How useful is the body mass index (BMI)?

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

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 three million people die 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.

Should we stop giving so much "weight" to BMI?

Maybe. Research suggests that BMI alone frequently misclassifies metabolic health, which is linked to how much fat a person has and how it is distributed. And, BMI may be particularly unreliable during pregnancy, for athletes, and the elderly.

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, the study mentioned above did not consider conditions that might also be relevant to an individual with an elevated BMI, such as liver disease or arthritis. In addition, BMI may be more useful at predicting future rather than current health. Those who are healthy and overweight or obese are more likely to develop diabetes or other negative health consequences over time, according to studies such as this one and this one.

And there's another problem: current BMI definitions of overweight or obesity were based largely on white populations. Yet body composition, including percent body fat or amount of muscle mass, can vary by race and ethnic group. So, BMI may help predict health status among people who are white, but may be less accurate for people in other racial and ethnic groups.

For example, defining obesity by standard measures of BMI tends to overestimate risk in Black individuals and underestimate it for those of Asian descent. This may lead to suboptimal counselling and treatment, and may ultimately increase healthcare disparities. The World Health Organization and the National Institutes of Health recommend different BMI cutoffs for overweight and obesity in people of Asian descent. Changes are being recommended for BMI cutoffs for other ethnic groups as well.

The 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.

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