A new psychology study provides evidence that being overweight makes people tired and sedentary, rather than vice versa. Life scientists placed 32 female rats on one of two diets for six months. The first, a standard rat's diet, consisted of relatively unprocessed foods like ground corn and fish meal. The ingredients in the second were highly processed, of lower quality and included substantially more sugar -- a proxy for a junk food diet.

Source: Does a junk food diet make you lazy? Psychology study offers answer