The Great Weight Debate
Is your body programmed to rebel when you try to lose pounds?
If losing weight and keeping it off weren't such a struggle, there would be no diet books, no Weight Watchers, and maybe even no daytime TV talk shows. Clearly, it's very hard to slim down and stay that way. But why? Don't our bodies want us to be fit and trim?
Well, maybe not. Some scientists and physicians have a theory to explain the body's seeming obstinacy: Each of us has a personal "set point," the weight our bodies prefer. Any deviation from that weight, the theory goes, and the body will employ tricky tactics to get right back to the set point.
While the idea of a set point is accepted by many (but not all) scientists, how that point may be set by the body is not well understood. Most believe that genes play a significant role; "obesity genes" may well be the blueprints that shape, or at least try to shape, the set point.
And if that turns out to be true? Researchers say it's no reason to throw up your hands and ditch a sensible diet and regular exercise. Healthy living is still within your reach. But the set point would help explain why a good-faith effort to lose weight can be so challenging.
Set point or not, the factors that determine weight gain or loss are simple:
— Calories Out
= Change in Weight
Calories are a measure of energy. If your calorie intake (in food) and output (in activity) are equal, your weight change will be zero. If you consume more calories than you burn, you'll gain weight, and conversely, if you burn more than you consume, you'll lose weight (about 1 pound for every 3,500 calories).
Calculating "calories in" is straightforward: Find out how many calories are in the foods you eat, and add them up (don't forget beverages like soda and juice). Figuring your "calories out"—your total energy output—is a bit more complicated. It is a combination of the calories you burn moving around (through exercise and other physical activity) and your "resting energy output." That's the amount of energy your body uses just to keep you alive, and it actually makes up the bulk of your total energy output. The body may not be able to dictate how much you exercise (although even that is debated), but research shows that the body can tweak your weight by changing how much energy you expend at rest.
When people talk about "metabolism," they often are referring to resting energy output. An efficient metabolism, known colloquially as a "slow" metabolism, spends the calories in the food you eat wisely, and so there are more leftover calories to put in the bank—that is, in fat tissue. In other words, if you're trying to lose weight, an efficient metabolism is a hurdle to overcome.
Your Stubborn Body
For a person with a high set point, any attempt to shed pounds will be met with resistance. What's worse, if you gain pounds, it appears that the set point can be reset over time to a higher weight. So although some people aren't born to be obese, once they become obese, they may find it difficult to lose weight.
"When you reach a new set point from a long-term change in lifestyle, the body wants to stay there," says Tsu-shuen Tsao, PhD, an assistant professor of biochemistry and molecular biophysics at the Albert Einstein College of Medicine in New York, who studies energy expenditure. "Set-point theory is most relevant for people trying to lose weight. For each pound you are trying to lose, it gets harder and harder; that is the essence."
A groundbreaking 1995 study examined metabolic efficiency in people who gained or lost weight. Participants agreed to either gain or lose 10 percent of their body weight. Scientists first measured each person's resting energy output using sealed "metabolic chambers" that allowed them to monitor tiny changes in the air's temperature and mixture of gases to determine how much energy was burned. After the participants gained or lost weight, the researchers measured their resting energy output again and found that the levels had changed, as the subjects' bodies apparently sought to return to their set points.
Those who gained weight now had less efficient metabolisms that burned energy more easily, promoting weight loss. The opposite happened with people who lost weight: They became more efficient metabolically, burning less energy while at rest as their bodies tried to get back to their higher normal weights. "Here is the kicker," says Tsao. "When they returned [to their normal weight], their bodies weren't quite [the same]. Their bodies were a little more efficient, which is a bad thing where weight loss is concerned. It's like your body has the memory of having undergone a period of starvation and says, 'Just to be on the safe side, let's be a little more efficient from now on.' "
Scientists think the body might have other ways to try to claw its way back to its set weight. For example, one study showed that when people went on a diet, the amount of circulating ghrelin—a hormone released by stomach cells that acts on the brain to make people hungry—went up. This might be one way that the set point manipulates the calories-in side of the energy equation.
Some scientists question the very existence of a set point; others believe that a set point can be reset to a lower weight. There is growing evidence suggesting that differences in physical activity, not in resting energy output, determine whether people regain weight. Opponents of set-point theory argue that, while resting energy output may slow down during active weight loss, it gets back to normal during weight maintenance. One 2002 study found that 77 percent of the weight regained by women who had initially lost pounds could be explained by their having a lower level of physical activity than those who had not regained weight, not by differences in resting energy output.
Other studies also suggest that physical activity may be the key to weight maintenance, not shifts in metabolic rate. There are two types of physical activity: exercise—like running, swimming, and lifting weights—and non-exercise activity, which includes pacing, fidgeting, and standing instead of sitting. A person's tendency to engage in non-exercise activity is largely subconscious and is believed to relate to brain chemistry. A 1999 study found that some people increase non-exercise activity when overfed while others just pack on extra pounds. A 2005 study by the same group found that obese people sit two hours longer a day than lean people, who burn an extra 350 calories just standing around those two hours. "People thought they had magic metabolism, but they are the twitchers and movers," says Cheryl Rock, PhD, RD, a professor of family and preventive medicine at the University of California–San Diego. She argues that there is surprisingly little hard scientific data to support set-point theory.
Even so, scientists are seeking to reset set points, in hopes of stemming the obesity epidemic. Brown fat, a type of tissue that burns calories instead of storing them and is used by infants to generate heat, was recently discovered in adults. If a set point exists, brown fat would definitely affect it, says Tsao. The more brown fat a person has, the less efficient his or her metabolism. Researchers found more brown fat in thin people than in overweight people, but the only way to activate it was to put people in a chilly environment—brown fat generates heat as it burns through calories. Next, scientists will be exploring how to tap into brown fat's calorie-zapping potential without the low temperatures.
Weight Isn't Everything
At the very least, set-point theory is far from "set," and a set-point view of body weight can be demoralizing to someone trying to get fit. "If someone throws up their hands and says they have a set point that keeps them at 250 [pounds], that's misusing science," says Rock. "Maybe they won't ever be 100 pounds, but they can still be active and eat better."
Tsao agrees that set-point theory should not discourage healthy living. "If you try to eat healthy and exercise regularly, then if you are a few pounds heavier, it's OK," says Tsao. "Body weight is not everything. If you eat healthy and exercise, you can maintain low cholesterol, low blood pressure, and blood glucose levels that are fantastic. People aren't killed by extra weight; they are killed by high blood pressure and high cholesterol."
While an intriguing theory, set point is neither an inescapable fate nor an excuse for abandoning diet and exercise regimens. Yet if you are doing all the right things for your health but still can't quite reach that weight goal, the set point is a useful reason not to beat yourself up. Someday, perhaps, scientists will learn how to reset the set point to a weight of your choice. Just don't give up healthy living in the meantime.