Is weight loss simply the difference between "calories in" and "calories out"?
In theory, managing your weight is simplicity itself. If you take in more energy ( in the form of food than you burn (through physical activity and metabolic processes), those extra calories are stored as excess weight. If you burn more than you eat, you lose weight. Calories in minus calories out. From a physicist’s perspective, this is inarguably true, since nature forbids you to either create or destroy energy. In practice, though, it’s a little more complicated – because the “calories out” part of the equation doesn’t behave as we expect.
Consider what would happen if you added one 60-calorie chocolate chip cookie to your daily diet. Since a pound of fat contains 3,500 calories, simple math suggests that you’d pack on about half a pound a month, or six pounds a year, or the rest of your life. But, as a 2010 paper in the Journal of the American Medical Association explains, this isn’t what happens. As you begin to gain weight, your body has to spend metabolic energy repairing, replacing, and supporting the cells in this new tissue. You start burning more calories without any change in physical activity. As a result, the weight gain slows down and eventually levels off after a few years at a total of six pounds – even if you keep eating that extra cookie for the rest of your life.
Unfortunately, the opposite happens if you start eating 60 fewer calories per day. Initially, you’ll lose weight. But now the body doesn’t have to expend any energy maintaining the lost tissue, so you burn fewer calories, and the weight loss levels off. If you now revert to your normal diet – which are what people usually do once they achieve their weight-loss goals – you’ll simply regain the weight.
It’s not just the weight you gain or lose that conspires to keep your body weight constant. For more than decade, researchers at Columbia University have been performing painstaking experiments in which volunteers check into a controlled hospital setting for months at a time. They’re fed only a liquid diet (40 percent of the calories come from corn oil, 45 percent from glucose, and 15 percent from the protein casein) so that their daily caloric needs can be computed exactly. With both obese and normal-weight subjects, the researchers control food to reduce or increase body weight by 10 percent and observe the metabolic consequences.
The most recent study, published in 2010 in the American Journal of Physiology, tested how the subjects responded to physical activity. When they lost weight, their muscles became about 15 percent more efficient – not simply because they were carrying less weight around, but because of changes in the ration of enzymes responsible for burning fat or carbohydrate as fuel. While greater efficiency sounds like a good thing, it means they were burning fewer calories, making it harder to maintain the low weight. In contracts, their muscles became 25 percent less efficient when they gained weight, once again pushing them back toward their starting weight. These findings provide a sober reminder of how hard it is to slim down once you’ve gained weight. But there are also some practical lessons we can draw from the Columbia research. The biggest change in muscular efficiency was observed at the lowest levels of exercise intensity, equivalent to the activities of day-to-day life as opposed to exercise. As a result, the researchers suggest, “The weight-reduced individual might ‘escape’ this increased efficiently by altering the intensity of exercise.” Exercising harder rather than longer might be the most effective strategy for keeping weight off.