Something about being poor makes people fat. Though there are many possible explanations for the income-body weight gradient, we investigate a promising but little-studied hypothesis: that changes in body weight canat least in partbe explained as an optimal response to economic insecurity. We use data on working-age men from the 1979 National Longitudinal Survey of Youth (NLSY79) to identify the effects of various measures of economic insecurity on weight gain. We find in particular that over the 12-year period between 1988 and 2000, the average man gained about 21 pounds. A one percentage point (0.01) increase in the probability of becoming unemployed causes weight gain over this period to increase by about 0.6 pounds, and each realized 50% drop in annual income results in an increase of about 5 pounds. The mechanism also appears to work in reverse, with health insurance and intrafamily transfers protecting against weight gain.