AYESHA RASCOE, HOST:
Body mass index, or BMI, has been the go-to way to determine if people are overweight. It simply requires measuring height and weight. But for years, doctors have wrestled with how much to rely on BMI. New research underscores some of the problems and is proposing an alternative. For more on this, we're joined by NPR health correspondent Will Stone. Welcome.
WILL STONE, BYLINE: Hey. Good to be here.
RASCOE: So first, what's behind the debate over BMI?
STONE: The problem here is really that BMI lacks nuance. It's basically an indirect measure of whether you have excess body fat, and there are certain agreed-upon cutoffs. A BMI of 25 to 29 is overweight, and obesity is defined as more than 30. And just for context here, 70-plus percent of the U.S. population falls into one of those two categories. But BMI can actually misinform people. For example, you may be classified as overweight, but that could be due to muscle mass. This is especially true for athletes, people who lift weights.
And on the other hand, BMI also can't distinguish if someone is in a normal range, meaning under 25, but does have excess body fat, especially high amounts around the belly. This visceral fat is particularly bad for health. So ultimately, all of this matters because doctors care about your risk of health problems, like heart disease or diabetes, and critics point to evidence showing BMI isn't necessarily reliable.
RASCOE: And is that what this new research is showing?
STONE: Yeah, that's part of it. This latest study is from a team at the University of Florida, and it was published in the Annals of Family Medicine. They looked at more than 4,000 adults between the ages of 20 and 49 and the risk of dying over the next 15 years, and they compared BMI with body fat percentage. Arch Mainous is one of the study's authors.
ARCH MAINOUS: Well, what we find is that BMI doesn't predict. We find that this direct measure of body fat and lean muscle mass does predict, and it predicts quite well.
STONE: So here they measured body fat with bioelectrical impedance analysis, BIA, and that uses an electrical current to estimate body composition, and you can buy scales that do this for a few hundred dollars. The accuracy does vary between machines, but Mainous argues it's still better than BMI, and he points to the results showing people in the higher body fat group had a 78% increased risk of dying.
MAINOUS: Even for cardiovascular or heart disease, what we find is that people who are above our standard thresholds on this have a 262% increased risk of dying in the next 15 years, which I thought was pretty dramatic.
RASCOE: And what were those standard thresholds for body fat?
STONE: OK. For men, it was anyone over 27% body fat, and for women, it was 44%. And those percentages are based on evidence, a review of studies. But the authors acknowledge there is no consensus about what the ranges should look like if body fat percentages do replace BMI one day, and that would likely vary based on age and other factors.
RASCOE: Do others in the medical community think replacing BMI as a measure is a good idea?
STONE: Well, the fact is, BMI is baked into our health care system. It's simple, it's inexpensive. But the issues with it are well recognized. The American Medical Association even updated its policy a few years ago to say doctors should not only rely on BMI when evaluating patients. I reached out to Dr. Michael Blaha about this. He's a cardiologist and researcher at Johns Hopkins University.
MICHAEL BLAHA: I think BMI still has utility in large populations where you need a quick measurement. It gives you a real good starting point about how overweight or obese someone might be, how much body fat they might have. It is true that if you have a very high BMI, you're going to have increased body fat. We know that. But in the clinical setting, we need something better.
STONE: Now, you know, this is just one study and there are limits. All-cause mortality. It can be a blunt way to measure health risk. Also, Blaha thinks a waist circumference, not just body fat, is another good approach. But big picture, with so many people dealing with weight-related chronic diseases, many doctors do agree it's critical to fine-tune how they measure this so they can accurately advise patients about their risk.
RASCOE: NPR health correspondent, Will Stone. Thank you so much, Will.
STONE: Thank you. Transcript provided by NPR, Copyright NPR.
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