Why BMI is BS

Every year when I get my physical, my doctor measures my height and weight. Then he takes those 2 numbers, plugs them into a formula (weight multiplied by 703, divided by height squared, in case you’re interested) and gives me my body mass index.

Otherwise known as BMI.

For years, BMI has been used by doctors, personal trainers and other health care professionals when deciding whether their patients are overweight. And according to the CDC, “BMI is a fairly reliable indicator of body fatness for most people.”

But is it? More and more experts are weighing in (ahem) that it’s not.

The ABCs of BMI

A little history: The BMI was introduced in the 1830s by a man named Lambert Adolphe Jacques Quetelet. He was a Belgian mathematician, astronomer and statistician. He was NOT a doctor.

And here’s where Mr. Quetelet went wrong … The BMI does not take into account the relative proportions of bone, muscle and fat in the body. And since bone and muscle are denser than fat, a person who has solid bones and a lot of muscle will have a high BMI.

So if you lift weights regularly, you may find yourself in the overweight, or even obese, category. Even though you’re far from it.

On the flip side, someone with little muscle but a hefty amount of fat may have a healthy BMI, but they’re actually at risk for certain health conditions.

So BMI doesn’t give us much insight into a person’s body composition. And it certainly doesn’t measure body fat … the real culprit when it comes to weight-related health risks.

BMI is meant to be a screening tool — nothing more, nothing less — to spot a POSSIBLE weight-related concern. It was never designed to be a stand-alone diagnostic tool.

In fact, a simple measure of your waist has been shown to be just as effective as BMI for predicting weight-related health risks.1 I’ve written about how to measure your waist accurately here.

Better than BMI

While BMI has its place as a screening tool, there’s something that’s been emerging as a stronger predictor of an individual’s cardiovascular health.

Fat mass.2

And there are a number of ways to measure yours. Here’s a quick breakdown of some of the more popular options:

  • Hydrostatic, or underwater weighing: The American College of Sports Medicine calls this the gold standard because it’s highly accurate. To calculate the body fat percentage, you’re weighed twice. First submerged in water (after expelling all the air from your lungs) and then on land. Those numbers are then entered into equations to arrive at your body fat percentage. The bad news: This is an expensive, time-consuming way of measuring — and you have to get into a bathing suit. Ouch.
  • Bioelectric impedance analysis (BIA): This method uses low-intensity electric currents through the body. Two electrodes are placed on the body (right hand and right foot). The faster the signal moves from one electrode to the other, the more muscle and less fat a person has. This also is expensive and not widely available for the general public.
  • Skin-fold analysis: Using special calipers, measurements are taken at different sites on the body. The tester pinches the skin at the location site and pulls the fold of skin away from the underlying muscle so only the skin and fat tissue are being measured. With calipers in hand, the tester takes several measurements on various parts of your body, such as upper arms, upper back, midsection and thighs. This is a pretty cost-effective way of getting measured, but it all depends on the skill level of the pincher, especially for overweight people who are more difficult to measure using this method.
  • BIA scale. In essence, this uses the same method as the BIA mentioned above, but the scale sends a current through the metal plates under your feet. They’re available at any department store. And they can be reliable … when used properly.

While there are plenty of ways to measure your body fat with reasonable accuracy, the most convenient one is the one you’re most likely to actually use.

My vote?

It would have to be for a high-quality BIA scale. It can be used in the comfort of your own home, and, when used properly, it can reliably measure changes in your body fat. No pinching. No bathing suit. No time-consuming travel to a testing center.

And when you’re getting frustrated that your regular scale isn’t budging, a body fat scale can show your progress. Because if you’re eating right and exercising (and losing fat and gaining muscle, which is a good thing), your body composition may be changing, but your body weight stays the same.

So all that good progress that doesn’t show on a regular scale may reflect better on a body fat scale.

Just be sure to use your scale according to the directions. Place it on a hard surface, enter your height, age and gender, step on it properly, and don’t use it after exercise, when you’re dehydrated, or after drinking anything. You’ll find all the details in the little instruction booklet that typically comes with a scale … the one we all tend to toss out without reading. Don’t.

Use it to set up and use your scale properly, and you’ll be rewarded with an accurate measure of changes in your body fat.

Steve Sisskind, M.D.

Hi, I'm Dr. Steve Sisskind, Chief Medical Officer & Founder at RealDose Nutrition.

As a young physician, I struggled because my patients came to me with serious health issues, but I didn't have the right tools to help them. Medical school taught me how to put "band aids" on their symptoms with drugs and surgery, but not how to address the root causes of their problems.

Years later I discovered a better approach... based on the fundamental idea that the power of nutrition can transform your health and vitality. But there's a lot of confusion... What foods should I eat? Which supplements should I take? What does the science say?

I have dedicated my life to answering these questions... And I share this knowledge with you every day here at RealDose Nutrition.

I invite you to connect with me by joining my free private community. I've helped thousands of people and I know I can help you too!

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References:1. Janssen I, Katzmarzyk PT, Ross R. Waist circumference and not body mass index explains obesity-related health risk. Am J Clin Nutr. 2004;79(3):379-84. PMID: 14985210.

2. Pourshahidi LK, Wallace JM, Mulhern MS, et al. Indices of adiposity as predictors of cardiometabolic risk and inflammation in young adults. J Hum Nutr Diet. 2015 Feb 10. [Epub ahead of print] PMID: 25677964.

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