Commentary to Accompany
Decades of clinical experimentation demonstrate that physical activity acutely increases fat oxidation across a range of intensity levels and durations. These observations led to attempts to optimize exercise intensity to maximize fat oxidation as a means to promote weight loss. The prevailing view is that modest intensity exercise should, by repetitively stimulating lipolysis and increasing fat oxidation, lead to a loss of fat mass and therefore weight loss. In a series of carefully controlled studies, Melanson (5) measured 24-h fat oxidation using whole room calorimetry in volunteers under two conditions, while sedentary and with planned bouts of low- to moderate-intensity exercise. In contrast to expectations, they were unable to show a difference in 24-h fat oxidation and conclude, "Moderate-duration exercise (≤1 h) has little impact on 24-h fat oxidation." They then go on to pose the question, "Why doesn't integrated 24-h fat oxidation increase with increased physical activity?" (5).
Importantly, the experiments of Melanson et al. were performed at energy balance and with a stable dietary macronutrient composition. Metabolic control systems act rapidly to maintain homeostasis by switching substrate flux and substrate uptake, storage, and lipolysis. If fat oxidation is increased during exercise but does not change over 24 h, enhanced glucose oxidation must occur in between to maintain balance of the stored fat and carbohydrate. By keeping subjects in energy balance and with a constant macronutrient intake, Melanson et al. allowed these control systems to balance fat oxidation to fat intake and carbohydrate balance to carbohydrate intake. So why should we exercise?
Day-to-day respiratory quotient matches the macronutrient content of the diet; within a single day, fat and carbohydrate oxidation varies widely. Fat oxidation is maximal after an overnight fast, and carbohydrate oxidation is maximal after a meal. This "flexibility" (3) in substrate oxidation is reduced in obesity, diabetes, and insulin resistance because of dual defects: decreased fat oxidation (9) and insulin resistance (1). Exercise training remodels muscle to increase the capacity for maximal fat oxidation and increases insulin sensitivity, thereby increasing the ability to turn on glucose oxidation after a meal. Recent work from our laboratory suggests that the capacity for fat oxidation, insulin sensitivity, and metabolic "flexibility" go hand-in-hand providing a strong link between aerobic fitness and the ability to burn fat (9).
For example, when dietary fat increases, even under conditions of energy balance, we are slow to turn on fat oxidation to match the increased fat load (6,8). This leads to a transiently positive fat balance (and negative carbohydrate balance), which over time could lead to weight gain. Immediately, physical activity increases the rate at which we are able to turn on 24-h fat oxidation (7); this may be one reason that increased physical activity is associated with lower risk of weight gain. Taken together, these data suggest that physical activity is very important in our capacity to buffer the positive fat balance that occurs with day-to-day variation in dietary fat that occurs on weekends, holidays, and so on.
So what can we take home from the results of Melanson etal.? Their work informs us that moderate exercise "under conditions of energy balance and with a stable macronutrient content of the diet" does not promote a negative fat balance. This is an important finding that leads to the next question, "How do we achieve weight loss?" By reducing calorie intake. Guidance on diet and caloric intake is not included in the new American College of Sports Medicine guidelines (2). People who are successful at losing weight share common behavioral strategies, including eating a diet low in fat, frequent self-monitoring of body weight and food intake, and high levels of regular physical activity (10), which brings us back to where Melanson et al. started. Exercise is frequently accompanied by an increase in food intake (4), leading to the exact conditions described by Melanson et al.: energy balance and no increase in fat oxidation leading to fat balance (5). The author suggests the revision of current guidelines to incorporate advice on reducing calories or the public will continue to believe that exercise alone will burn enough fat to help people lose weight - a common goal for the two thirds of Americans who are overweight or obese.
STEVEN R. SMITH
Molecular Endocrinology Laboratory
Pennington Biomedical Research Center
Baton Rouge, LA
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