July 2008
Where BMI falls short
Researchers have long relied on the body mass index (available to ConsumerReports.org subscribers), since it factors in not just weight but also height. But as obesity science evolves, it's becoming increasingly
clear that there is simply no one-size-fits-all definition of a healthy BMI.
To begin with, age, ethnicity, and gender affect how the numbers add up. For example, a moderately high BMI appears to increase mortality risk more strongly in Asian men and women—as seen in the large Korean study—than it does in African-American women. And "BMI is not a precise health measure in older people," says Michael Leitzmann, M.D., an investigator with the National Cancer Institute. Being underweight is more likely to be a concern in that group because aging affects both appetite and the ability to absorb nutrients from food.
BMI also doesn't tell you much about fitness. Some couch potatoes have a fine BMI, while many moderately overweight people are very active.
In fact, the inadequacy of our language about weight became clear recently when researchers coined the term "normal-weight obesity" to describe supposedly healthy-weight people with a high percentage of body fat. That group is much more likely than muscular people of any weight to be at high risk for type 2 diabetes and heart disease, according to a recent Mayo Clinic study.
Other measures, such as waist circumference or the ratio of waist to hip size (available to ConsumerReportsHealth.org subscribers), may more accurately reflect body fat and health risk than weight or BMI. But none of those tools predicts longevity or quality of life as effectively as does a careful assessment of lifestyle factors such as diet and exercise habits.
To begin with, age, ethnicity, and gender affect how the numbers add up. For example, a moderately high BMI appears to increase mortality risk more strongly in Asian men and women—as seen in the large Korean study—than it does in African-American women. And "BMI is not a precise health measure in older people," says Michael Leitzmann, M.D., an investigator with the National Cancer Institute. Being underweight is more likely to be a concern in that group because aging affects both appetite and the ability to absorb nutrients from food.
BMI also doesn't tell you much about fitness. Some couch potatoes have a fine BMI, while many moderately overweight people are very active.
In fact, the inadequacy of our language about weight became clear recently when researchers coined the term "normal-weight obesity" to describe supposedly healthy-weight people with a high percentage of body fat. That group is much more likely than muscular people of any weight to be at high risk for type 2 diabetes and heart disease, according to a recent Mayo Clinic study.
Other measures, such as waist circumference or the ratio of waist to hip size (available to ConsumerReportsHealth.org subscribers), may more accurately reflect body fat and health risk than weight or BMI. But none of those tools predicts longevity or quality of life as effectively as does a careful assessment of lifestyle factors such as diet and exercise habits.
