Diet :: Not eating enough -- or often enough.

While overeating and undereating may seem like contradictory nutrition mistakes, they are related.
"If you don't eat at regular intervals throughout the day, you risk disrupting your blood sugar and insulin levels, which in the end can promote fat storage and lower your metabolism -- both of which lead to weight gain," Brandeis says.

The solution:

Eat something every four hours and never let yourself "starve" from one meal to the next, Brandeis says.