Is BMI an accurate measure of obesity?

Body mass index (BMI) is based on a formula that uses both weight and height to estimate associated health risks. It was invented between 1830 and 1850. Due to its ease of use, it is the most widely used diagnostic tool to identify weight problems within a population, whether individuals are underweight, overweight or obese. Though it does not actually measure body fat percentage, your BMI is a general measurement that can help guide you in determining a diet and exercise program.

However, the body fat percentage you get from a body fat scale or hand-held body fat device can vary depending on many factors. How warm or cold your body temperature is at the time you take the test, how hydrated you are when the test is taken. In fact, it's difficult to get an accurate body fat measurement from most commercially available body fat analyzers. A hydrostatic body fat test is the best and most accurate way to have our body fat taken. A hydrostatic body fat test is where you are submerged in water. This kind of  testing is a more exact measurement because bone, muscle and connective tissue, known as lean mass, sinks while body fat floats, giving you a more accurate measurement.

Body Mass Index is accurate most of the time. However, it may overestimate or underestimate body fat. Common examples:

• BMI doesn't distinguish between muscle mass and body fat. Muscle mass weighs more than fat, which is why many pro football players have been labeled "obese" because of their high BMI. In fact, they have a low percentage of body fat.
• BMI is not always accurate in the elderly, who often have less muscle and bone mass. Their BMI might lie within a normal range, but they could still be overweight.
• BMI may relate differently to some ethnic groups. For instance, Asians may be at risk for health problems at a lower BMI than Caucasians.

The chance for error means BMI should be only one of many gauges used to assess a person's weight status and health. The National Institutes of Health recommends three factors to doctors in assessing whether their patients are overweight:

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

Many health experts say body fat percentage is a better indicator of weight status than BMI. But body fat can be difficult, and sometimes costly, to measure.

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Some people who have an "overweight" BMI may have a normal amount of body fat with a large muscle mass, and others who have a "normal" BMI may have excess fat and reduced muscle mass. However, on average, these statistics hold true and are a warning sign to all Americans who consume more food than necessary.

Body mass index (BMI) is a calculated ratio between your height and weight. This formula is a quick measure of body fat that fits most of the population. However, muscular athletes often find themselves on the overweight or obese portion of the BMI chart because it does not take into consideration the actual percentage of body fat. Muscle is more dense than fat, but the chart does not differentiate where the weight is coming from.
 
For example, a defensive back who is six feet tall and 220 pounds with 5% body fat is labeled as borderline obese on the BMI chart. For athletes, a more accurate measure of body fat is done with a skinfold caliper or underwater weighing.

(This answer provided for NATA by the Appalachian State University Athletic Training Education Program.)