Why BMI Isn't Always the Best Measure of Body Composition — and What to Use Instead

You already know that being overweight is bad for your health, raising your risk of heart disease, certain cancers and myriad other conditions. And you know that BMI, or body mass index, is the best measure of whether you're overweight — right?

BMI is one piece of the puzzle when it comes to knowing whether or not you're at a healthy weight. (Image: PhotoTalk/iStock/GettyImages)

For years, BMI has been the gold standard for determining whether your weight falls in a healthy range. The formula is your weight in kilograms divided by the square of your height in meters (this can be hard to figure out on your own, but you can calculate it here.)

But now, research suggests that where you store your weight may be just as important as how much you weigh. A study of over 155,000 postmenopausal women published July 2019 in the medical journal JAMA Network Open found that women with a normal BMI (between 18.5 and 24.9) whose waist circumference was over 35 inches were almost a third more likely to die from cancer or heart disease than women with smaller waists who were also at a healthy weight.

"A BMI can give us important health information, but it's not a perfect indicator — our research indicates that what's deep within your abdominal area is just as, if not more, important," study co-author Wei Bao, MD, PhD, an epidemiologist in the College of Public Health at the University of Iowa in Iowa City, tells LIVESTRONG.com.

So Why Is Belly Fat So Toxic?

It's actually not your muffin top itself that's dangerous. "This type of stomach fat is known as subcutaneous fat, since it's right underneath your skin," explains Pamela Peeke, MD, MPH, an assistant professor of medicine at the University of Maryland and author of Body for Life for Women. It may not look good, but it's no more dangerous than the fat that's on your arms or thighs.

The real villain, Dr. Peeke says, is visceral fat, aka the stuff that pads your internal organs. This type of fat is more metabolically active, which means it causes your body to secrete stress hormones like cortisol that in turn increase your blood pressure and blood sugar, raising your risk for a whole slew of diseases.

"You can get a very accurate idea of whether or not you're in the healthy zone by looking at all three measures — your BMI, your waist circumference and your waist-to-hip-ratio."

The Best Way to Measure Body Fat

The best way to tell how much visceral fat you have is to have a CT or MRI scan done on your abdominal area, says Dr. Bao. But that's expensive. An easier option is simply to grab a tape measure and check your waist circumference.

Using a cloth measuring tape, measure your waist at your belly button. (Make sure the tape's parallel to the ground.) An ideal, healthy measurement for women is below 35 inches, and below 40 inches is the aim for men, says Dr. Bao.

If you're borderline, it's a good idea to also calculate your waist-to-hip ratio: Measure your waist at your belly button and your hips at their widest point, then divide your waist size by your hip size. A healthy waist-to-hip ratio is under .9 for men and under .8 for women, according to the National Center for Biotechnology Information.

While there are other ways to measure your body fat, like using skinfold calipers, these aren't always accurate and can be pricey.

The solution? "You can get a very accurate idea of whether or not you're in the healthy zone by looking at all three measures — your BMI, your waist circumference and your waist-to-hip-ratio," says Dr. Bao.

3 Ways to Blast Belly Fat

The good news is that belly fat responds well to a mix of healthy eating and exercise. Here, three small changes that can help you trim down your waistline, stat.

1. Time your eating. Dr. Peeke advises her patients to eat all their daily calories within a 12-hour window — usually between 7 a.m. and 7 p.m. This type of eating pattern — known as time-restricted eating or intermittent fasting — can help reduce levels of visceral fat, according to a paper published June 2014 in Nutrition Research Reviews. It also prevents late-at-night noshing, which is another risk factor for developing visceral fat, adds Dr. Peeke.

2. Nix the nightcap. If you can't put a kibosh on alcohol entirely, limit it to an occasional treat. An August 2017 Korean study published in the journal Public Health Nutrition found a link between boozing it up and abdominal obesity. "Alcohol raises your body's insulin levels, which in turn encourages it to store fat around your abdomen," explains Dr. Peeke. "It also loosens your inhibitions, making it more likely you'll eat everything that's not tacked down."

3. Take your workout up a notch. High-intensity interval training (HIIT) — bursts of all-out exercise with short breaks in between — has been shown to be much more effective at getting rid of visceral fat than regular, moderate activity, notes Dr. Peeke. Indeed, a February 2018 review published in the journal Sports Medicine looked at 39 studies and concluded that HIIT significantly reduced both total and abdominal body fat. Try to incorporate it into your cardio activity for about 20 minutes twice a week. "It's easy to do — if you walk, for example, then do a five-minute warm-up, then 30 to 60 seconds at a much higher pace or walking uphill, bring it back down for a minute, then ramp it right back up again for 30 to 60 seconds," recommends Dr. Peeke.

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