Dear Mr. Dad: My 17-year old son is very athletic and in great shape. But his BMI puts him in the obese category. His pediatrician isn’t worried, but I am. How important is BMI?

A: BMI (Body Mass Index) has been around for more than 100 years, and over that time, medical professionals have found that it’s a fairly good indicator of a patient’s overall health and disease risk. Roughly speaking, it’s a ratio between height and weight.

A number of studies have found that for predicting obesity, BMI is 95% accurate for men and 99% accurate for women. And studies conducted at the National Heart, Lung, and Blood Institute (an agency within the U.S. Department of Health and Human Services) found that a high BMI is associated with numerous health risks, including Type II (adult onset) diabetes, high cholesterol, high blood pressure, heart disease, stroke, osteoarthritis, sleep apnea, gall bladder disease, cancer (breast, colon, pancreas, kidney, thyroid, and others), and mental health issues like depression and anxiety.

That said, BMI is far from perfect. For example:

  • BMI can’t tell the difference between fat and muscle. Since muscle weighs more than fat, a guy like your son probably has less body fat than a thinner guy who gets little or no exercise. The pediatrician has obviously determined that your son’s weight is coming from muscle, not fat
  • BMI doesn’t measure overall body fat or tell you where that fat is located. This is the biggest problem with BMI. A variety of studies have found that the distribution of fat is much more important than the amount. For example, an apple-shaped person (who carries his or her extra weight around the belly) has far greater health risks and a greater chance of dying than a pear-shaped person (who carries that extra fat in the butt and hips) with the same BMI.

If you’re really concerned about your son’s weight, his waist circumference is a better indicator than BMI of obesity and health risks. Researchers at the National Institutes of Health have found that at the same BMI level, men with a waist measurement of 102 centimeters (40 inches) or more, and women with a measurement of 88 centimeters (35 inches) or more, have a greater risk than those with smaller waist measurements of developing cardiovascular disease, diabetes, and other serious health issues.

For readers who are on the other end of the scale, so to speak, low BMI may actually be more deadly than being overweight, according to a recent study in the Journal of Epidemiology and Public Health. Joel Rey, the study’s lead researcher, said that adults who have a BMI of under 18.5 have 1.8 times times the risk of dying compared to adults with a “normal” BMI (18.5 to 24.9). People who are obese (BMI of 30-35) have 1.2 times the risk of dying, and those with BMI of over 35 have 1.3 times the risk of dying. Obesity, says Rey, is “an important public health and individual health issue. But in the process we’ve neglected the influence of being underweight on mortality.”

Health risks associated with low BMI include:

  • Increased risk of being deficient in important vitamins and nutrients
  • Increased risk of getting sick or developing infections (being underweight reduces immune system function)
  • Increased risk of developing respiratory problems and lung disease.
  • Increased risk of miscarriage. Women with a low BMI before getting pregnant are 72% more likely to have a miscarriage during the first trimester than those whose BMI was normal, according to a study published in BJOG (the International Journal of Obstetrics and Gynaecology). Taking supplements (including folic acid and prenatal vitamins), cut the miscarriage risk in half.

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