For example, metabolism plays a significant role in determining our weight. Resting metabolic rate refers to the amount of energy the body burns when not engaged in physical activity; it accounts for approximately 70 percent of the calories we burn each day. About 40 to 80 percent of the influence for resting metabolism is apparently inherited. In the journal Nature Medicine, Jeffrey Friedman, director of the Starr Center for Human Genetics, writes, "The commonly held belief that obese individuals can ameliorate their condition by simply deciding to eat less and exercise more is at odds with compelling scientific evidence indicating that the propensity to obesity is, to a significant extent, genetically determined."
This inherited weight range, known as the set point, is the weight your body settles at when you're eating in response to signals of hunger or fullness and engaging in some level of physical activity. Our set point acts like a thermostat, seeking to maintain our natural body weight within a range of 10 to 20 pounds. When we take in less food as fuel, our body deals with this reduction by slowing down to conserve energy. Metabolism is lowered, reducing the rate at which calories are burned. Within 24 to 48 hours of beginning a calorie-restricted diet, metabolic rate decreases 15 to 30 percent. Our body has successfully slowed itself down to defend against this self-imposed famine. By contrast, when our body takes in more food than it needs as fuel, the metabolism speeds up and burns calories more quickly. In her book Health at Every Size: The Surprising Truth about Your Weight, Linda Bacon, a physiologist specializing in nutrition and weight regulation, explains that when this mechanism is working properly, it functions as a force that pulls you back to your comfortable range whenever you veer away; however, if you consistently override your body's signals of fullness, this system becomes broken. The goal is to find your healthy weight, keeping in mind that even if we all ate the same and exercised the same, we wouldn't weigh the same. Weight is a complicated matter, which can be affected by a variety of factors, including medical issues, such as thyroid problems or polycystic ovary syndrome, the side effects of medications, poverty, stress, and lack of sleep.
Beyond the psychology of dieting and our largely inherited physiology, we're still driven by the evolutionary pressures that drove our ancient hominid ancestors—hunters and gatherers, who had to make the most of every bite to survive. Sometimes their food was plentiful, but during times of scarcity, their bodies adapted by lowering their metabolism to conserve every calorie consumed. Following a period of scarcity, their bodies became even more efficient at storing fat in preparation for the next famine. These fat-layered bodies, better able to adapt to scarcity, were likelier to reproduce. As a species, therefore, we've inherited a predisposition to hold onto fat after each period of scarcity. Today, our bodies can't distinguish between hunger caused by famine and hunger caused by a self-imposed diet—and they react to the latter as if it were the former. The "failure" of diets is actually a "success" in terms of species survival!
When dieting for weight loss, our bodies respond to the perceived famine by feeding off fat and muscle. Muscle is the metabolically active part of our body: the more muscle we have, the more calories we can burn. Since every weight-loss attempt includes the loss of both fat and muscle (but what's regained is only fat), dieters burn even fewer calories, which makes it easier to gain weight and results in a higher fat-to-muscle ratio. Repeated dieting attempts may significantly increase the percentage of body fat over time. In fact, in 2007, Traci Mann and her colleagues at UCLA conducted a comprehensive and rigorous metanalysis of 31 long-term studies of obesity treatment for Medicare patients. They found that despite losing 5 to 10 percent of their starting weight in the first six months, the vast majority of dieters had regained all the weight—and within four or five years, one-third to two-thirds of subjects had regained more weight than they'd lost.
In 1993, after it was discovered that less than one percent of dieters could maintain their weight loss for five years (the criterion for success), the Federal Trade Commission charged 17 companies, including Weight Watchers, Jenny Craig, and Nutrisystem, with making false and deceptive claims about the safety and efficacy of their programs. While many programs claim that they aren't diets, whenever food is manipulated for the purposes of weight loss, it is, in fact, a diet, and not one of them has produced research demonstrating long-term results.