Obesity is increasing rapidly in many human populations. The explanations for this trend are complex, but the adaptive perspective is often overlooked. Adipose tissue is an adaptation that stores energy and allows individuals to buffer shortfall. From an evolutionary perspective, we should expect plasticity in fatness, as individuals respond to the demands and parameters of their environments. I will review our recent work showing that European starlings (Sturnus vulgaris) rapidly gain weight in response to uncertain access to food. Crucially, our results show that this weight gain is driven not by an increase in calorie consumption, but rather by re-allocation of energy from other functions, notably somatic maintenance and repair. I will then show that in humans, uncertain access to food is also correlated with higher body mass (at least in women, and in Western populations). Again, this effect does not appear to be driven by increased calorie consumption. Having uncertain access to food has large long-term health costs, and I argue that is plausibly due to the re-allocation to fat of energy that would otherwise have been used for somatic maintenance and repair. Understanding the adaptive logic of energy allocation, including when energy is directed to fat instead of to other functions, appears central to understanding health inequalities.