Fat activist tells teen girl she will gain back every pound she lost through diet and exercise.

The majority of people who "diet" to lose weight probably do gain most of it back, if not even more.

The reason for this is that most people think of a "diet" as a temporary restriction. They go on a "diet", lose weight, and then go off the "diet". This approach is doomed to failure. The first thing to realize about the word "diet" is that it describes what you eat, whether you're trying to lose weight or not. For any given diet and level of physical activity, there is a equilibrium point. If you are at a stable weight and activity level and alter your diet to have fewer calories, you will lose weight until your body becomes more efficient (largely as the result of not hauling as much blubber around), and you reach a new, lower equilibrium point. If you go back to your old diet, you'll regain every pound you lost.

Assuming no major changes in muscle density, a thinner person requires fewer calories to maintain the same weight. This isn't "damaging your metabolism". It's a simply consequence of doing less work hauling around dead-weight. As people become thinner and fitter, their bodies become more efficient at moving. A 150 kg porker does a lot more work to haul himself down the sidewalk than a 75 kg marathon runner.

The consequence of all this is that, if you want to remain at a lower weight, there must be permanent changes to your diet and physical activity level. You might be able to accelerate weight loss temporarily with temporary changes to diet and activity, but if there are no long-term changes your weight will return to where it began.

The girl asking this question describes how she changed her diet drastically and increased her activity level. I don't know if she's at a stable equilibrium point yet, but it's safe to say that she's not going to go back to her old weight unless she quits exercising and goes back to her old diet. It's true that healthy people can have some fat on them, but it's almost impossible to be healthy at a weight where you're getting teased. Quite frankly, the average weight of teenagers (especially in the U.S.) is currently on the porky side.

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