well this is depressing

http://well.blogs.nytimes.com/2010/03/01/in-obesity-epidemic-whats-one-cookie/

so this article takes the position that good but small changes in diet and exercise (like those heralded by Michelle Obama and just about everyone else) don't equal long term weight loss because your body adapts:

A person’s weight remains stable as long as the number of calories consumed doesn’t exceed the amount of calories the body spends, both on exercise and to maintain basic body functions. As the balance between calories going in and calories going out changes, we gain or lose weight.

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.

well, hell.