You’ve probably heard of the BMI (Body Mass Index), which is a tool commonly used to determine if a person is at a healthy weight.

There’s a problem with that method – it is a simple height-to-weight ratio, and does not take into consideration one’s body composition. Athletes and bodybuilders with low body fat often are labeled as overweight or even obese on the BMI scale because they carry a lot of muscle.

Despite its flaws, the BMI has been used as a standard measure of individual health for years. According to the Centers for Disease Control and Prevention, a “healthy” BMI is 18.5-24.9, an overweight BMI is 25-29.9, and an obese BMI is 30 or higher.

In recent years, researchers have started to see that BMI is not such a good indicator of overall health after all.

Last month, a study that showed the serious implications of using BMI as a health measure was published.

Among the findings:

• Using BMI to gauge health incorrectly labels an estimated 74,936,678 adults in the US as cardiometabolically unhealthy or cardiometabolically healthy.
• Of that number, 54 million Americans have been classified as “unhealthy,” even though they are not.
• More than 30 percent of those with BMIs in the “normal” range — about 20.7 million people — are actually unhealthy based on their other health data.
• More than 2 million people who are considered “very obese” by virtue of having a BMI of 35 or higher are actually healthy. That’s about 15 percent of Americans who are classified as very obese.

 

Now, the results of another study are in, and this one found that a low BMI and high body fat percentage (not just high BMI) are independently associated with increased mortality in both men and women.

Researchers looked at the link between BMI and death rates over several years among nearly 50,000 women and 5,000 men age 40 and older in the Canadian province of Manitoba.

Unlike many previous studies, the researchers did not rely on BMI as a proxy for fat. They were also able to estimate total body fat directly, because they specifically selected individuals who had previously undergone an X-ray test (DXA for dual-energy X-ray absorptiometry) to determine if they had decreased bone density and possibly osteoporosis.

The researchers found that the skinniest women, with a BMI less than about 22.5 (a group that includes both those who are underweight and normal weight) had a 44% higher risk of dying during the approximately seven-year follow-up period.

At the other end of the range, women with more than 38.7% body fat had 19% higher death rates.

Among men, the thinnest group (those with a BMI less than about 23.8) had 45% higher death rates during a follow-up period of about 4.5 years. Men in the highest body fat group (more than 36% body fat) were at 59% higher risk of dying during the study period.

The study authors explained their findings:

The major finding was that when BMI and body fat percentage were included in the same fully adjusted models, low BMI and high body fat percentage were both associated with increased all-cause mortality. Mortality increased as BMI decreased and body fat percentage increased. This suggests that after adjustment for BMI, higher adiposity may have a detrimental effect on survival and that after adjustment for body fat percentage, a lower BMI may likewise be associated with decreased survival.

Being too thin and not preserving muscle mass creates an unhealthy situation, explained Dr. William D. Leslie, professor of medicine and radiology at the University of Manitoba and one of the authors of the study:

Being underweight is a marker for illness in some individuals at the same time that being overweight and obese is not optimal for health.

There were no clear health differences between individuals of different weights to suggest why those at the extremes had higher rates of death. Rates of conditions such as heart disease, diabetes, and lung disease were similar between the BMI groups.

A small subset of the individuals in the study had both excess fat and low BMI because of inadequate muscle mass, which Dr. Leslie said is a “double whammy in terms of adverse effects on health.”

Previous studies have suggested that adults whose BMIs place them in the overweight and mildly or moderately obese categories had a lower risk for mortality than those who had BMIs in the normal range. This finding has been referred to as “the obesity paradox.”

The authors emphasized that BMI may be an inappropriate measure for body fat, and that this limitation may help explain the obesity paradox.

It is also important to reiterate that a high BMI can be caused by either high body fat OR a good amount of muscle mass (or in some people, both). A person with a high BMI due to high body fat will likely face more health risks (such as Type 2 diabetes and heart disease) than a person with a high BMI who has low body fat and significant muscle mass. It is possible to be very thin, but still have high body fat, which can also lead to health problems.

BMI is a seriously limited (and arguably seriously flawed) measure of body composition and health, and scales don’t tell the whole story either, so what is a person who wants to know if they are in healthy weight range to do?

Better ways to assess your weight (or progress if you are trying to lose body fat) are by taking your measurements periodically,  observing how your clothes fit, and by how you look and feel.

If you would like to find out what your body fat percentage is, there are various ways to do that. Two very accurate methods are DEXA scans or hydrostatic weighing, but they are only conducted at certain fitness and research facilities. Or, you can purchase a bioelectrical impedance scale or skinfold calipers to measure your body fat at home (some gyms also offer those methods). This online calculator can help you estimate body fat, but it isn’t as accurate as the other methods mentioned above.

Related Reading

BMI Is Broken: System Mislabels 54 Million as Unhealthy

Why You Should Ditch Your Scale