Set-point theory is pretty much settled medical fact. The mechanism involves leptin, and you can easily see processes that defend bodyweight change in both dire

Set-point theory is pretty much settled medical fact. The mechanism involves leptin, and you can easily see processes that defend bodyweight change in both directions. Though it will more aggressively defend weight loss than weight gain. In addition prey animals will defend against weight gain more aggressively than non-prey animals. Which makes sense from an evolutionary standpoint. If a lion gets fat he doesn't have nearly as much to worry about than if a gazelle gets fat.