Well, this stinks. Excerpt:

But bodies don’t gain or lose weight indefinitely. Eventually, a cascade of biological changes kicks in to help the body maintain a new weight. As the JAMA article explains, a person who eats an extra cookie a day will gain some weight, but over time, an increasing proportion of the cookie’s calories also goes to taking care of the extra body weight. Eventually, the body adjusts and stops gaining weight, even if the person continues to eat the cookie.
Similar factors come into play when we skip the extra cookie. We may lose a little weight at first, but soon the body adjusts to the new weight and requires fewer calories.
Regrettably, however, the body is more resistant to weight loss than weight gain. Hormones and brain chemicals that regulate your unconscious drive to eat and how your body responds to exercise can make it even more difficult to lose the weight. You may skip the cookie but unknowingly compensate by eating a bagel later on or an extra serving of pasta at dinner.

So it’s not simply a matter of doing small things to cut calorie intake and hoping they add up over time. Biology conspires to undermine that strategy. More:

“We need to know what we’re up against in terms of the basic biological challenges, and then design a campaign that will truly address the problem in its full magnitude,” Dr. Ludwig said. “If we just expect that inner-city child to exercise self-control and walk a little bit more, then I think we’re in for a big disappointment.”