BMI Doesn't Really Tell You Anything About Your Health

Millions of obese Americans are metabolically healthy, while many "normal" weight Americans aren't.
Big Cheese Photo via Getty Images

Your weight actually isn’t a very reliable predictor of heart health or diabetes risk, according to a new study from obesity researchers.

Nearly half of overweight people, 29 percent of obese people and 16 percent of morbidly obese people were found to be metabolically healthy when the researchers examined data for 40,000 people. This means those folks weren’t at risk for type 2 diabetes or heart disease.

On the other hand, more than 30 percent of people in the “normal” weight category were metabolically unhealthy, which means they were at risk for these kinds of diseases.

These findings add another crack to the foundation of medical orthodoxy on obesity. Despite long-standing, well-researched associations between obesity and chronic disease, a growing body of research is pushing back on the notion that a person who is overweight is, by definition, also unhealthy.

Instead of a one-size-fits-all approach to weight and health, the study suggests that a person can be “fat but fit” or thin and unhealthy. People who research this “obesity paradox” also note that people who are overweight or obese are more likely to survive chronic diseases than normal weight or underweight people.

As with most obesity studies, the researchers relied on measuring body mass index, or BMI, a calculated proportion of weight to height that divides people into normal weight, overweight, obese or underweight categories.

BMI looms large in the discussion over obesity and health, despite being a pretty unreliable metric. Health insurance companies use BMI to figure out how to offer financial discounts or penalties to policy holders, while doctors use BMI to diagnose people with overweight or obesity and tell them that they need to lose weight to protect their health.

"There have been many misuses of BMI throughout the decades, despite very good evidence, like ours, that shows it’s a flawed measure,” said study co-author A. Janet Tomiyama, director of the Dieting, Stress and Health Laboratory at the University of California, Los Angeles. "We’re hoping that this study will be the final nail in the coffin for BMI."

The problem with BMI

The creator of BMI, a 19th-century mathematician named Lambert Adolphe Jacques Quetelet, never dreamed that the ratio would be used to assess an individual person’s health or weight, reported NPR. Instead, he created it as a shorthand calculation to assess obesity on the population level, rather than an individual one.

One main issue with BMI is that it can’t tell how much of a person’s weight is body fat and how much of it is bone, muscle and water. Professional athletes are a clear example of this problem: Take LA Clippers player Blake Griffin, for instance. He's six-foot-ten and weighs 251 pounds, making him technically “overweight” according to the BMI ratio. But anyone who’s seen Griffin play ball knows that couldn’t be further from the truth:

The obese and healthy, misclassified

Based on their data, the study’s researchers estimate that approximately 19.7 million Americans who think they are obese may actually be metabolically healthy. For this population, they wrote, the “obese” label is a distracting misclassification that could negatively harm their overall health.

When healthcare providers prescribe weight loss for this group, they aren't just wasting patients' time and effort, they're also contributing to stigma and "high levels of anti-fat bias," according to the researchers.

Overall, they estimate that about 74.9 million Americans are misclassified in some way because their BMI number doesn’t match up to their true metabolic health status.

"I’m sure some will defend the BMI as a relatively quick and cheap measure, but why should 'quick and cheap' win out over accurate, especially when we’re talking about the health of millions of Americans?" said co-author Jeffrey Hunger, a doctoral student specializing in weight-based stigmatization at the University of California, Santa Barbara.

“I’m sure some will defend the BMI as a relatively quick and cheap measure, but why should 'quick and cheap' win out over accurate?”

What we should measure instead

The researchers say a better way to assess dangerous weight gain would be to do things like measure a person’s body fat percentage or calculate a person’s waist to height ratio.

The body fat percentage measurement does what the BMI doesn’t: it makes a distinction between body fat, which can drive hormonal disorder, diabetes and heart disease, and everything else, like bones and muscles that help you stay fit and strong. Healthy body fat percentages can also vary according to sex, athletic ability and age.

The waist to height ratio, on the other hand, makes a distinction between abdominal fat, which is the most metabolically active and dangerous kind of body fat, from the fat found on hips and thighs, which is not associated with disease or a shortened life.

Tomiyama added that beyond weight, there are other routine, simple measures of health that doctors can take to evaluate metabolic health, like measuring blood pressure or blood sugar levels.

Critics of the “fat but fit” notion might say that the “healthy” obese people in Tomiyama and Hunger’s study are merely at a temporary stop on the way to their final destination: chronic disease and early death. Hunger notes that while their analysis doesn’t address this question (as it only measures one point in time, not long-term data), the inclusion of people over 60 years old, some of whom were also obese yet metabolically healthy, suggests that this isn’t the case for everyone.

"Although our data can’t speak directly to to this issue, I suspect that lifestyle factors such as being active are more important than BMI in predicting the stability of metabolic health,” he said.

"It’s definitely not a death sentence," Tomiyama agreed. "There are millions of folks that are perfectly healthy despite having an overweight or obese BMI."

Tomiyama and Hunger are continuing their research on obese but healthy people. If you’re interested in participating, check out the HE.R.O. study, conducted by the University of California, Los Angeles.

The research was published Feb. 2016 in the International Journal of Obesity.

Also on HuffPost:

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