Is Your BMI Lying to You?
Body mass index (BMI) is a formula for measuring optimal weight. It uses your height to adjust for your weight as it compares you to other individuals and populations.
For instance, two men may weigh 200 pounds, but if one is 5-foot-5 and the other is 6-foot-2, then weight alone is obviously a very poor predictor of which one is in fact overweight. BMI is the most common method of classifying normal versus overweight versus obese individuals. If your BMI is between 18.5 and 24.9, you’re considered to be in a healthy weight range for your height. If your BMI is between 25 and 29.9, you’re considered overweight. And, if the figure is 30 or greater, you’re considered obese. To determine your BMI, just use one of the quick BMI calculators available on the Web.
While BMI has been a more helpful measure of obesity than weight alone, when it comes to comparing obesity rates in two different cities or states, it can be a misleading indicator of health in individuals. That’s because BMI does not take into account the distribution of body fat. Remember, fat concentrated in the belly is much more dangerous than fat concentrated directly under the skin. For instance, you might have a professional athlete with a great deal of muscle mass who has an elevated BMI but little belly fat, or a severely overweight person with a high BMI who has fat predominantly concentrated under the skin. Both of these people may well be at low risk for prediabetes, diabetes, and heart disease. Conversely, you can have someone with a normal BMI who carries a dangerous amount of visceral fat in a potbelly. That person’s BMI might be normal because of thin arms and legs and little weighty muscle — but that person is nevertheless at increased risk for heart disease and many other diseases as well.
So if BMI can’t accurately predict if your fat is dangerous, what is a more accurate measure? The answer is your waistline. Two measures of waistline are commonly used: One is simply your waist circumference measured where it’s smallest, usually just above your belly button. (The waist circumference cutoff for the diagnosis of prediabetes in a woman is 35 inches; in a man, 40 inches.) The second measure is the ratio of your waist circumference to your hip circumference — known as your waist-to-hip ratio. To find it, measure your hips at the widest part of your buttocks, then divide your waist circumference by your hip circumference. For example, if your waist is 34 inches and your hips are 32 inches, divide 34 by 32; your waist-to-hip ratio would be 1.06. If your ratio is greater than 0.95 for men or 0.8 for women, you fall into the apple-shaped category, and it’s time to do something about that potentially lethal belly.