When we are in positive energy balance from consuming more calories than we expend, we convert surplus calories into fat that we store in fat cells. When we are in negative energy balance from spending more calories than we consume, we burn some of this fat. This calories-in-calories-out equation, however, is regulated by hormones, which in turn are strongly affected by diet and by other factors including psychosocial stress, the microbes in our gut, and, of course, physical activity.
