Calories are the energy in food. Calories can come from carbs, fats, and proteins. The body uses calories to fuel all activities or stores most of them as body fat.

Weight gain and weight loss may have been attributed to reasons related to calories and willpower. However, modern obesity research disagrees. Scientists now believe that the cause of this condition may be related to the hormone leptin. Leptin resistance, in which your body does not respond to this hormone, is now believed to be the leading cause of fat gain in humans. This article explains leptin and its relationship to obesity.