Your diet has far more influence on developing your ideal lean body mass and overall health than exercise. It accounts for about 80 percent of the health benefits derived from a healthy lifestyle. It is also more important to weight loss. As for exercise, resistance exercise results in more weight reduction than aerobic exercise. Only resistance training increases your BMR (basal metabolic rate). You should do both but do not substitute exercise for diet.
NBH, PhD., Director of Education & Research