If you have been to see your doctor lately, he or she probably calculated your body mass index, or BMI, to determine whether you are at a healthy weight.
Based on height and weight, BMI is the most widely used method of assessing overweight and obesity. But a recent report has called the validity of BMI into question.
The article, published in the journal Science last week, suggests that BMI is not the best way to determine whether a person is overfat or the impact on their health.
The authors make their case by citing previous research that shows that many people with a high BMI aren’t necessarily overfat and that some people with a “normal” BMI may actually have high levels of body fat.
Furthermore, having a high BMI, even in the obese category, isn’t always associated with poor health. In fact, being overweight but physically fit is associated with a lower risk of death than having a normal BMI but being unfit.
This is all true, but that doesn’t mean that BMI isn’t useful. Since it is so easy to measure, BMI is widely used to determine the prevalence of obesity in the population. This works because most people with a high BMI really are overfat.
But this isn’t always the case. Some people with a high BMI are heavy because of excess muscle. Athletes tend to fall into this category, and can be misclassified as overweight or obese when they are actually lean. Even when an elevated BMI does reflect excess fat it doesn’t tell where that fat is stored.
Visceral fat, stored deep in the abdomen, is associated with greater health risks than subcutaneous fat, the fat beneath the skin you can pinch. For these reasons, many people are critical of BMI.
But BMI is one of many indicators of obesity and health and should be combined with other measurements. Your doctor wouldn’t make an assessment of your heart health based solely on your blood pressure. He or she would include your blood cholesterol and glucose, among other tests. Similarly, BMI shouldn’t be used in isolation to determine weight status or health risk.
The addition of another simple measurement, waist circumference, can make the BMI more informative. The combination of a high BMI and high waist circumference (above 40 inches for men and 35 inches for women) almost always indicates excess fat and that the fat is stored in the abdomen. This indicates a high level of visceral fat, suggesting a greater health risk.
But BMI and waist circumference don’t give a complete assessment of health. Another important component of what is called “metabolic health” is physical activity.
Participation in regular exercise is the major difference between people who are overweight and healthy and those who are overweight and unhealthy.
Even people who are lean but inactive have a greater risk of chronic conditions like diabetes, heart disease, and cancer than those who are “fit and fat.”
The criticisms of BMI are based on the fact that alone it isn’t always indicative of obesity and poor health. Adding waist circumference helps, but assessing physical activity is also important.
At your next appointment make sure you discuss your BMI, waist circumference, and exercise habits with your doctor as you would any other test results.
In the meantime, make it your goal to achieve and maintain a healthy body weight through good nutrition and at least 30 minutes of physical activity each day.
Brian Parr, Ph.D., is an associate professor in the Department of Exercise and Sports Science at USC Aiken where he teaches courses in exercise physiology, nutrition and health behavior.