Science of Losing Weight

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23 Mar 2024

Losing weight leads to better health? Not necessarily

There is no evidence to support the idea that losing weight alone lowers the risk of heart disease and cancer, but other lifestyle changes can.

If the number on your scale qualifies you as overweight or obese, you should shed those extra pounds to become healthier—right? It’s a belief many hold, but one science doesn’t support.

The issue is not merely academic. Dissatisfaction with one’s weight is so widespread experts consider it a global mental health concern. A review published in February found 30 percent of young girls worldwide admit to having unhealthy beliefs and eating habits, including purging after a meal. And on any given day in the United States, 17 percent of adults are on a diet, according to 2020 figures from the Centers for Disease Control and Prevention—up from 14 percent a decade earlier. More importantly, recent reports suggest behavioral changes like increasing exercise yield more health benefits than losing weight.

“There is no strong evidence to suggest that being higher weight automatically leads to poorer health or that losing it leads to health improvements,” says Jeffrey Hunger, an assistant professor of psychology at Miami University of Ohio, who in recent years reviewed the evidence and concluded public health policy needs to step away from the scale. “Society has this false assumption that higher body weight equals poorer health,” he says, noting this is the dominant message coming from government, medical organizations, and the media.

Katherine Flegal, a longtime nutrition scientist at the CDC who is now retired, says that health benefits are largely lacking in the consensus statement on weight loss by the U.S. Preventive Services Task Force, an independent, authoritative body of scientists. The group recommends intensive behavioral interventions as a weight-loss tool for adults with a body mass index of 30 or above, the current cutoff for obesity. But its summary of research states there is “no evidence” that cardiovascular disease, cancer, or death are reduced by shedding pounds.

This is not for lack of trying. One of the largest studies investigating the link between weight loss and a lower risk for heart attack, stroke, and other cardiovascular conditions came up empty. The decade-long, 16-center clinical trial in the early 2000s was sponsored by the National Institutes of Health and included more than 5,000 high-weight people with type 2 diabetes. Those randomized to the intensive lifestyle intervention (reducing calories and increasing physical activity) did lose a bit more weight—6 percent of body fat compared with 3.5 percent for the control group by the study’s end. But the loss did not result in fewer cardiovascular events.

On the other hand, two studies published last week highlight behaviors with solid evidence connecting them to improved health and longevity. One linked greater adherence to a healthful, plant-based diet with lower rates of cancer, cardiovascular disease, and death. The other found people taking 8,000 or more steps even a few days each week had lower 10-year mortality compared to non-walkers.

But the message that weight loss is not the best path to health is one many don’t want to hear, Flegal says, noting the belief that high weight is “dangerous and terrible” is baked into contemporary Western culture. Flegal herself was pilloried nearly two decades ago after a paper she coauthored at the CDC associated obesity—including extreme obesity—with fewer than 5 percent of excess deaths, far lower than prior estimates. Her research also associated being overweight (but not obese) with slightly fewer deaths than in normal-weight individuals.

Flegal and her colleagues were attacked for their findings, with one article in the New York Times citing critics who described the research as “deeply flawed,” even though the CDC stood by the figures.

BMI is not a reliable indicator of health

Even what weight should be considered “normal” demands further scrutiny, some experts say. Currently, people with a body mass index between 18 and 25 are described as normal; those between 25 and 30 are classified as overweight; and people with a BMI of 30 and above have obesity. Lines like these were drawn somewhat arbitrarily beginning in the 1980s, Flegal says, including one effort at the National Institutes of Health that was based on the then-average weight of people in their twenties. This was rooted in the assumption that young adults represent the ideal, she says.

It's true that the average weight for Americans has risen over the years. In 2000, 31 percent of American adults had obesity, according to the NIH. Today in the U.S., some 42 percent are labeled as having obesity, which the medical system categorizes as a disease, with fewer than 20 percent of adults currently falling into the normal-weight category. The reasons for the rise are not well understood and likely include genetic, socioeconomic, and environmental factors as well as behaviors.

Flegal is troubled that a person sitting at home can cross an arbitrary line and “suddenly they have this disease,” whereas a diagnosis for other medical disorders requires a careful physician evaluation and, often, blood tests and imaging.

Using weight as the key indicator of a person’s health does a disservice to those across all BMIs, says Miami University’s Hunger. When he and his colleagues evaluated data from more than 40,000 participants of the U.S. government’s annual nutrition survey, they found nearly half the people in the overweight group and 29 percent of those with obesity have many of the markers of good health, from blood pressure to cholesterol. What’s more, 30 percent of the “normal” weight people did not.

Higher weight individuals without health problems—known in medicine as metabolically healthy obesity—are often more physically active and have good cardiorespiratory fitness, scientists wrote in a review in BMJ Open Sport & Exercise Medicine last year. “It is high time to advocate this lifestyle change beyond its influence on energy balance,” the researchers concluded.

Why maintaining significant weight loss is difficult

People with very large bodies may experience some health benefits by losing weight. Patty Nece, a person with severe obesity who chairs the drug-industry-supported, patient-advocacy group Obesity Action Coalition, says the osteoarthritis in her knees worsens when her weight goes up. Research has shown sleep apnea may also improve with weight loss. And complications after hip-replacement surgery are higher for those with a BMI over 40.

But diet studies following people for a long period of time have not linked substantial, sustained weight loss to meaningful improvements in blood pressure, cholesterol, triglycerides, and other markers of health, researchers wrote in Social and Personality Psychology Compass.

These adaptations benefit a person who is actually starving but work against someone hoping to slim down before their college reunion. “Despite eating so few calories, you start gaining the weight back, and others blame you for lacking willpower,” Mann says. This happened to Nece, who recalls once eating as few as 900 calories a day and exercising regularly at the gym, yet “my weight continued to go up.”

Glucose is one blood marker some studies have associated with weight loss. But even here, Hunger says, the facts are murky. Studies such as the widely publicized Diabetes Prevention Program, begun in the late 1990s, instructed participants with prediabetes to lose 7 percent of their bodyweight through calorie restrictions and to exercise 150 minutes a week. After 10 years, compared to the control group, those interventions delayed the development of diabetes by 34 percent. This led the American Diabetes Association to recommend them, including 7 percent weight loss, in people with prediabetes.

But Mann and others believe it’s less likely that the improvement came from shedding such few pounds—12 for a 170-pound woman—than from the increased physical activity. “Exercise is an underappreciated component of that study,” she says.

Last week, after reviewing the data on diabetes, researchers at Northwestern University urged healthcare professionals to move beyond weight as a factor and instead start screening all adults ages 35 to 70 for the disease. This contrasts with current recommendations that limit testing to people in that age range who fall in the overweight or obese category.

Instead of focusing on weight-loss as a path to improved health, everyone would do better to ignore the scale and put their attentions on behaviors instead, says A. Janet Tomiyama, professor of psychology at the University of California, Los Angeles, and a coauthor of the Social and Personality Psychology Compass study.

“Moving the body more, getting a good night’s sleep, adding one more fruit or vegetable to your meal, and getting a handle on your stress—these are things that make anyone healthier, no matter their weight,” Tomiyama says, even if they don’t result in the loss of a single pound.

They’re called healthy behaviors for a reason: “They make you healthier,” Mann asserts.
“Exercise doesn’t make you healthier because it makes you thinner. The health benefits show up even if you don’t lose any weight.”