Healthy people who have endured considerable weight loss over a number of months as a result of food deprivation can put away astonishing amounts of food. After volunteers in a famous laboratory hunger study carried out by Anselm Keys returned to eating freely, they began to gorge themselves on 10,000 calories a day. Yet, no matter how hungry at the outset, people do not ordinariy continue to stuff themselves so resolutely that they swell to the gargantuan proportions of circus exhibitionists. We have an almost irresisible urge to eat, but we also have at least some built-in controls that reduce our appetite for food and that limit the accumulation of excess fat. In one study, prisoners volunteered to stuff themselves until they had increased their body weight by 20 percent. Upon achieving this goal, they were allowed to eat as much or as little as they wished. Most of them immediately began to consume only a few hundred calories a day until they had gotten back to their original weights. Another indication that our bodies must be equippped with some kind of "foodstat" (like a thermostat) is that the average person gains relatively little weight over a life span. Between ages eighteen and thiry-eight, Americans usually put on no more than ten or twenty pounds while eating their way through twenty tons of food. Nutritionists estimate that to keep the weight gain to such a small percentage of food consumed means that the foodstat operates with a tolerance of less than 1 percent. Impressive as this may sound, the human foodstat cannot be relied on to keep people from eating too much. Putting on ten to twenty extra pounds by age thirty-eight often means being ten to twenty pounds overweight at age thirty-eight. According to the National Center for Health Statistics, 24.2 percent of adult men and 27.1 percent of adult women weigh 20 percent more than is good for them. What is really remarkable about the modern-day incidence of obesity is that it persists--despite fashions and aesthetic standards that demean fat persons, despite an intense educational effort conducted by public health authorities linking obesity with cardiovascular diseases, and despite billion-dollar industries devoted to fitness, diet foods, and weight control. With half the adult population of Western nations on one diet or another, I think one has to conclude that the foodstat doesn't work very well under the present circumstances. And the reason for this seems clear enough: For most of the time that hominids have been on earth, it wasn's foodstats that kept them from becomimg overweight. It was lack of food.