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 the 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.

In “Calories and Corsets: A History of Dieting over 2,000 Years” (Profile Books, 2011), Louise Foxcroft surveys human beings’ relationship with weight and body shape. While there is a long and rich history of fascination with body shape and the relationship between diet and health, only in the past 100 years has it turned into an unwieldy obsession.

The book is a British publication, but Ms. Foxcroft draws extensively on American studies, trends and policies. She traces the history of popular diets, from Fletcherism in the Edwardian era to Atkins in the ’70s and then again in the early 2000s. “At the height of its popularity,” writes Ms. Foxcroft, “it was estimated that 1 in 11 North American adults was on the diet.”

Unfortunately, even diets that successfully promote weight loss seem to have little long-term success. Ms. Foxcroft cites a report by the American Psychological Association that examined 31 diet studies and “found that, after two years of dieting, up to two-thirds of dieters weighed more than they did before they began their regimen.”

The National Weight Control Registry tracks approximately 10,000 people who have lost weight and kept it off. Established in 1994 by Rena Wing when she was at Western Psychiatric Institute and Clinic (she’s now at Brown University’s medical school) and James O. Hill of the University of Colorado, it “is the largest prospective investigation of long-term successful weight-loss maintenance,” according to the registry website.

In a 2011 story in the New York Times Magazine, Kelly Brownell, director of the Rudd Center for Food Policy & Obesity at Yale University, pointed out that “while the 10,000 people tracked in the registry are a useful resource, they also represent a tiny percentage of the tens of millions of people who have tried unsuccessfully to lose weight.”

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, as Ms. Foxcroft points out, we knew that 2,000 years ago: Hippocrates, the ancient Greek physician, “understood … that high food intake meant that a lot of hard work was needed for it to be properly assimilated. A failure to balance an excess of either [food or exercise] would upset the body’s metabolism and disease would surely follow.”

In “Why Calories Count: From Science to Politics” (University of California Press, 2012, $30), Marion Nestle, a professor in the nutrition, food studies and public health department at New York University, and Malden Nesheim, professor emeritus and former director of the division of nutritional sciences at Cornell University, write, “Body weight is the net result of the balance between the calories you eat and those you use, store, put out, burn off or, more formally, ‘expend’ … in metabolism and physical activity.”

In other words, if you eat fewer calories than you expend, you will lose weight; eat more and you will gain 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 one pound of fat.

If weight were a purely aesthetic characteristic, there would be no justification for incessant scrutiny, much less government intervention. 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, and “[Extreme weight loss] is not realistic for many, many people. It is the exceptional person, and even that, the word exceptional, even seems to be blaming. It’s just the exception.”

Instead, many people take off significant amounts of weight only to regain them. 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. Ms. Nelson pointed out that many of the healthiest diets emphasize whole foods, which means that people have to cook from scratch.

Ms. 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.

Ms. Bacon is just one of a significant number of health professionals and advocates who are skeptical about the relationship between weight and disease, but even health experts who disagree emphasize the environmental factors involved in weight gain.

In “Why Calories Count,” Ms. Nestle and Mr. Nesheim call the United States an “eat-more” food environment, arguing that “Since the early 1980s the U.S. food environment has changed in ways that encourage eating in more places at more times of day in larger portions.”

While every snack might be a personal choice, people have simply not evolved to defend themselves against this kind of environment. “Food choices,” they write, “are only partially about personal responsibility. … More efforts should focus on ways to improve the food environment to make it easier for people to make healthier choices.”

Yet public health campaigns continue to focus on diet and exercise as key to weight loss despite so much evidence to the contrary. “Weight of the Nation’s” two slogans, repeated throughout the miniseries and emblazoned on the DVD package, place the blame squarely on weight and on the people who have too much of it: “To win, we have to lose,” and “the first step starts with you.”

Nutrition experts and activists generally had more praise for the third and fourth installment of the miniseries, which emphasized the serious problem of food marketing, particularly junk food ads targeted at children. Still, most of the airtime was devoted to local community and individual solutions, such as creating more bike and walking paths, playgrounds and physical education classes — developments that could improve the lives and health of all people, no matter their weight.

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