Body mass index (BMI) is a measure of human physical fitness that is designed to provide a standard metric for evaluating individuals’ weight, relative to their height. Specifically, BMI is calculated as weight divided by height (squared), as shown below. As such, BMI can be thought of as the ratio of weight to height, per unit of height.
BMI = kg/m2 or, BMI = lbs/in2 * 703
Based on BMI, an individual’s weight status can be classified as underweight, normal, overweight, or obese. Some professionals use an additional weight category of “extreme obesity” (or “morbidly obese”) when classifying weight status.
Weight Classifications Among Adults
Within adult populations, healthy body mass index values range from 18.5 to 24.9. Adults with a BMI of less than 18.5 are considered “underweight.” BMI values of between 25.0 and 29.0 classify individuals as “overweight,” and BMI values of equal to or greater than 30.0 classify individuals as “obese.” These weight classifications are based upon research findings associating higher BMI scores with increased incidence of health complications such as heart disease, diabetes, and asthma.
Weight Classifications Among Children
Although the same formula for calculating BMI is used with children and adults, the process for identifying normal versus unhealthy growth patterns differs between populations. Because body mass index changes substantially with age, rather than using specific cutoff values, weight classifications for children between ages 2 and 20 are determined by plotting body mass values along growth charts for age and sex. Children whose BMI values fall between the 15th and 85th percentiles on BMI-for-age-and-sex growth charts are considered to be of healthy weight. Children whose BMIs fall above the 85th or 95th percentiles for their age and sex are classified as “overweight” or “obese,” respectively. Similarly, individuals with BMI values below the 15th percentile are considered to be “underweight.”
Alternative Methods For Classifying Weight
Alternative methods exist for determining weight status including waist circumference, skin fold measurement, underwater weighing (hydrostatic weighing), and bioelectrical impedance. Waist circumference is a general method for determining unhealthy weight status and simply involves measuring a person’s waist circumference. For women, a waist circumference greater than 35 inches is considered unhealthy and 40 inches is unhealthy for men. Skin fold measurement involves using a caliper device to measure the thickness of a fold of skin and its underlying layer of fatty tissue. Specific locations that are to be representative of overall body fat include back of arm, front of thigh, and lower abdomen. Hydrostatic weighing is a method where an individual’s “dry” weight is obtained while wearing minimal clothing. Next, the subject is lowered into a water filled tank where they are weighed underwater. These two weights are then compared to determine the person’s body fat percentage. Bioelectrical impedance uses medical technology to determine body composition including percentage of fat, muscle, and water.
Why Use BMI?
Compared to procedures such as skin fold measurement, waist circumference measurement, underwater weighing, and bioelectrical impedance, body mass index proves to be a relatively uncomplicated, inexpensive, accurate, and reliable tool for classifying weight status. Whereas techniques such as underwater weighing and bioelectrical impedance require both extensive training and sophisticated equipment, BMI calculations require only a scale, height rod, and basic mathematical skills. Furthermore, unlike skin fold and waist circumference measurement, body mass index can be consistently calculated over time and by different people.
However, body mass index calculations are associated with a number of limitations as well. Although highly correlated with other assessment techniques for weight status, BMI calculations do not directly measure body fat percentage. BMI may overestimate body fat in individuals with higher than expected muscle mass (i.e., athletes) and may underestimate body fat percentage in individuals with little muscle mass (i.e., older adults). Similarly, BMI does not take into account the distribution of body fat within an individual, which is highly correlated with certain types of health risks. Thus, two individuals with identical BMI scores may have dramatically different body compositions.
As with any categorical system, specific values and percentiles separating healthy from unhealthy weights are, to some degree, arbitrary. That is, the prospective health status of individuals with respective BMIs of 28 (designated as “overweight”) and 30 (designated “obese”) is probably more similar than the different labels would suggest. Although BMI shares this fundamental problem with other measurement systems that utilize categorical systems, weight status as measured by BMI has repeatedly been shown to be a robust predictor of health problems in the research literature. Currently, BMI is one of the most widely used, accurate, and cost-effective methods for classifying weight status.
References:
- Cole, T. , Bellizzi, M. C., Flegal, K. M., & Dietz, W. H. (2000). Establishing a standard definition for child overweight and obesity worldwide: International survey. British Medical Journal, 320(7244), 1240–1243.
- Centers for Disease Control and Prevention (CDC). (2000).
- Growth charts. Retrieved from http://www.cdc.gov/growthcharts/
- Centers for Disease Control and Prevention (CDC). National Center for Chronic Disease Prevention and Health (n.d.). Chronic disease prevention. Retrieved from http://www.cdc.gov/nccdphp/
- Department of Health and Human Services. National Heart, Lung, and Blood http://www.nhlbi.nih.gov/
- Kraemer, C., Berkowitz, R. I., & Hammer, L. D. (1990).Methodological difficulties in studies in obesity: Measurement issues. Annals of Behavioral Medicine, 12, 112–118.