BMI

Like many recycled cyclists, I have a story about a body mass index (BMI) that was much different than the one I currently can calculate. In the "pre" version of my BMI, I was out of shape, overweight, and not using much judgment in nutritional matters. Of course, I could rationalize how I looked in the mirror, and the occasional diet plan would knock enough weight off sporadically to give me false hope that I might find a sustainable non-exercise way to stay trim.

Now, in the "post" version of my BMI, my body fat percentage has fallen more than 10 percentage points, clothes that used to cling now hang, and muscle contours are clearly discernible. Yet, my BMI number has only fallen a few points. Why is this?

BMI has many problems as an accurate measurement of overweight in active people. Interestingly, part of its design was to account for the fact that most members of the general public will overestimate their height and understate their weight in the process of calculating their BMI. Therefore, the BMI scale was designed to accommodate these tendencies. This means that active people, who generally know their actual height and weight, will appear to have higher BMIs than average (they will calculate as relatively shorter and heavier than a sedentary peer who just estimates his or her height and weight as taller and lighter than actual).

In addition, the BMI measure does not take into account the fact that muscle weighs more than fat. If you train in the hills a lot and are generally a mesomorph (someone who adds muscle mass easily, a jock), you will gain muscle mass while maintaining or shrinking your fat stores. In either eventuality, you will weigh more despite having accomplished a generally commendable thing -- losing fat and gaining muscle.

Finally, BMI does not address anything like cardiovascular or respiratory health. So, a sedentary smoker may have a great BMI of 23 but an LDL cholesterol that is dangerously high and an inability to climb a flight of stairs without becoming winded. BMI is not a measure of health, in other words. It is a potential signal, but not a precise one. In fact, many who think hard about these things are beginning to drop BMI as a measure. It is too crude, even as a public health tool. For instance, BMI in Africa is quite low, but life expectancy and incipient health are both worse than for places with higher BMIs but better health and social infrastructures.

For cyclists, BMI should also be shunned as a measure of fitness or health, I think. My experience these past years has been of losing fat, gaining muscle, and maintaining my weight (even increasing it slightly this past year). Yet, despite gaining weight, my power:weight ratio seems better than ever, because what I've gained has been muscle. My legs are better-defined, small areas of flab have disappeared, and I continue to shrink in profile. I've become leaner and meaner, BMI be damned.

Cyclists are typically in-tune with their health. You probably know your cholesterol levels, your resting heart rate, your body fat percentage, your maximum heart rate. You probably have a nutrition plan, and a training plan. You don't smoke, and probably drink less alcohol than your counterparts who don't ride. And, best of all, you know how you feel when striving to do your best on your bike. This is the real test of fitness and BMI, the bicycle mass index. When you fly up a hill, you know your BMI is right. Otherwise, it's just a number.