The BMI measure has given us the obesity paradox—being overweight or even slightly obese protects us against an early death. But iron out the anomalies, and the true impact of extra weight is revealed
Have that last piece of pizza. You know you want to, and anyway, medical science tells us it might even be good for you. People who are a little overweight—pleasantly plump, perhaps—have the greatest protection against a range of chronic health problems and could even live longer.
It’s known as the obesity paradox. According to BMI (body-mass index) measures, obesity is defined as any score over 30. But those who are merely overweight—with a BMI of 25 to 29.9—are at the lowest risk of an early death, so it seems as if that little bit of fat around the middle helps you live longer. In a U-shape of risk, the overweight enjoy the same protection as the slim, who have a BMI score of 18.5 to 24.9.
Even stranger, people classified as underweight with a BMI of 18.5 or lower are at the highest risk, like the morbidly obese with a score of 35 or higher, of an early death. And young people with a low BMI of 18.5 to 24.9 are more likely to suffer arterial stiffness, a precursor of cardiovascular disease.1
“Obesity alone may confer a survival benefit independent of age, medical care or therapy . . . perhaps the definition of obesity needs to be revisited,” said researchers at the Naval Medical Center in San Diego.2
Who’s obese of these two: Charlie, the tragic hero of the movie The Whale, or Hollywood action man Tom Cruise? The answer is that both are, at least according to their BMI scores. Charlie, weighing in at 600 lb (272 kg), has a BMI score of 77, and Tom’s BMI is 31.5—his muscular frame weighs 201 pounds (91 kg) and stands 5 feet 7 inches tall (169 cm).
It’s hard to argue with the science that uses the BMI score, the go-to measure for predicting health and longevity. While doctors used to rely on simple weight and height calculations to judge body fat, they now use the more precise BMI score, which is arrived at by dividing weight in kilograms by height in meters squared.
It doesn’t directly measure body fat, but it provides a reasonable guide, says America’s Centers for Disease Control and Prevention (CDC).
This is all great news for the 30 percent of American adults who are overweight and who are already tucking into an average of three fast-food feasts every week. Even the 42 percent of Americans who are obese don’t seem to have too much to worry about, with their weight problems increasing their risk of premature death by a marginal 3 percent. Only the morbidly obese need to start cutting back on the carbs.
Putting that into numbers, the World Health Organization estimates that obesity and excess weight are directly responsible for 2.8 million deaths a year around the world. Sounds like a lot, but with the world’s population tipping 8 billion, that’s just 0.035 percent of people dying from weight-related health problems every year.
When it comes to body fat, there’s fat and there’s fat. We all know about the calorie-storing white fat that seems to accumulate around our stomach and thighs, but biologists have recently discovered the body is also made up of another fat, and it’s completely missed by the BMI measure.
Brown adipose tissue (BAT), or brown fat, burns fat and helps maintain a healthy body temperature. Babies have plenty of brown fat, and it keeps them alive when they’re born, but biologists had assumed we lose it as we mature.
In fact, we retain brown fat, although some of us have more of it than others. In one study, the fat was detected by PET (positron emission tomography) scans in just 10 percent of people—but those with detectable amounts had half the rate of type 2 diabetes, plus lower blood pressure, congestive heart failure and coronary artery disease.1
Having detectable amounts of BAT protects against obesity—and even if you are overweight, the fat reduces your risk profile for heart and metabolic diseases to that of someone with a healthy weight.
It seems that BAT—which tends to accumulate around the neck and shoulders—burns calories to maintain body temperature.
We may be able to transform white fat to brown, at least according to some early theories, by exposing ourselves to extreme cold. Easier ways could be by adding more BAT-inducing foods and drinks to our diet, such as green tea, resveratrol, curcumin and grape extract.2
None of this makes sense on a fast-food planet where the primary exercise is video gaming and the overweight and obese outnumber the slim.
It certainly didn’t make sense to Ryan Masters, an associate professor of sociology at the University of Colorado at Boulder. “I’ve been suspicious of the claims of the obesity paradox,” he said, and so he decided to find out what’s really going on.
He suspected the BMI score was at the heart of the paradox. The BMI doesn’t take into account different body compositions—it doesn’t recognize the muscle mass of sportspeople or weight-lifters, for instance—nor does it consider the length of time someone has been at their current BMI score.
It also takes a Western view of body types. The Inuit people of Greenland have long torsos and short legs, which means they weigh more for their height than white people, on whom BMI measures are based. It puts most of them in the BMI’s obese category—and yet disease is a rarity in this people who are physically active and are still hunter-gatherers.3
Masters re-analyzed data that captured the BMI scores and health of more than 17,000 Americans who had participated in the National Health and Nutrition Examination Study (NHANES) between 1988 and 2015, during which time 4,468 of them died.4
Around 20 percent of the participants who were healthy according to their BMI score had been overweight or obese the previous decade. Conversely, 37 percent of the participants had only recently become overweight or obese, so they didn’t have any health problems and were not more likely to die prematurely.
Health and BMI levels “are not like a light switch,” Masters said, and the real impact of obesity can take many years to materialize.
When these anomalies are ironed out, the true extent of obesity’s damage is revealed. Being overweight or obese is responsible for 16 percent of all deaths every year, making it at least five times more deadly than current estimates suggest.
Excess weight and obesity raise the risk of premature death by anything from 22 percent to 91 percent, Masters calculates. If he’s right, obesity is directly responsible for 10.7 million deaths around the world every year, a sorry legacy of fast foods.
The U-shaped paradox disappears and instead a straight upward-moving line emerges, with the lightest at the lowest risk and the heaviest at the greatest risk of early death. The supposedly protective effect of being a little overweight also vanishes.
“Existing studies have likely underestimated the mortality consequences of living in a country where cheap, unhealthy food has grown increasingly accessible, and sedentary lifestyles have become the norm,” he said.
Perhaps it’s premature to throw out the BMI measure, but its deficiencies need to be recognized, and more direct measures of body fat—such as bioelectrical impedance, which uses electrical currents—need to be considered.
Even simpler measures, such as waist size, are better indicators of disease risk. One study found that a waist size of 40 inches in men and 35 inches in women is a much stronger predictor of heart dysfunction. In other words, the classic apple shape suggests a higher risk than a pear shape, where fat accumulates around the hips, and it’s a far more accurate predictor than BMI, say researchers at the Intermountain Medical Center Heart Institute.5
One thing’s for sure: being overweight or even slightly obese is no longer a free pass. They are both killers, despite what the BMI researchers might be telling you.