National Guidelines For Body Mass Index

If you wonder if your weight is appropriate, calculate your Body Mass Index. BMI provides a commonly used indicator of underweight, overweight and obesity. Although the BMI has a few limitations, it offers a simple estimate of whether your weight jeopardizes your health. Weight and height measures are accurate, easy and inexpensive to take, but their ratio does not reveal how much of the weight is fat or whether the location of the fat is an added risk. Guidelines for interpretation of your BMI have been proposed by several national and international organizations.

Identification

BMI describes your relative weight for height by the formula BMI = weight in kilograms divided by height in meters squared. Another way of calculating BMI is BMI = (weight in pounds) divided by (height in inches squared) times a conversion factor of 703, according to the Centers for Disease Control and Prevention.

History

The World Health Organization first recommended a three-tier classification of overweight with BMI cutoff points of 25, 30 and 40 in 1995. The International Obesity Task Force introduced different terminology and an additional cutoff point of 35. In 1998, an expert panel consisting of members of the National Heart, Lung, and Blood Institute, together with members of the National Institute of Diabetes and Digestive and Kidney Diseases, defined overweight and obesity in terms consistent with those first introduced by the World Health Organization.

Function

Interpret BMI based on standard weight status guidelines for adults older than 20 years. BMI gauges your risk for chronic diseases associated with excess body fat. At BMI values below the healthy range, signs of illness and reduced reproductive function appear. Obesity-related diseases and increased mortality become evident above the healthy range. The higher your BMI, the higher your risk for developing heart disease, high blood pressure, type 2 diabetes, certain cancers and some breathing problems.

Guidelines

A person with a BMI less than 18.5 is categorized as underweight. A BMI of 18.5 to 24.9 represents a normal weight. A BMI of 25 to 29.9 is considered overweight. A BMI of 30 or higher is categorized as obese, according to the National Heart, Lung, and Blood Institute. Some guidelines categorize a BMI of 40 or higher as morbidly obese.

Limitations

Although the correlation between BMI and body fatness is fairly strong and consistent, it does vary by sex, race and age. BMI may overestimate body fat in persons such as athletes who have dense bones and a muscular build. It may underestimate body fat in older persons who have lost muscle mass. With the same BMI, women tend to have more body fat than men. Differences in body composition also exist among people of various ethnic groups.

For children, BMI is calculated the same as for adults but is interpreted differently. Because children experience constant growth and their normal growth may vary widely within a normal growth curve, BMI interpretation for children and teens depends on age and gender and is assessed as it changes over time. The CDC BMI-for-age growth charts facilitate the translation of your child's BMI into a percentile for her sex and age. BMI is only one of a number of factors, such as waist circumference, sedentary lifestyle and high blood pressure, to consider in assessing risk of chronic diseases related to overweight or obesity.