Why is losing weight – and keeping weight off – such a puzzle? Granted, obesity is a complex and multifactorial problem at the population level. But at an individual level, the root cause seems to be quite simple: it is chronic positive energy balance. Simple, perhaps, but not easy. We now know that alterations in energy intake or energy expenditure can activate ancient feedback mechanisms that have evolved to preserve energy balance. And unfortunately for us, this system doesn’t seem to be biased toward supporting our beach body goals. Instead, it is adapted for survival. This is one reason why, for example, exercise interventions usually do not result in as much weight loss as would be predicted based on the projected energy expenditure produced by the activity. Changes in hormones like leptin, elicited by the energy deficit, cause hunger to ramp up. Consequently, studies have shown that many people who participate in structured exercise interventions wind up eating more foo