This paper tries to understand the underlying causes of the rapid increase in obesity rates over recent decades. In particular, we propose a dynamic general equilibrium model to derive the quantitative implications of a decline in the relative (monetary and time) cost of food prepared away from home on the caloric intake of the average American adult over the last forty years. Two channels that lower this relative cost are considered. First, productivity improvements in the production of food prepared away from home. We and that this channel is qualitatively consistent with expenditure trends in food items, but falls short of accounting for the magnitude of the observed changes. We then consider actual declines in income taxes and in the gender wage gap, which increase the cost of preparing food at home from scratch. Our model accounts for three quarters of the observed changes in calorie consumption, and is consistent with trends in aggregate food expenditures, time use, and key macroeconomic variables. Our results indicate that changes in the relative cost of food prepared away from home play an important role in our understanding of the increased weight of the American population during the last 40 years.