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How useful is the body mass index (BMI)?

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December 14, 2020 at 11:49 PM

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November 29, 2020 at 6:23 PM

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Do you know your BMI? Increasingly, people know theirs, just as they know their cholesterol.

If you don’t know your BMI, you can use a BMI calculator available online, including this one at Harvard Health Publishing. All you need is your height and weight. Or, you can calculate it yourself, using this formula:

BMI = (Weight in Pounds x 703) / (Height in inches x Height in inches).

So, now that you know your BMI, is it worth knowing? What are you going to do with it?

What your BMI means

To understand what your BMI means, it’s useful to take a step back and understand what it’s measuring and why it’s measured.

BMI is a calculation of your size that takes into account your height and weight. A number of years ago, I remember using charts that asked you to find your height along the left side and then slide your finger to the right to see your “ideal weight” from choices listed under small, medium, or large “frame” sizes.

These charts came from “actuarial” statistics, calculations that life insurance companies use to determine your likelihood of reaching an advanced age based on data from thousands of people. These charts were cumbersome to use, and it was never clear how one was to decide a person’s “frame size.”

BMI does something similar — it expresses the relationship between your height and weight as a single number that is not dependent on “frame size.” Although the origin of the BMI is over 200 years old, it is fairly new as a measure of health.

What’s a normal BMI?

A normal BMI is between18.5 and 25; a person with a BMI between 25 and 30 is considered overweight; and a person with a BMI over 30 is considered obese. A person is considered underweight if the BMI is less than 18.5.

As with most measures of health, BMI is not a perfect test. For example, results can be thrown off by pregnancy or high muscle mass, and it may not be a good measure of health for children or the elderly.

So then, why does BMI matter?

In general, the higher your BMI, the higher the risk of developing a range of conditions linked with excess weight, including:

  • diabetes
  • arthritis
  • liver disease
  • several types of cancer (such as those of the breast, colon, and prostate)
  • high blood pressure (hypertension)
  • high cholesterol
  • sleep apnea.

According to the WHO, nearly 3 million people dye yearly worldwide due to being overweight or obese.  In addition, independent of any particular disease, people with high BMIs often report feeling better, both physically and psychologically, once they lose excess weight.

And here’s why BMI may not matter

It’s important to recognize that BMI itself is not measuring “health” or a physiological state (such as resting blood pressure) that indicates the presence (or absence) of disease. It is simply a measure of your size. Plenty of people have a high or low BMI and are healthy and, conversely, plenty of folks with a normal BMI are unhealthy. In fact, a person with a normal BMI who smokes and has a strong family history of cardiovascular disease may have a higher risk of early cardiovascular death than someone who has a high BMI but is a physically fit non-smoker.

And then there is the “obesity paradox.” Some studies have found that despite the fact that the risk of certain diseases increases with rising BMI, people actually tend to live longer, on average, if their BMI is a bit on the higher side.

Should we stop giving so much “weight” to BMI?

Maybe.  There are studies suggesting that BMI alone frequently “misclassifies” metabolic health.  For example, found that:

  • More than half of those considered overweight by BMI had a healthy “cardiometabolic profile,” including a normal blood pressure, cholesterol, and blood sugar.
  • About a quarter of people with normal BMI measures had an unhealthy cardiometabolic profile.

Actually, this should come as no surprise. BMI, as a single measure, would not be expected to identify cardiovascular health or illness; the same is true for cholesterol, blood sugar, or blood pressure as a single measure. And while cardiovascular health is important, it’s not the only measure of health! For example, this study did not consider conditions that might also be relevant to an individual with an elevated BMI, such as liver disease or arthritis.  In addition, more recent studies (such as this one and this one) suggest that those who are healthy and overweight or obese are more likely to develop diabetes or other negative health consequences over time.

Bottom line

As a single measure, BMI is clearly not a perfect measure of health. But it’s still a useful starting point for important conditions that become more likely when a person is overweight or obese. In my view, it’s a good idea to know your BMI. But it’s also important to recognize its limitations.

Originally published by Robert H. Shmerling, MD on June 22, 2020

https://www.health.harvard.edu/blog/how-useful-is-the-body-mass-index-bmi-201603309339

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