How does waist size affect BMI?

How does waist size affect BMI?
According to recent statistics, over two-thirds of adults in the U.S. are overweight or obese. While the “middle-age spread” is considered by many to be an inevitable aspect of aging, it should be recognized that along with an increasing waistline comes increased health risks. The development of an “apple shape” to our bodies is an indication of increased deposits of visceral fat. As compared to a more obvious “beer belly”, visceral fat lies deep in the body around the abdominal organs. It is of significant concern as it has been linked to an increased risk for cardiovascular disease and type 2 diabetes. Additionally, in women it is associated with an increased risk of breast cancer and the need for gallbladder surgery.

What is the BMI and what does it tell us about our health? The body mass index (BMI) is a measure of body fat based on height and weight. The American Heart Association and National Institutes of Health consider this the primary tool in assessing obesity. The BMI calculator on the National Heart, Lung, and Blood Institute’s (NHLBI) website receives 1.6 million visitors a month and ranks #1 on Google. The NHLBI has even made a BMI Calculator App available for the iPhone. Based on the results of BMI calculation, individuals are ranked from being underweight, normal, overweight or obese. In general, the higher the BMI, the greater the risk of developing certain diseases. For example, someone with a BMI of 30 or higher, would be in the “obese” category—a group that has been shown statistically to have an elevated risk of heart attack and stroke.

Shortcomings of BMI in assessing obesity. The BMI provides meaningful information in most people, but it does have some limitations in assessing obesity since it depends solely on the weight and height of a person. For example, some people are naturally stocky and have a BMI in the overweight category, when in fact their weight is due to muscle mass and a heavier bone structure rather than excess fat. The BMI can also underestimate body fat in an older person who has lost muscle mass. Conversely, even with a BMI in the normal range, someone could be carrying more body fat than is good for him or her. As a result, it is best to use a second assessment tool to get a more accurate assessment of obesity.

What measuring your waist can tell you. The recommended tool to use along with the BMI to assess body fat distribution is waist circumference. It correlates well with an individual’s visceral fat and the risk of obesity-related morbidity and mortality. According to the National Institutes of Health, when the BMI is between 25 (overweight) and 35 (obese), a high waist circumference is associated with an increased risk for type 2 diabetes, high cholesterol, hypertension and cardiovascular disease.

How is the waist measurement performed? Follow these steps and consult this table to assess your risk:

  1. Place a tape measure around your bare waist, just above the hip bones.
  2. Make sure that the tape is level all the way around and fits snugly and not pushing into the skin.
  3. Relax, breath out, and measure your waist.

Women with a waist measurement of 35 inches (89 centimeters) or more indicates an unhealthy concentration of belly fat. In men, a waist measurement of 40 inches (102 centimeters) or more is cause for concern.

How else can body fat be measured? In most instances, a combination of BMI calculation and waist measurement provides enough information to assess one’s health risks due to obesity. Other ways of assessing for obesity include measuring skinfold thickness with calipers, underwater weighing, electrical impedance, and computed tomography. However, these methods tend to be impractical, expensive, or unreliable. If your personal assessment indicates that you need to work on your “belly fat”, next week we’ll look at some ways of doing this.

If you have any more questions just Ask Hanna, our health advisors are here to help.
Image: ©Shutterstock / Kyryk Ivan

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