Extra calories cause weight gain--but how much?

HOW MUCH WEIGHT WOULD AN INDIVIDUAL GAIN BY eating an extra chocolate chip cookie every day for life? One approach to answering this question, frequently used in textbooks and scientific articles, is based on the assumption that a pound (454 g) of fat tissue has about 3500 kilocalories (kcal). Thus, a daily 60-kcal cookie would be expected to produce 0.2 kg (0.5 lb) weight gain in a month, 2.7 kg (6 lb) in a year, 27 kg (60 lb) in a decade, and many hundreds of pounds in a lifetime. This of course does not happen. In this article, the physiology of weight gain and loss is reviewed, and the amount of reduction of caloric intake necessary to avoid becoming overweight or obese is estimated.