In 1998, Roy Baumeister co-published a paper suggesting that self-control decisions drew on some limited resource. After resisting cookies and chocolates, he found, subjects had less self-control at a subsequent task. In Willpower, Baumeister and John Tierney convincingly describe another addendum: willpower depends on glucose as an energy source.
As a control, some subjects drank a "large, tasteless concoction of low-fat dairy glop." But the researchers found that both the ice cream shakes and the joyless glop reversed the effects of depletion. It wasn't pleasure that rejuvenated willpower, it was the calories, a discovery they confirmed by measuring glucose levels after self-control tasks, and, by comparing the effects of lemonade with sugar versus lemonade with Splenda.