Have you ever heard of an Obesogen?
It is a subclass of Endocrine (hormone) disrupting chemicals that might predispose individuals to become obese. I know most people think that you become obese over time by consuming more calories than you burn, but this is not the whole story. Other factors, like hormones can come into play. Check out this study.
A researcher exposed one of two groups of mice to a substance called perfluorooctanoic acid (PFOA). Low doses of exposure to this substance led to the exposed group having elevated serum leptin, insulin, and being overweight in mid-life when fed the same diet. If you think that a chemical like this is rare, you would be wrong. It has been commonly manufactured in the US since the 1940s and can be detected in the blood of 98% of the general U.S. population. It has been detected in carpet cleaners, microwave popcorn bags, water, food, and cookware (particularly Teflon). Check out this picture of two of the mice. The mouse on the right is clearly much slimmer than the exposed mouse on the left.