Why is weight loss often temporary?

The United States is well into its fourth decade of the “obesity epidemic,” and no matter how loudly we repeat the refrain “eat less and exercise more,” the numbers on our collective scale keep creeping upward.

Is weight gain caused by individuals' poor diet and lack of exercise? Or is it an unavoidable effect of an abundant food supply, out-of-control marketing and unlucky genetics? And if most of the evidence points to the latter, why do government agencies continue to use tax dollars to promote solutions that have no hope of working?

In May, HBO aired “Weight of the Nation,” a miniseries produced with the Institute of Medicine, the Centers for Disease Control and Prevention, and the National Institutes of Health. The four-part series made an impassioned case for the dangers of overweight and obesity, and the need to act now before it is too late for a generation of Americans.

The campaign was intended to start a national discussion about weight and health, but responses have been mixed, as many nutritionists and public health activists have called the miniseries out for its “fat-shaming” rhetoric and emphasis on individual responsibility.

Currently, two-thirds of Americans 20 and over are overweight — approximately 150 million people — and a third are obese. The United States weight-loss market was worth more than $60 billion in 2010, showing that Americans have spent a lot of money trying to slim down.

Although it seems we know little about how to successfully diet, the medical and scientific community tend to agree on the basics: Take in fewer calories than you expend, and you will lose weight.

But what scientists are just beginning to understand is how people's bodies change as they gain or lose weight, altering metabolism and even feelings of hunger, and making the overall equation much more complicated than 3,500 calories equals 1 pound of fat.

If weight were a purely aesthetic characteristic, there would be no justification for incessant scrutiny, much less government intervention.

But higher percentages of body fat are correlated with higher risks of health problems, from Type 2 diabetes and high blood pressure to breathing problems and certain cancers. As a result, body mass index charts have become ubiquitous in doctors' offices and on health websites. However, the index does not measure body fat percentage. It simply estimates it. Putting so much emphasis on the risks of being overweight can obscure the fact that thin people are often at risk for these diseases as well.

Linda Bacon, nutrition professor in the biology department at City College of San Francisco and an associate nutritionist at the University of California, Davis, points to other factors that have a strong correlation with health: relative wealth, social connectedness and activity level.

“Weight,” she said, “doesn't play as large a role (in health) as we thought, nor is it controllable as we thought. Good health is much more achievable and sustainable through improved health habits.”

Her campaign, “Health at Every Size,” emphasizes the idea that there is natural diversity in body shapes and sizes, that food should be a source of pleasure, not anxiety, and that people should focus on improving habits such as eating and exercising for the sake of their health, not body weight.

Jennifer K. Nelson, lead dietitian at the Mayo Clinic in Minnesota, takes a different tack. She regularly evaluates diets for the Mayo website and says a wide variety will work, if strictly followed, because they all restrict calories. The best ones also will emphasize increased activity, but what really matters is whether the diet is sustainable.

Realistically, she said, very few people can stick to extreme diets for any length of time.

Instead, many people take off significant amounts of weight only to regain it. A number of studies suggest that this type of weight cycling may actually cause serious health problems, including cardiovascular disease and diabetes.

There is greater consensus on the positive benefits of healthful behaviors, such as cooking at home and exercising. Nelson pointed out that many of the healthiest diets emphasize whole foods, which means that people have to cook from scratch.

Bacon, meanwhile, encourages people to enjoy whatever they eat, try to relearn what it feels like to be full and to pay more attention to their bodies' individual cues. She's also a huge proponent of exercise, but not as a weight-loss tool.

“Exercise is one of the most potent things we can be doing to improve our health,” she said. Most people who lose weight by exercising also eventually regain it, she said, but studies have shown that exercise alone has a huge positive effect on health.

“Weight,” Bacon said, “is not the problem we should be attacking. We're never going to succeed if it's about trying to wipe out a demographic.”