Abstract：Abdominal fat-reducing effect of exercise training has been commonly reported. Since exercise appears as an energy consuming behavior, it is intuitively thought that the outcome of abdominal fat decrease by exercise training is mediated by what has been commonly called “fat-burning effect.” Thus aerobic training at moderate intensity is widely recommended for obesity management. Such interpretation raises several controversies. First, if fat burning contributes size reduction of fat-storage tissues, skeletal muscle should be the first tissue showing size reduction due to increased breakdown of carbon-based fuel. However, exercise training increases muscle mass with enhanced intramuscular triglyceride storage. In a contrary, exercise training decreases abdominal fat storage despite fat cell contains very few mitochondria that are required for fat burning during exercise. Second, increasing exercise intensity results in energy reliance on carbohydrate than fat. Yet, anaerobic-based high-intensity intermittent exercise has better fat-reducing effect than aerobic training at moderate intensity with the same energy consumed. Furthermore, the 24-h fat utilization is not altered by exercise at either low or high intensity compared to resting condition. It is therefore unjustified of saying that fat burning contributes to the actual fat reducing effect of exercise training. Third, oxygen is required for fat burning. However, lowering oxygen availability by chronic hypoxia exposure has been consistently shown to decrease body fat in humans and animals. A significant fat reduction without weight changes occurs when swimmers switched their training site from sea-level to altitude at the same daily training volume. Oxygen deficit due to systemic hypoxia (16%) increases postprandial blood distribution to skeletal muscle, suggesting that increased deliveries of insulin and ingested carbon source towards skeletal muscle against adipose tissue might be the major determinant of body composition in humans. Conclusion: Exercise training increases energy demand of skeletal muscle against adipose tissue resulting favorable fuel distribution towards muscle than adipose tissue, which may be a better explanation for the abdominal fat reducing effect of exercise training. This concept can better explain the fact that anaerobic training produces better fat reducing benefit than aerobic training with similar energy expenditure. (Full text)