Body Mass Index. Is it useful or deceptive?

Philip J. Goscienski, M.D.

August 2005

Wouldn't it be nice to have a single numerical measurement that would tell you how healthy you are? Body Mass Index (BMI) is a relatively new way of looking at body weight - or overweight. It does give a clue to overall health, but in a limited way. It's nothing more than an expression of weight compared to height. In other words, if you weigh more than you ought to for your height, you'll have a high BMI.

To figure out your BMI, simply divide your weight in kilograms (pounds divided by 2.2) by the square of your height in meters (inches divided by 39.4). Spare yourself the math by going to one of the web sites that does it for you, such as www.caloriecontrol.org or www.webMD.com. A normal BMI is between 18.5 and 24.9. At 25.0 or more a person is considered overweight, and obesity starts at 30.0.

Those figures don't apply to everyone. A well-muscled but very healthy athlete can have a high BMI. Someone whose muscle mass has been replaced by fat, as in many older persons, may have a normal BMI, but be far from healthy. We can't apply the same BMI standards to all races. Given the same percentage of body fat as Caucasians, African-Americans and Polynesians have a higher BMI, and some Asian groups have a lower BMI.

Stone Age people had high BMIs because they were heavy-boned and carried more muscle than the average person today.

What about measuring body fat directly? If you exercise properly while cutting back on calories you're likely to melt away some fat as you add some muscle and end up weighing the same! That can be very frustrating if you only see the results on the scale. It would be useful to know that your fat percentage is going down but the current methods of calculating body fat are either inconvenient or inaccurate. Underwater weighing is the gold standard but it certainly isn't convenient. The accuracy of measuring skinfold thickness using calipers depends on the skill of the person doing the measuring, the age of the subject, the quality of the calipers and the method of calculation. An impedance analyzer looks like a bathroom scale but measures the difference in the flow of electricity between fat and other body tissues. Results may differ from one day to another depending on time of day, relation to meals, water intake, etc.

You don't need a calculator in order to figure out if you're at risk of diseases that are associated with being overweight. What is your waist size? If you are man with a waist circumference greater than 40 inches or a woman whose waist is larger than 35 inches you are at greater risk of heart disease, stroke, type 2 diabetes and cancer, no matter what other factors are present. And it doesn't matter if you're seven feet tall!

Philip J. Goscienski, M.D. is the author of Health Secrets of the Stone Age, Better Life Publishers 2005. Contact him at drphil@stoneagedoc.com.