The Body Mass Index & Waist Size

High levels of body fat are associated with increased risk of chronic diseases such as type II diabetes, hypertension, and cardiovascular disease. Body mass index (BMI) and waist circumference are two of the easiest and most practical methods for determining body fat levels and resulting health risks. Together, the body mass index and waist size are powerful tools for assessing increased risk of disease.

Calculating BMI

Calculate your body mass index by dividing your weight in kilograms by your height in meters squared. Body weight can be converted from pounds into kilograms by dividing by 2.2. Multiply your height in inches by 2.54 to convert into centimeters. Next, divide that number by 100 to achieve a height value in meters. For any given height, the body mass index will increase with escalating body weight.

Measuring Waist Size

Circumference measurements are an equally uncomplicated assessment tool. However, to ensure accuracy, these measurements must be taken at specific locations on the body. Anatomical landmarks are useful tools for identifying and remembering circumference measurement locations. Waist circumference, for example, should be taken above the hip bone line and at the narrowest point of the waist. If there is no obvious narrowest point, measure at the navel. Wrap a flexible cloth measuring tape around your waist and pull tight without causing any indentation of the skin. Your waist size can be recorded in inches or centimeters.

Determining Health Risk

The National Institutes of Health (NIH) identify six weight categories in the body mass index. Underweight is a BMI range under 18.5, normal weight between 18.5 and 24.9, overweight between 25.0 and 29.9, grade I obesity between 30.0 and 34.9, grade II obesity between 35.0 and 39.9 and grade III obesity over 40. As weight increases beyond the normal range, so does the risk of developing chronic diseases such as type II diabetes, hypertension, stroke, cancer and cardiovascular disease, according to the National Heart Lung and Blood Institute (NHLBI) at the NIH. Waist circumference adds another dimension to this analysis. Metabolic disease risks increase in women with a waist size over 32 inches (80 centimeters) and in men at over 37 inches (94 centimeters). These risks are significantly increased as waist size advances beyond 35 inches (88 centimeters) in women and 40 inches (102 centimeters) in men, according to Debasis Bagchi and Harry G. Preuss in their book "Obesity: Epidemiology, Pathophysiology, and Prevention." The Obesity Education Initiative asserts that an elevated waist size, even in someone with a healthy BMI, can be a marker for increased risk of these same chronic diseases.

BMI v. Waist Size

Although BMI is not an absolute measure of obesity, a strong correlation exists between the body mass index and body fat in the general population. As a tool for identifying obesity trends and disease risk, BMI serves its purpose well. Where the body mass index falls short, however, is in distinguishing general obesity from abdominal obesity. Excess body fat in the abdominal area is far worse in relation to disease risk than fat deposits elsewhere on the body. Abdominal obesity, independent of body mass index, has been strongly associated with development of cancer, osteoarthritis, type II diabetes, elevated blood pressure, high cholesterol and cardiovascular disease, according to Bagchi and Preuss. In fact, for a given waist circumference, overweight, obese and normal-weight persons have comparable health risks, according to a March 2004 article by Ian Janssen in the American Journal of Clinical Nutrition. Waist circumference is thus a useful addition to the body mass index, contributing a level of depth and analysis that further identifies individuals at risk for disease.

Lowering Your BMI

You can reduce your risk of developing obesity-related diseases just by losing 10 percent of your body weight, according to the NHLBI. There are simple lifestyle modifications that, if implemented now, will help you lose body fat and maintain a healthy weight as you age. The National Institutes of Health recommend 30 minutes of moderate-intensity exercise most days of the week to maintain health. Significantly lowering body fat may require up to 60 minutes per day, ideally in a combination of cardiovascular exercise and strength training. Diet, too, plays a vital role in lowering body weight. A study published in the American Journal of Clinical Nutrition in June 2003 found that consuming a diet rich in fruits, vegetables, reduced-fat dairy and whole grains and low in red meat, fast food and soda were associated with smaller gains in BMI and waist circumference. This combination of diet and exercise is the most effective way to keep your BMI under control and reduce your risk of disease.