Body Mass Index (BMI) is a statistical measure of an individual's scaled weight according to his/her height. It is a simple index of weight-for-height and is widely used by medical, health and fitness professionals to classify underweight, overweight and obesity in adults.
BMI is calculated by dividing weight (in kilograms) by height (in meters) squared. The World Health Organization defines overweight as having a BMI value of 25.0 to 29.9, while a BMI of greater than 30 is considered obese. A BMI value of 18.5 to 24.9 is generally considered normal, and a BMI less than 18.5 is defined as underweight. The cutoff points were chosen as a result of numerous research studies, both observational and epidemiologic in nature, which relate BMI to risk of disease and premature death. For children and adolescents, weight status must be determined by comparing the child's BMI with age- and gender-specific values (BMI growth curves).
Advantages of BMI
BMI is generally considered the best way to determine if an individual is at a healthy weight. Using BMI is popular because it is simple, quick, effective and applies to adult men and women, as well as children. BMI is a useful tool for quickly assessing weight classification. While it does not directly measure body fat, it is more accurate at approximating degree of body fatness than weight alone. In addition, you do not have to be of an exact weight or measurement, to be considered 'normal.' There is a range within each classification to allow for different body types and shapes. For example, you can be 10 to 15 pounds heavier than a same height counterpart and still fall within a normal weight range.
Disadvantages of BMI
BMI has limitations. Because it is not a measure of body fatness, very muscular individuals often fall into the overweight category when they are not overly fat. Additionally, BMI may place individuals who have lost muscle into the healthy weight category. Measuring BMI for very short people or pregnant women is not appropriate. It is believed that excessive abdominal fat is more health threatening than hip or thigh fat. A woman with a waist circumference greater than 35 inches and a man with a waist circumference of greater than 40 inches may be at an increased risk for developing high blood pressure, stroke and heart disease. Therefore, the National Institute of Health (NIH) has asked physicians to measure patients' waistlines.