When you visit your doctor for a physical, it's likely that your exam will include a calculation of your body mass index, or BMI. It's also likely that your doctor will use the resulting score to assess your weight status and related health risks. But BMI is simply a measure of weight in relation to height, which doesn't necessarily reflect body fatness, especially in individuals. And even in large populations, where BMI is more closely correlated with adiposity, factors such as degree of physical fitness may trump body weight when it comes to disease and mortality risks.
"Often people don't understand what BMI is," says Dr. Steven Heymsfield, obesity researcher at the Pennington Biomedical Research Center. "It was never intended to measure an individual."
Calculating the curve
The index was the brainchild of Lambert Adolphe Jacques Quetelet (1796-1874), a Belgian mathematician and statistician who wasn't interested in obesity — he was interested in characteristics of the "average man." When trying to devise a formula that comprised the distribution of human body sizes, he found that weight tended to increase with height squared. Quetelet's index provided a measurement of body shape that was independent of height across a population.
"Repeated studies have shown that, almost invariably, BMI provides the best measure of shape as a predictor of adiposity in a population," explains Heymsfield. "If you take the entire U.S. population and plot percent body fat against BMI, you'll find a close correlation between the two."
Because of this correlation, the measurement was transformed from an index for epidemiological studies to a measurement to assess individuals. The problem with this, says Heymsfield, is that there is a wide range of actual percent body fat for any particular BMI score. In one study looking at data from the National Health and Nutrition Examination Survey, Heymsfield found that men with a BMI of 27 had body fat that ranged from 15% to 30%, which reflects a large difference in actual fatness. Because BMI is based solely on relative weight, it doesn't take into account bone structure or muscle mass.
A person who is overweight or obese will have a high BMI, but a person with a high BMI is not necessarily overly fat.
Apples and pears
Obesity is linked to a host of medical problems, including heart and liver disease, diabetes, and uterine and colon cancer. In an effort to identify people at risk of mortality from obesity-related disease, the National Institutes of Health established recommended weight guidelines based on BMI. But the drawback is that individuals with a different body type, gender, age, race, nutritional status, fitness level and mortality risk are all lumped together in the same category.
A person's body fat tends to increase with age, while muscle mass decreases — a change that may not be reflected in a corresponding change in BMI. Women tend to have more body fat than men with the same BMI score. And body composition of lean muscle mass, fat tissue and bone structure varies across racial and ethnic groups. In addition, two people with the same BMI (and even the identical percent body fat) can have very different patterns of fat distribution — "apples" versus "pears." Pear-shaped people tend to carry excess fat in their hips, where it is relatively inert, whereas apple-shaped people tend to carry excess fat in their abdominal region, where it is metabolically active and places them at higher risk for obesity-related diseases.
"Some people say that we should just throw out BMI and measure waist circumference to determine health risk," says Heymsfield. "The 2013 Obesity Guidelines say that we should use both. If you have a high BMI and abdominal obesity, you're at risk." He adds that there is a critical range, that BMI and waist circumference can be used together as a first step to determine health risk for people who fall in the ambiguous overweight or low obesity range. However, a BMI of 35 and above (reflecting severe obesity) is a clear indicator of elevated risk.
High BMI or low fitness?
The higher the BMI, the less chance someone has of being metabolically healthy. And in studies across large populations, a high BMI is correlated with a high risk of obesity-related disease, says Christian Roberts, director of the Exercise Physiology and Metabolic Disease Research Laboratory at UCLA. But, he emphasizes, "When you control for fitness levels, the effects of BMI go away. So the question remains: Is it a high BMI that is the problem or is it really low fitness?"
Studies have shown that aerobic fitness and strength training are more important than BMI in reducing disease risk and that obese people who exercise have a lower mortality rate than obese people who don't exercise. Regardless of a patient's BMI, Roberts recommends that, as a routine vital sign, physicians ask about levels of physical activity. He adds, "All physicians should recommend exercise … and prescribe physical activity as a therapy to reduce the risk of chronic disease."
Weight guidelines based on BMI
Body mass index, or BMI, was devised to give a measurement of weight that is independent of height across populations. Using the metric system of measurement, BMI is calculated by dividing weight in kilograms by height in meters squared. To adjust BMI to the standard system of measurement, divide weight in pounds by height in inches squared, then multiply the result by 703.
Weight guidelines based on BMI:
Underweight = below 18.5
Normal weight = 18.5-24.9
Overweight = 25.0-29.9
Obese = 30.0 and above
For the obese category, there are three grades:
Grade 1 = 30-34.9
Grade 2 = 35-39.9
Grade 3 = 40 and above