Beyond the scale: how to measure whether you’re fat or not, from the Harvard Health Letter
Most of us have our private ways of assessing how fat we are. We feel our pants getting snug—or loose, if we're lucky. But there are more objective ways to answer the question. The January 2009 issue of the Harvard Health Letter provides a guide to three measures of fatness.
Body mass index, or BMI, is computed by taking your weight in kilograms and dividing it by the square of your height in meters. The BMI is easy to calculate, and in most people, it correlates reasonably well with overall body fat. It's also a good measure of health risk: as a rule, when BMIs go up, so do deaths, particularly from cardiovascular disease. But BMI doesn't distinguish whether the pounds are from fat or from fat-free tissue like muscle and bone. BMI also doesn't tell us about the type of fat we're carrying—a significant shortcoming, as the type of fat that builds up in the abdomen is believed to be particularly unhealthful.
Waist measurement puts a different spin on obesity: it's no longer about weight or total body fat, but about the metabolically active fat that collects around the organs in our abdomens. Waist circumference is a better predictor of diabetes than BMI and a good indicator of heart disease risk. Measuring it identifies the sizable group of people who pass muster when it comes to BMI but whose large waists put them at higher risk. Still, waist measurement hasn't become part of routine medical practice for several reasons. For one thing, there's some uncertainty about exactly where the waist should be measured, although navel-level is widely accepted. Moreover, the definition of too large a waist may need revision: some studies show that health risks start well before the current cutoffs of 40 inches for men and 35 inches for women. Finally, given all the other information that's collected on patients—blood pressure, cholesterol levels, BMI—it's not certain that adding a waist measurement to the mix would affect treatment decisions.