Is your body mass index a useful measurement? Kevin Hall, creator of the NIH Body Weight Planner, dispels some myths about shedding pounds. (The Washington Post)

Obesity has been on the rise worldwide for decades, putting people at risk for developing diabetes, heart disease and several forms of cancer. Today, two thirds of adults and one third of children in the United States are overweight. As the numbers on the scale get bigger, so does the diet and weight-loss industry, which is now worth tens of billions of dollars. What doesn’t seem to be increasing, though, is people’s understanding of obesity and how to lose weight. Here are a few of the myths clouding the facts.

1. Body mass index is useless.

The body mass index, or BMI, is a simple and widely used method for classifying whether a person is overweight or obese. It’s calculated by dividing a person’s body weight by height squared, which helps account for the fact that taller people weigh disproportionately more than shorter people if they have the same percentage of body fat.

BMI is often criticized because it doesn’t differentiate muscle from fat. Muscular people can therefore erroneously be classified as overweight. For example, Dallas Cowboys quarterback Tony Romo’s BMI makes him borderline obese.

(Armand Sciberras/Getty Images)

But despite its limitations and notorious counter-examples, BMI is highly related to body fat and correctly categorizes people as having excess body fat more than 80 percent of the time. Additional simple measurements such as waist circumference may be even more informative because they provide information about where fat is distributed in the body.

2. All people with obesity are unhealthy.

The idea that people with obesity can’t be healthy has been repeated widely. CNN told readers that there’s “no such thing” as healthy obesity. Forbes called it “a myth.” “You can’t be fit and fat,” Time reported.

But in reality, fat’s location in the body may be more important for health than the total amount of fat. People who are “pear-shaped” tend to store fat in their buttocks and flanks and are at less risk of disease than those who are “apple-shaped” and tend to accumulate fat around the belly. Especially bad is the “visceral fat” around the organs as well as the fat in the liver. So being obese but pear-shaped may be less risky than being overweight or normal weight but apple-shaped. This realization has led to the recent concept of “metabolically healthy obesity.”

Genetics primarily determine where fat is stored in the body. Men, especially those with South Asian ethnic backgrounds, have a greater proportion of the dangerous visceral fat than women. Targeting the loss of body fat from one region to another is difficult, but overall weight loss results in predictable parallel reductions of all of the various fat deposits.

Additionally, exercise may help counterbalance obesity’s negative effect on health. Physically fit and active people who are obese have a similar or decreased risk of cardiovascular disease and death as people who are less fat but also less fit. Therefore, physical inactivity may be as great a risk to health as obesity, and people should be encouraged to be active even if it doesn’t result in weight loss.

3. Having a healthy body weight is all about personal responsibility and willpower.

People of normal weight sometimes like to take credit for avoiding obesity, suggesting that fatness arises from some combination of gluttony and sloth. As all those medical experts on Internet comment sections recommend, just eat less and exercise more!

Alas, if only such advice were effective. Consider exercise. Even when people participate in a supervised exercise program, weight loss is much less than would be expected from the calories burned during exercise. On average, exercising women experience no weight loss, and many people actually gain weight. This may be the result of compensatory decreases in other physical activities — such as collapsing on the couch for a few hours after a 30-minute run. Alternatively, hunger may increase, and it doesn’t take much food to offset the calories expended during exercise.

Why not just count calories and eat less? Unfortunately, even with the best diet-tracking apps, people tend to greatly underestimate how much food they consume. Furthermore, calorie intake tends to fluctuate widely, with swings often exceeding 1,000 calories from one day to the next. How would someone know if they made a dent in their calorie intake using such imperfect tools? Of course, making large calorie cuts would be easier to detect, and this is what most people do when they “go on a diet” to lose weight. When this happens, biology resists weight loss by increasing appetite and hunger.

4. Dieting causes the body to go into “starvation mode,” slowing its metabolism and halting weight loss after several months.

The idea that dieting can actually be counterproductive for weight loss is a trope that appears in just about every fitness publication, and warnings of the weight-loss plateau abound.

But although it’s true that metabolism does slow down when people cut calories, that offsets less than half of the decrease in diet calories over the first six months. It takes several years for metabolic slowing to fully offset the average dieter’s reduction in calories and result in a weight plateau. The fact that most people experience a weight plateau much earlier, typically after six to eight months of dieting, means that something else must be happening to thwart their continued weight loss.

In truth, the dreaded weight plateau is much more likely the result of a gradual loss of adherence to the original plan — people are actually eating many more calories when their weight loss stalls than when they started to diet. Why this happens is not fully understood, but biology probably plays a major role. For example, we know that weight loss results in hormonal changes that influence feelings of hunger and fullness, as well as alter how the brain responds to food cues in the environment. Food may actually become more rewarding. These changes influence overall food intake and may occur below our level of conscious awareness. People, therefore, may honestly report that they are sticking to their original diet when the weight plateau occurs, but objective measurements demonstrate otherwise.

5. All diets are doomed to fail.

Beware of diet,” Slate warns, noting that your chance of keeping the pounds off is no higher than it is of surviving metastatic lung cancer. Even actress Gwyneth Paltrow’s Goop lifestyle Web site warns that most weight-loss efforts are doomed.

This myth exists because, statistically, most people tend to regain at least some portion of their lost weight after a few years. This is especially true if they consider dieting a temporary strategy for losing weight. However, when diet changes are part of a persistent lifestyle modification, many people lose weight and keep it off over the long haul.

A recent study showed that eight years after adults began a diet and exercise program, more than half maintained weight loss of greater than 5 percent — an amount believed to be clinically beneficial. Furthermore, almost 40 percent of people lost more than 10 percent of their initial body weight after one year, and about 65 percent of those maintained more than 5 percent weight loss after eight years.

In addition to frequent weight monitoring, the secret to their success may be physical activity. Although exercise may not be very effective for inducing weight loss, it’s often a major contributor to maintaining lost weight. This may be because, compared to losing weight, only a relatively modest change in calories is required to keep weight off.

For example, the National Institutes of Health’s Body Weight Planner calculates that a 40-year-old woman who weighs 200 pounds requires a little more than 1,000 calories to be cut from her daily diet to lose 40 pounds in six months. But subsequently maintaining the 40-pound weight loss requires a permanent change of only about 350 calories per day, which can be attained with one hour of daily walking.

Twitter: @KevinH_PhD

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