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The Medical Evidence
Here's the medical evidence. According to a study in The New England Journal of Medicine, people who jotted down what they ate realized that they had been underestimating their daily intake by about 1,050 calories. No, that is not a typo. Added to your daily menu, 1,050 calories will put on an extra pound every three and a half days.

Researchers at the Center for Behavioral Medicine in Chicago found that when overweight people were asked to keep food diaries for 10 weeks, the most diligent recorders lost more weight than the lax ones. The study, published in Health Psychology, showed that subjects gained about 500 percent more weight per week than usual over the Thanksgiving, Christmas and New Year's holidays—with the exception of those who were consistent food diarists. As this group entered the dangerous season to be jolly, they began keeping notes even more faithfully than before. Instead of gaining weight, they actually lost, on average, about 10 pounds more than the others.

Even if you have a hard time keeping track of details and can't imagine recording every bite for the rest of your eating life, doing so for a short time could give you a jump-start on weight loss. A study in Obesity Research showed that regardless of how carefully subjects kept their food diaries, they dropped much more weight during their two most consistent weeks of note-taking than their least.