Why Your BMI Doesn't Tell the Whole Story
How well do BMI, waist size, and other measurements gauge obesity?
BMI's Drawbacks continued...
Frail or inactive people: Some people with BMIs that are in the normal range actually have lots of body fat though they may not look out of shape. Think of that as being fat on the inside.
"They have very low levels of muscle and bone -- often elderly people, those in poor shape, sometimes those who are sick. They tend to be somewhat frail. 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."
Very inactive people who rarely exercise may be of normal weight and BMI but be "overfat," Atkinson says. "They don't have as much lean body mass as they should, and they have more fat than they should."
Ethnic differences: Major ethnic differences exist regarding BMI, but few people are aware of this fact, including some doctors, Kahan says. "It's certainly something that needs to be communicated better to the public and the health care community -- and in a way that people can do something about it."
For example, Asian Americans tend to develop health risks, including the risk of diabetes, at significantly lower BMIs than whites. According to the Joslin Diabetes Center in Boston, a healthy BMI for Asians ranges from 18.5 to 23.9, a full point lower than the standard range of 18.5 to 24.9.
For Asians, a BMI of 24 or more signals overweight, compared to the standard of 25. More strikingly, Asians are considered obese at a BMI of 27 or higher compared to the standard BMI obesity measure of 30 or higher.
Genetics plays a role in body fat, Atkinson says. Asian ethnic groups, from Japanese and Chinese to Cambodians and Indians from India, tend to lay down fat in the midsection, he says. "When they start gaining weight, they gain it first in their abdominal cavity."
People of Indian descent are at highest risk, according to Atkinson. "With a lot of Indians who come to the United States and start living the American lifestyle, their weight and BMI may not make them look fat, but they are particularly at risk," he says. "They have excess fat in their visceral cavity, in their abdominal fat, and that's associated with diabetes, hypertension, metabolic syndrome, and all those kinds of things."
"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," Atkinson says.
In contrast, many African-Americans may have high BMI measures, but no associated health risks, Kahan says. "Because African-Americans tend to have a little bit more lean body mass -- bone and muscle -- than, for example, Asian Americans, they very often have a deceptively higher BMI than other populations."