We develop a theoretical model to identify conditions under which price and income changes are most likely to change weight. Although it is intuitive that raising the price of high-calorie food will decrease consumption of such goods; it is not clear that such an outcome will actually reduce weight. Our empirical analysis demonstrates a case where a tax on food away from home, a food intake category blamed for much of the rise in obesity, could lead to an increase in body weight; a finding which emphasizes the need to employ economic modeling when developing public policy to reduce obesity.