results in derision. However, the term weight loss is misleading because body weight incorporates fat, muscle, bone, glycogen and water. That exercise can cause the short-term loss of water or glycogen is not in question. However, it is more controversial as to whether physical activity is effective at causing long-term losses in body fat, particularly for obese individuals. The evidence against exercise induced fat loss comes from studies showing that increased physical activity results in an increase in energy intake. If the form of that energy is in the types of foods that lead to the development of insulin resistance, such as foods containing fructose, then body fat loss is unlikely. In fact under these conditions it is probably that weight gain will occur due to deterioration in the metabolic efficiency of the energy storage and oxidation pathways.
The ability of exercise to cause weight loss has been extensively studied, and in general clinical trials show that physical activity alone is not effective at causing reductions in body fat. However, most individuals do experience weight loss on an exercise regimen, because it is often combined with a period of calorie restriction. Forced calorie restriction in itself has also been shown to be ineffective at causing body fat loss. However, when forced calorie restriction occurs, individuals often concomitantly decrease intakes of fructose containing foods such as cakes and biscuits and it is this, not the calorie restriction, that ultimately causes body composition improvements. Evidence for this comes from studies showing that low carbohydrate diets and high quality diets cause body fat loss without the need for energy restriction. In fact such diets cause weight loss to an equal degree as similar diets with additional exercise regimens.
A study published in the International Journal of Obesity1 investigated the association between physical activity and body composition in healthy adolescents aged 14 to 18 years. The results of the study showed that physical activity was inversely associated with percentage body fat. This supports other studies that have reported similar findings in large scale epidemiological studies. However, this does not mean that the physical activity is the cause of the improvements in body composition. Athletes for example may eat higher quality foods. This contention is supported by further results in the same study. When the authors controlled for energy intake the association between physical activity and body composition disappeared. However, energy intake was negatively associated with body fat. Therefore physical activity increases energy intake, but creates a paradox in that this is also associated with lower body fat percentage and improved body composition.
So why does a higher energy intake result in a better body composition? This may relate to the fact that those interested in physical activity, are also more likely to eat higher quality diets. In fact anecdotally it is interesting to speculate that based on current recommendations, athletes would tend to eat higher protein diets that were rich in fruit, vegetables and whole grain foods. In addition, while physical activity may not be effective at reversing abdominal adiposity, it is effective at preventing detrimental metabolic changes caused by fructose. This is because following exercise, fructose ingestion can result in hepatic glycogen synthesis, rather that stimulating de novo lipogenesis. This scenario is protective of insulin resistance because it removes the source of the lipids that may interfere with insulin signalling. High quality diets and not exercise or energy restriction may therefore be responsible for causing the improvements in body composition.
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