Body Mass Index, BMI, calculated from height and weight, provides a useful measure of overweight and obesity, according to the National Heart, Lung and Blood Institute. BMI provides a fairly reliable indication of body fatness for most people and can serve as a screening tool for weight categories associated with health problems. The higher the BMI, the higher the risk for certain chronic diseases. Knowing a person's BMI can provide information to help assess disease risk and plan steps to minimize or prevent health problems related to weight status.
Identification
BMI equals weight in kilograms divided by height in meters squared. Another way to define BMI is the ratio of weight in pounds divided by height in inches all multiplied by the constant 705, according to Kathleen Mahan and Sylvia Escott-Stump in "Krause's Food, Nutrition, & Diet Therapy." The BMI defines the level of adiposity. A score of 20 to 25 is associated with the least risk of early death.
Weight Status
Weight status, defined by the BMI calculated value, includes underweight below 18.5, normal weight 18.5 to 24.9, overweight 25.0 to 29.9 and obese 30.0 and above, according to MayoClinic.com. A higher risk of serious health problems is associated with a BMI greater than 40. Although one in three adults in America is considered obese, even a modest weight loss can improve or prevent health problems related to obesity.
Health Risks
Body weight and fat distribution relate to disease risk. Disease risk and risk of early death increases as BMI moves above or below the normal range of 18.5 to 24.9. Health risks related to obesity include diabetes, hypertension, cardiovascular disease, sleep apnea, some cancers, gallbladder disease, osteoarthritis, respiratory problems and pregnancy complications. An estimated 300,000 deaths each year are attributed to obesity-related diseases, according to Eleanor Whitney and Sharon Rolfes in "Understanding Nutrition."
Limitations
BMI is useful for most people, but it may overestimate body fat for athletes and other people who have a muscular build. For instance, a man weighing 245 lbs and 6 feet 3 inches tall with only eight percent body fat might appear to be overweight according to his BMI, but his weight is primarily muscle. BMI may also underestimate body fat for older persons and others who have lost muscle mass.
Although accurate weight and height measures are easy to take, they omit a couple of important pieces of information related to disease risk. How much of the weight is fat and where the fat is located are perhaps as important as the ratio of weight to height.
Considerations
Because of its limitations, doctors, dietitians and other health professionals use additional methods such as fatfold measures, hydrodensitometry which involves comparing weight on land and under water and bioelectrical impedance analysis using low density electrical current to more accurately estimate body fat and fat distribution.
BMI for children is calculated the same way as for adults but interpreted differently. Because of the growth curves for children and adolescents aged 2 to 20 years, the meaning of a specific BMI value for a child depends on the child's age in years and months and whether the child is a boy or girl. Consult a doctor, a dietitian or the CDC growth charts to interpret the BMI for a child.
References
- National Heart Lung and Blood Institute: Assessing Your Weight and Health Risk
- CDC: Healthy Weight - it's not a diet, it's a lifestyle!
- "Understanding Nutrition, Ninth Edition"; Eleanor Noss Whitney and Sharon Rady Rolfes; (2002)
- MayoClinic.com: Obesity Definition
- "Krause's Food, Nutrition, & Diet Therapy, 10th Edition"; Mahan and Escott-Stump; (2000)
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