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. However, BMI should be used along with other measurements as it does have limitations.
About BMI
BMI is generally considered the best way to determine if an individual is at a healthy weight. It is calculated by dividing weight (in kilograms) by height (in meters) squared. While it is more accurate at approximating body fat than weight alone, BMI does not directly measure body fat. Using BMI is popular because it is simple, quick, effective and applies to both men and women. For children and adolescents, weight status must be determined by comparing the child's BMI with age- and gender-specific values (BMI growth curves).
BMI Meaning
The World Health Organization defines a BMI value of 18.5 to 24.9 as normal. Overweight is defined as having a BMI value of 25.0 to 29.9, while a BMI of more than 30 is considered obese. A BMI of 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. As BMI rises above 25, both blood pressure and cholesterol rise, while high density lipoprotein (HDL) or "good" cholesterol, decreases.
BMI Limitations
Unfortunately, BMI has many limitations. Because it is not a measure of body fatness, muscular individuals often fall into the overweight category when they are not overly fat. In contrast, 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 more than 35 inches and a man with a waist circumference of more 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.
In general, BMI is used to classify overweight and obesity as well as to estimate relative risk for disease when compared to normal weight. Along with other measurements, and keeping in mind its limitations, BMI is a useful tool for quickly assessing weight classification.
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