An often neglected aspect of exercise is its hormonal dimension. Research published Nov 2010 In the Journal of Obesity, found HIIT more effective in shredding body fat than other modes of exercise. Why? The reason may be that HIIT is anabolic. It boosts growth hormone and testosterone more than cortisol, which is particularly important for men. By contrast, protracted aerobic exercise is catabolic. The main hormonal effect of running for an hour is to spike levels of cortisol .Excess cortisol can suppress the immune system, Increase belly fat, and is also proven to contribute to muscle wasting. The Feb 2009 issue of the Journal of the International Society of Sports Nutrition reported that male subjects following a 6 week program of HIIT (15 min. per day 3 days per week & supplementing with amino acids) gained an average of 2 pounds of lean muscle without lifting weights. If you want a leaner, more muscular body your best bet is to harness the hormonal magic of HIT.