Why BMI (Body Mass Index) Doesn’t Tell the Whole Story
You’ve probably heard the term BMI (body mass index). It’s based on your height and weight, and it’s widely used to determine if you’re in a healthy weight range. But as it turns out, BMI alone may not be the best way to size up your shape.
Taking a Closer Look at BMI
Calculated from a person’s height and weight, BMI breaks down into four categories:
- Underweight: BMI below 18.5
- Normal: BMI of 18.5 to 24.9
- Overweight: BMI of 25 to 29.9
- Obese: BMI of 30 or higher
But how useful is this number really?
“Probably for 90% or 95% of the population, BMI is just fine as a general measure of obesity,” says Richard L. Atkinson, MD, a researcher and editor of the International Journal of Obesity.
But some critics take a different view. Scott Kahan, who directs the National Center for Weight and Wellness, says, “Traditionally, we define obesity by a certain cutoff on the BMI scale.” But judging whether a person is obese based only on their size is old-fashioned and not terribly useful, he says.
A football player who is very muscular could have a high BMI and yet their body fat is actually low. — Scott Kahan, MD
Kahan specializes in helping people manage excess weight that can lead to health problems, including diabetes and heart disease. At his center, measuring BMI is only a starting point. He sees people who are overweight but healthy, and their BMI doesn’t truly reflect their health risks.
“They’re heavy. Their BMI puts them in the obesity range. And yet on every level that we look at, their health is actually quite good,” he says. “Their cholesterol and blood pressure are excellent. Their blood sugar is excellent. They don’t seem to have any health effects associated with their excess weight.”
Although BMI is useful as a quick screening tool by a doctor or nurse, Kahan says, it’s not enough to look at only that number.
A football player who is very muscular could have a high BMI and yet their body fat is actually low. — Scott Kahan, MD
BMI’s Drawbacks
Your BMI doesn’t reveal anything about the makeup of your body, such as how much muscle vs. fat you have. That’s why conclusions based only on this number can be misleading, especially when it comes to the following:
How muscular you are: A few people have high BMIs but don’t have much body fat. Their muscle tissue pushes up their weight. An example: “A football player or a body builder who is very muscular. Their BMI shows up pretty high, and yet their body fat is actually pretty low,” Kahan says.
Your activity level: Someone who is very inactive may have a BMI in the normal range and have lots of body fat, though they may not look out of shape.
“They have very low levels of muscle and bone — often elderly people, those in poor shape, sometimes those who are sick. Their BMI can look in the normal range, even though they have quite a lot of body fat in comparison to their lean body mass,” Kahan says. “Ultimately, they have similar risks as people who carry lots of body fat and have a high BMI.”
Your body type: Are you an apple shape or a pear shape? The location of your fat makes a difference to your health. Generally, it’s the belly fat, or the “apple” shape, that has a higher health risk. When fat settles around the waist instead of the hips, the chance of heart disease and type 2 diabetes goes up. Fat that builds up on the hips and thighs, or the “pear” shape, isn’t as potentially harmful.
Your age: The notion of an ideal BMI may shift with age. “People who are older probably should have a little more fat on them, [but] they shouldn’t have a BMI of 30,” Atkinson says.
He points out that late in life, people who are “a little bit overweight” tend to have a better survival rate than leaner people. The reasons for that aren’t totally clear, but it may have to do with having reserves to draw on when fighting off an illness. It’s hard to tell for sure, since many things affect your health.
Your ethnicity: There are a lot of differences in BMI and health risk among ethnic groups. For example, Asian-Americans tend to develop health risks, including the risk of diabetes, at lower BMIs than whites. A healthy BMI for Asians ranges from 18.5 to 23.9, a full point lower than the standard range. And Asians are considered obese at a BMI of 27 or higher, compared to the standard BMI obesity measure of 30 or higher.
People of Indian descent face higher health risks at relatively lower BMIs, Atkinson says. “The standard definition of overweight is a BMI of 25 or above. But if you’re from India, your risk of diabetes starts going up with a BMI of about 21 or 22.”
In contrast, many African-Americans may have a high BMI, but without the health risks that usually go along with it. Compared to whites with the same weight and BMI, African-Americans tend to have less visceral fat (fat around their organs) and more muscle mass, Atkinson says. Therefore, an African-American with a BMI of 28, which the standard chart calls overweight, might be as healthy as a white person with a BMI of 25.
Beyond BMI
So what other tools can you use besides BMI? You may want to get out your measuring tape.
Waist size: For an accurate measurement, the tape measure should go around your waist at the top of your hip bones in your lower back and go around to the belly button.
To help prevent health problems from being overweight, men should keep their waist size to no more than 40 inches. Women should stick to no more than 35 inches. Again, there are some ethnic differences. Asian men should keep their waists no more than 35.5 inches and Asian women to no more than 31.5 inches, according to the Joslin Diabetes Center.
Waist-to-height ratio: This compares your waist measurement to your height. It may be even more helpful than waist circumference alone, Kahan says. The goal is for your waist circumference to be less than half of your height.
Other ways to measure body fat that may be more accurate than using BMI alone include the waist-to-hip circumference, skinfold thickness measurement, and ultrasound. Your doctor can help decide if these further tests may be needed.
