The association between physical activity and body fat is well established. For example, a prospective study1 that followed 84,511 men and 203,987 women for 5.1 years categorised the subjects into groups based on their physical activity levels. Inactive, moderately inactive, moderately active and active subjects then had their waist circumference and body weight measured at baseline and on completion of the study. As expected, physical activity level was predictive of waist circumference in men and women, but not body weight. This suggests that physical activity is inversely associated with central adiposity rather than total body fat. Evidence like this is often used to support the contention that increasing physical activity will help obese individuals lose weight. However, care has to be taken in the interpretation of these results because cause and effect cannot be ascribed from an association.
Increasingly, research suggests that obesity is caused by metabolic dysfunction. Low quality diets high in refined carbohydrates such as fructose, containing nutrient poor calorie dense foods increase de novo lipogenesis and the transport of fatty acids to skeletal muscle. Here intramuscular fat stores accumulate and are converted to signal molecules that desensitise the insulin receptor. This reduces the uptake of glucose by muscle tissue for oxidation during exercise, and shifts compartmentalisation of glucose to the adipose tissue, particularly in the abdominal area, for storage. The increase in white fat accumulation then causes the production of cytokines that results in systemic inflammation and disease. Importantly, the skeletal muscle has a reduced ability to oxidise glucose because the insulin receptors in skeletal muscle are not able to function correctly for effective glucose uptake. This decreases glycogen storage in skeletal muscles and also decreases the stimulus for protein synthesis.
Understanding the cause of obesity shows that there are valid biochemical reasons why the metabolic dysfunction that causes obesity could decrease the ability to exercise. Applying this logic to the known inverse association between physical activity and waist circumference suggests that perhaps, it is obesity that causes the lack of physical activity, rather than the lack of physical activity that causes the obesity. This assertion is supported by evidence in the literature that shows that physical activity is not able to cause long-term body weight reductions in obese individual. However, improving the quality of the diet, through diets such as the Mediterranean diet is able to reverse the metabolic dysfunction which facilitates the up regulation of natural homeostatic mechanisms to allow weight loss. Once insulin sensitivity is increased, the ability of muscle to oxidise glucose and store glycogen will increase energy reserves required for exercise.