Body Mass Index

Is BMI an Accurate Measure of Obesity?

It's important to note that although BMI is accurate most of the time, it may overestimate or underestimate body fat. For example, BMI doesn't distinguish between body fat and muscle mass, which weighs more than fat. Many NFL players have been labeled "obese" because of their high BMI, when they actually have a low percentage of body fat.

The BMI is not always accurate in elderly adults, who have often lost muscle and bone mass. Although their BMI might be within a normal range, they could still be overweight. BMI may also relate differently to various ethnic groups. For example, Asians may be at risk for health problems at a lower BMI than Caucasians.

Because of the possibility for error, BMI should be just one of many gauges used to assess a person's weight status and health. The National Institutes of Health (NIH) recommends that doctors assess whether their patients are overweight based on three factors:

  1. BMI
  2. Waist circumference - a measurement of abdominal fat
  3. Risk factors for diseases associated with obesity, such as high blood pressure, high LDL ("bad") cholesterol, low HDL ("good") cholesterol, high blood sugar, and smoking

Many health experts say that body fat percentage is a better indicator of weight status than BMI. But body fat isn't always as easy, or as inexpensive, to measure. Tests such as skin-fold measurements (in which a technician pinches a fold of skin on the patient's body to measure the subcutaneous fat layer just beneath the skin), dual energy X-ray absorptiometry (DEXA, which measures bone density), or bioelectrical impedance (which measures the opposition to a flow of electric current through the body -- impedance is low in lean tissue and high in fat tissue) are more precise, but they must be done by a trained medical professional.

Next, we'll learn about the history of BMI.