Should You Care About Your Body Mass Index?

Find out what this number really says—and doesn’t say—about your body and health.

You probably know how much you weigh, but whether the number on the scale means you’re healthy depends on many factors — and your height is just one of them.

Your body mass index (BMI) is a score that uses both your height and weight to provide a rough estimate of how much body fat you have. Depending on your BMI number, you might be classified as underweight, normal weight, overweight, or obese.

Research has consistently shown that people with a high BMI have a greater risk of serious health problems, including heart disease, diabetes, and certain cancers. The thinking is that a higher BMI means more body fat, which can contribute to health problems.

But "most" is the operative word. BMI is an effective tool at tracking large populations over time, but what it means to each individual - to you - is where things get more complicated.

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The Problems with BMI

BMI was never intended to be a measure of individual health, according to a report in Social Theory & Health. It was developed nearly 200 years ago by a mathematician as an equation to measure the “average man.”

Later on, about 50 years ago, a public health researcher used the equation to estimate body fat in populations. Eventually, cutoff points were assigned to BMI numbers to classify people as normal, underweight, overweight or obese

BMI can be a useful tool for measuring rates of overweight and obesity in a population over time. But using it to diagnose individuals is more problematic. Your BMI can give you a rough estimate of your body fat percentage, but it’s far from perfect.  

“BMI isn’t going to give you the whole picture, but it’s a piece of the puzzle,” says New York–based dietitian Jessica Cording, R.D.N.

For example, BMI doesn’t account for your bone structure, gender, genetics, or any conditions like osteoporosis. Nor does it actually tell you how much fat you’re carrying around.

Since muscle is denser than fat, it’s possible to be very fit and have a BMI in the overweight or obese category—though that’s most apt to happen if you’re an athlete or extremely muscular.

Older adults should also know that their BMI might appear normal when they would actually benefit from gaining some weight. That might be the case if you've lost a lot of muscle mass because of aging. In fact, some studies have found that elderly people who are overweight, according to their BMI, actually live longer than their thinner peers.

BMI also doesn’t tell you anything about where you carry your body fat, which may be more important in determining your disease risk. Fat carried around your lower body (i.e., “pear-shaped” people) is less dangerous than fat carried around your abdomen and organs (i.e., “apple-shaped” people), according to report from Harvard Health.

It also seems that BMI’s association with disease risk can vary depending on your race and ethnicity. Some researchers have even suggested custom BMI scales for different genders and races.

In short: Consider BMI a tool, but not the final word on your health.

How to Calculate Your BMI

The easiest way is to use an online calculator, such as this one provided by the Centers for Disease Control and Prevention (CDC). All you need to do: Enter your height and weight, and click a button.

If you prefer the DIY method, you’ll need to use this formula:

BMI = [Your weight in pounds ÷ (Your height in inches)²] x 703

Either way, you’ll be given your BMI, which is interpreted as follows:

• Underweight = less than 18.5
• Normal weight = 18.5 to 24.9
• Overweight = 25.0 to 29.9
• Obese = 30.0 or greater

What to Do with Your Number

Whatever your score, don't ignore it. "BMI gives you some indication of lean versus fat mass," explains Sonya Angelone, R.D.N., a spokesperson for the Academy of Nutrition and Dietetics.

Most people with a BMI in the overweight or obese range will benefit from losing some weight, and those who are underweight may need to gain weight. But before you make any major changes to your diet or exercise routine, talk to your doctor.

Regardless of what BMI category you fall into, your doctor should look at other factors to get a more holistic picture of your health status. Other numbers to look at might include: 

• Waist-to-hip ratio. This helps determine where you are carrying your body fat (whether you are more “pear-shaped” or “apple-shaped”). Research published in the Annals of Epidemiology found that waist-to-hip ratio might be a better gauge of obesity in people over age 70.
• Blood pressure
• Cholesterol levels
• Blood sugar and Hemoglobin A1c 

Recommended reading: 5 Best Ways to Take Your Body Measurements

These factors, along with family history and lifestyle habits, will give a much clearer picture of your overall health and disease risk.

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