Why Your BMI Doesn't Tell the Whole Story
How well do BMI, waist size, and other measurements gauge obesity?
BMI's Drawbacks continued...
Compared to whites with the same weight and BMI, African-Americans tend to have less visceral fat and more muscle mass, Atkinson says. Therefore, an African-American person with a BMI of 28, which the standard chart defines as overweight, might be as healthy as a white person with a BMI of 25.
Age and BMI: The notion of an ideal BMI may shift with age. "People who are older probably should have a little more fat on them -- not obese, they shouldn't have a BMI of 30," Atkinson says. But, he points out, 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 upon when fighting off an illness. However, it's hard to tell for sure since many factors affect someone's health.
Big BMI but Healthy Anyway?
Kahan sees some people who are fat but healthy; their BMI doesn't truly reflect their health risks.
Researchers describe such people as "MHO," or "metabolically healthy obese," Kahan says. "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. 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."
Measuring BMI is only a starting point when patients come to Kahan's center. There are many other ways to assess their body fat and health risks, he says.
Where's the Fat?
Are you shaped more like an apple, or a pear? The location of your fat may make a difference to your health.
There are exceptions, but generally, it's the abdominal fat, or the "apple" shape, that's metabolically riskier. When fat settles around the waist instead of the hips, risk of coronary artery disease and type 2 diabetes is more likely, according to the NIH.
"That tends to be more metabolically active fat, or fat that deranges your metabolism and your health parameters and ultimately leads to more risk," Kahan says.
Fat that accumulates on the hips and thighs -- the "pear" shape -- isn't as potentially harmful. "Many women have lower body obesity, so their hips and thighs are larger, but their waists are not so big," Atkinson says. Fat lower down on the body doesn't carry the same risk as belly fat, he says.
Other Ways to Measure
What else is there besides BMI? You may want to get out your measuring tape.
Waist Size: This is as simple as it gets: wrapping a tape measure around your waist. For an accurate waist measurement, Kahan says the tape measure should encircle your waist at the top of your hip bones in your lower back and go around to the belly button.