A new book tackles the questions of why it's so hard to lose weight — and why it's even harder to keep the pounds off.

In Rethinking Thin, Gina Kolata, a science writer for The New York Times, examines trends in America's diet industry and some of the most basic assumptions about health, dieting and body weight.

Kolata says that scientists have a much better understanding of the factors that lead to obesity than of what it takes to lose weight permanently.

One factor is that different people have different biological "set points" and their bodies' metabolism will speed up or slow down to keep their weight in that range, Kolata explains to Michele Norris.

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