What Your BMI Doesn’t Tell You
BMI's Drawbacks continued...
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.