post by Bill Gardner
The Lancet has a superb series on the global obesity epidemic, including an article summarizing the nonlinear dynamics of metabolism that govern weight loss. One key point is that there is a vicious feedback mechanism in weight gain:
increased daily energy intake will result in greater weight gain in the 100 kg man than in the 80 kg man and a greater fraction of the weight change will be body fat
That is, the more body fat you already carry, the easier it is for your body to convert extra calories into additional body fat. The reason is that a kg of fat burns less energy in a given day than does a kg of lean flesh, so as the proportion of lean body fat declines, your body burns energy less intensively.
The problem is that eating just a bit more than we need seems to give us a feeling of well-being. If you are lean, a small overshoot will produce a small weight gain. As you get fatter though, the small overshoot that signals satisfaction will lead to accelerating weight gains. If I've understood this right, this may explain why it seems to so hard to recover lean weight following large weight gains.
This is clearly a problem at any age, but it really freaks me out thinking about kids. Modest weight gains set them up for worse problems as adults.