Great apes in zoos have a history of receiving diets resembling those of humans in industrialized societies, with ensuing obesity and comorbidities. We analyzed adult zoo body mass (BM) data of 1290 chimpanzees, 141 bonobos, 231 Bornean orangutans, 186 Sumatran orangutans, and 719 Western gorillas in comparison to BM of free-ranging conspecifics. Females and males considered overweight or obese were 62% and 45% in chimpanzees, 14% and 3% in bonobos, 88% and 81% in Bornean and 74% and 72% in Sumatran orangutans, and 67% and 12% in Western gorillas. The higher degree of overweight in females might also be linked to a lack of reproductive activity. Historical improvements, if at all, were evident in gorillas. We hypothesize that the main cause of obesity is the use of diet items that are low in fiber, high in easily digestible carbohydrates, and therefore highly palatable (such as cultivated fruit and pelleted or extruded compound feeds high in starch), instead of the recommended use of higher-fiber items. As zoo animals do not choose diets for themselves but are provided with the items they can choose from, it is the responsibility of zoos to provide primates with diet items that can safely be used in a group setting without the incentive of overeating and the risk of monopolization by dominant individuals, that is, most plausibly, high-fiber diets. For the optimal welfare of such long-living taxa, the focus under human care should shift from sheer longevity to health span, for which the prevention of obesity is key.