Willpower, I read in a January 2012 New Scientist article by psychologist Roy Baumeister ("Weak will comes from tired mental muscles"), is more like a muscle than an ethical attribute.
So rather than seeing willpower as a moral quality, the scientific view is that it is like a muscle that tires. After you exert self-control, you have less willpower so you are less able to resist a new demand. Self-control is only temporarily weakened and can recharge after a rest. Willpower resembles a muscle also in that it can be strengthened by exercise.
Thus just as muscles need rejuvenation after being used, so does willpower. Researchers have found that after exercising one's willpower, there isn't as much power to resist temptation next time around.
Even a few minutes of exerting self-control is enough to cause a decline in performance on a subsequent, seemingly unrelated test. That might suggest human willpower is scarce, but, again, no: willpower is like a muscle, and when a muscle gets tired, an athlete may cut back effort to conserve what remains. In fact, willpower looks as if it is indeed a kind of energy, tied to levels of the chemical glucose used to carry energy from the digestive system and fat stores to muscles and other organs. Neurotransmitters, that enable brain cells to fire, are made of glucose.
When it comes to using willpower in a weight loss effort, it turns out that eating is the key to maintaining resolve.
This glucose research also suggests why dieting is so fiendishly difficult. In order to resist tempting foods, we need willpower but to have willpower, we must eat. The essence of dieting (restricting food intake) robs us of the psychological strength needed to succeed. Perhaps dieters should concentrate on filling up with healthy food so they have the willpower to resist fattening stuff.
My wife used to be a psychotherapist in private practice. She had quite a few clients with eating disorders. When I told her about this article, it wasn't new news to her. Laurel said that she recommended eating frequent small meals to those clients, which keeps the glucose level (and hence willpower) up.
I've lost about ten pounds this year using a somewhat similar approach. I eat when I'm hungry, but only when I'm hungry (mostly; I succumb to cravings sometimes). Losing weight and feeling better as a result encourages me to keep up my low-key diet.