Almost half of Americans are on a diet -- not surprising, since two-thirds are overweight or obese, a frightening statistic that inspired Mayor Michael R. Bloomberg to push through a ban on large soft drinks in New York City. The country is preoccupied with calories. McDonald's, for instance, is now posting them. But our widespread hope for weight loss makes us vulnerable to all kinds of promises, even ones that aren't true, when it comes to food.

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