One night a few years ago, Emma Lewis sprained her ankle. She had spent the evening dancing, then walked several miles home wearing what she calls “slightly inadvisable shoes.”

The next morning, Lewis went to see her doctor. But her damaged ankle was not his main concern. Instead, he told Lewis that she ought to be getting more exercise and losing weight.

Like many Americans who are overweight or obese, Lewis dreads seeking medical help. Doctors inevitably focus on her weight rather than her symptoms. If her doctor had asked, Lewis would have told him that she already gets plenty of exercise—she’s a weight lifter who can squat 220 pounds. She also follows a whole-grain, vegetarian diet. Her blood pressure and blood sugar are normal. But the doctor had assumed that because she was obese, she was automatically unhealthy.

It’s time for doctors to stop using weight to judge the health of the people under their care. To do so is both lazy and ineffective. A recent analysis published in the International Journal of Obesity found that health risks for about 75 million Americans are misclassified by their weight category. That study measured metabolic health by looking at six indicators: blood pressure, blood sugar, insulin resistance, cholesterol, triglycerides, and C-reactive protein (a sign of cellular stress). By all those criteria, researchers found that 29% of obese people and 47% of overweight people are healthy. Meanwhile, over 30% of normal-weight people are at risk for diabetes and heart disease.

Yet many doctors continue to prescribe weight loss to all their overweight and obese patients. In doing so, they promote an intervention with a high rate of failure. And they sideline recommendations that are proven to improve the health of their patients in the long term.

Despite the popularity of dieting, most weight loss is temporary. Long-term studies show that most dieters reach their lowest weight six months to a year after starting a diet. Then they gradually regain all the lost weight, and often add more.

Weight loss is difficult to maintain because the brain uses a variety of powerful tools to return the body to its normal state. Our brains can reduce the energy that muscles burn while increasing feelings of hunger and reminding us how rewarding it feels to eat. These responses can last for at least six years, which is the longest they have been measured, and probably forever. For this reason, weight-loss maintenance is difficult even for people who have been trying to stick to a lower weight for years.

Making any significant change in our day-to-day lives is hard. But it is harder still to change our habits while also fighting the brain’s weight-regulation system. Although willpower is effective at limiting food intake over days to months, it works poorly over years to decades. That’s because applying willpower requires deliberate effort, which can’t be sustained all the time.

The brain system that promotes regaining weight, by contrast, is always on duty. For the most part, it influences our behavior unconsciously, encouraging us to load up on snacks or get a second serving at the buffet if our weight has fallen below its set point.

A more effective strategy would be for doctors to focus on habits that are easier to improve, such as exercise. No matter how much people weigh, more physical activity predicts better health. According to research published in the Journal of the American Medical Association (JAMA) in 1999, people who exercise, even moderately, are 200–250% less likely to die prematurely than people who rarely get up from the couch, whether they fit into the normal-weight, overweight, or obese categories.

Doctors need to recognize that it is possible to carry more pounds and still have good cardiovascular fitness. People who are obese and fit have only 21% more health risks than fit people who are at an average weight, a difference small enough that it might be due to chance. These data indicate that exercise habits are much more important than weight in determining the risk of early death. The JAMA research also suggests that exercise has a stronger effect on health for obese people than it does for normal-weight people. That makes it particularly important for doctors to encourage their obese patients to exercise.

It is true that obesity is correlated with low fitness across populations, but not at an individual level. Among middle-class professionals surveyed in 1999, for example, 9% of normal-weight people had low levels of cardiovascular fitness as measured by a treadmill test. Over 19% percent of overweight people were not fit, according to the test, and 51% percent of obese people posted low levels of fitness. Such results may be leading doctors to mistake a correlated factor (obesity) for the causal factor (fitness) that explains most of mortality risk.

Because exercise reduces the amount of glucose in the blood, physical activity also makes a big difference in preventing and treating diabetes. One study, published in The Lancet in 2008, examined how diet, exercise, or diet plus exercise affected 577 people at increased risk for developing diabetes in China.

Over the 20-year follow-up, exercise had the strongest effect, reducing the onset of diabetes by 43% compared to people who had no intervention. The diet-plus-exercise group showed the same improvement as those who just exercised.

Meanwhile, a 2006 review of existing research by the Cochrane Collaboration, recognized experts in evaluating medical evidence, found “that exercise improves blood sugar control and that this effect is evident even without weight loss” in people with diabetes. A similar Cochrane review concludes more broadly that “exercise improves health even if no weight is lost.”

In sum, weight is not a very good predictor of health. Doctors’ continued confusion on this point has serious consequences. If people believe that weight is the best indicator of their health, they are more likely to become discouraged when they stop making progress toward a target weight and give up their new, healthier habits. Meanwhile, people like Emma Lewis, who rightfully experience anxiety about their doctors’ weight biases, are more likely to avoid seeking medical help when they need it.

If doctors want to do right by their patients, they should stop chasing the mirage of universal thinness. Instead, they should focus on helping patients create healthy, sustainable lifestyles that involve plenty of exercise, healthy food choices and minimal stress—and on learning to view their patients’ health as more than just a number.