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Jan. 13 2009 12:00 AM

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” for men and 35” 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.
The waist-to-hip ratio (WHR) is a simple calculation: waist circumference divided by hip circumference. A small waist combined with big hips yields a smaller number than a big waist with small hips — and smaller is better, when it comes to WHR. For women, the risk for heart disease, stroke, and other health problems starts to climb at a ratio of about 0.85, so that is often set as the cutoff for a “good” ratio. For men, the cutoff seems to be about 0.90. Waist circumference has eclipsed WHR, but the WHR may be ready for a comeback. Research shows that WHR is more strongly associated with heart disease than waist circumference alone. By taking hip circumference into account, the ratio is more sensitive to the difference between dangerous abdominal fat and the less harmful layer of fat we carry under the skin throughout the body.
Also in this issue:
  •        Stomach acid suppression
  •        Statins and high CRP
  •        Selenium and prostate cancer prevention
  •        Lifestyle choices and death rates
  •        Hot hands and warm hearts
  •        Skin cancer exams
The Harvard Health Letter is available from Harvard Health Publications, the publishing division of Harvard Medical School. Subscribe or by calling 877.649.9457 (toll-free).


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