The Body Mass Index, which is abbreviated BMI, is an assessment tool that compares weight to height to determine if you are underweight or overweight. In adults, a BMI value under 18.5 is underweight, a value between 19 and 24.9 is normal weight, a value between 25 and 29 is overweight, and a value of 30 or above is obese. However, the BMI only utilizes two variables, height and weight, and does not directly measure body fat. This can lead to inaccurate categorization for some people.
BMI Basics
The BMI is the most commonly used weight-to-height index. A BMI value provides a broad view of your body composition, particularly how much fat your body carries. Health professionals use the BMI to assess whether you are overweight or underweight and to determine your risk for diseases related to body fatness such as type 2 diabetes, heart disease, gallstones and high blood pressure. There are separate BMI indexes for children, teenagers and adults. Clinicians use BMI values for young children to assess progress along normal growth curves.
Strengths of the BMI
The BMI correlates strongly with body fat measurements for adults and can be calculated quickly. The categories of underweight, overweight and obese are based on actual Centers for Disease Control study of the relationship between weight, the development of disease and mortality. In this way, scientists find BMI values useful as a standard measure of body composition when comparing large groups of people.
Weaknesses of the BMI
The BMI does not utilize variables that directly measure body fat; it only takes into account height and weight. In particular, the BMI can be misleading for athletes. Athletes generally have a higher amount of muscle mass than the general population, thus skewing BMI values upward even though their body fat values are typically lower than normal. The BMI is not appropriate for pregnant or lactating women, and it may underestimate body fatness in older people as they typically have lower muscle mass.
Alternative Assessment Tools
Treat the BMI as a broad indicator of body composition. More detailed assessment of body composition can be obtained by utilizing multiple-site skin fold testing or bioelectrical impedance. Both of these methods measure body fat directly. Skin fold testing measures the thickness of subcutaneous fat by pinching folds of skin at various body sites and averaging the results. Bioelectrical impedance relies on the difference in electrical transmission rate through fat versus muscle tissue to calculate body composition.
References
- "Krause's Food, Nutrition, and Diet Therapy: Therapy," 11th Ed.; Dietary and Clinical Assessment; Kathleen Hammond; 2004
- Centers for Disease Control: Body Mass Index
- National Heart Lung and Blood Institute: Body Mass Index
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