TMI About BMI

I don't know what this means.

If you've ever been anywhere near a gym, personal trainer or fitness magazine, you have, no doubt been exposed to the notion of Body Mass Index (BMI). It's a simple calculation that produces a number that allegedly defines how fat (and therefore healthy) you are. Awesome, right?

What Is It?

In essence it's a proxy, a rough way of estimating how much of your body-mass is made up of fat tissue. It works like this:

Your mass in Kilograms divided by your height in Meters squared:

Ideally, the result should be between 20 and 25. Otherwise you would fall somewhere on this scale:

It's that easy!

Smell too easy to you? Yeah, me too.

What's Wrong With It?

Well the most obvious shortcoming is that it tries to quantify something (body fat percentage) without actually measuring that thing. This would be understandable if it was prohibitively difficult to measure body fat. If that were the case, it would make sense to use some other trick as a proxy - an alternate measurement that can provide reliable information about the thing you're interested in.

So we have two claims now:

1. Body fat is too difficult to measure directly.
2. BMI is a reliable proxy for body fat.

Let's take them one at a time.

Measuring Body Fat

As with most things, the whole idea of fat is more complicated than we might think. Firstly, there's more than just one kind of fat. Actually there are three kinds. When it comes to the kind of fat which, when found in excess, is strongly correlated with all sorts of health risks, the one we're talking about is Abdominal or Visceral Fat. That's also the one that piles up on your belly (or hips & thighs if you happen to be of the female persuasion). So that's also the one that will most likely be of interest to people going to the gym.

Visceral Fat

When it comes to measuring the fat content of your body, there are actually quite a few different ways of doing it, ranging from the impractical to the useless. And some others.

On the impractical side of things, we have the Dual-energy X-ray absorptiometry technique. This method involves a full body scan using two different X-rays in order to construct an image of your body's composition. Although this is an excellent tool for finding out exactly how much fat you've got, it's cumbersome and expensive. It's not exactly something you can do at home, or even at the gym. A qualified radiologist has to do it, not a personal trainer. And access to this sort of technology is better reserved for people who need it for medical purposes - like measuring bone density in patients suffering from Osteoperosis. Chubby-checking is probably low on their list of priorities, unless you're in obvious danger.

Skin-fold technique.

On the useless end is the skin-fold technique. As the name suggests, a section of skin is pinched with a pair of calipers, and its thickness is measured. While this is a perfectly good way of measuring the quantity of Subcutaneous Fat in your hypodermis, that's not the fat we're looking for. Visceral Fat doesn't live there.

But there is a ray of hope, and it comes in the form of a infra-red light! A beam of old-fashioned IR light can penetrate the skin and muscle tissue of your upper arm and, in doing so, reveals the quantity of Visceral Fat stored inside the muscle! Now, as we said, people tend not to store much fat in their biceps, but it's a pretty reliable extrapolation from the quantity of fat in your bicep (as compared to a statistical average) to the quantity in your abdominal region.

Fat Scanner Booth

The clear advantage of the IR beam technique is that it's reliable, inexpensive and easy. it can be done with a hand-held device, by a chimpanzee wearing spandex (or a personal trainer). Although it's not the sort of device you'd be likely to buy for home use, odds are your gym has at least one lying around. (If you live in South Africa and go to Virgin Active gyms like I do, that little Discovery Health booth that measures you in all sorts of ways, and records the results on your account, has one of these devices built in.)

Right, so, measuring body fat isn't really all that hard. Although getting a precise reading is difficult, it's easy enough to get a good, reliable result.


Reliability of BMI

Here's the thing: BMI was originally developed as a statistical heuristic for comparing relative body fat levels in populations against each other. And it seems to be pretty good at that. But it wasn't really ever intended for use by individuals as a diagnostic tool.

Firstly, it's based on some pretty heavy-handed assumptions about distribution of body fat in relation to height and mass. While it may describe the average person fairly well, outliers (like athletes, for example) will be poorly represented: weight-lifters generally register as "Obese" on the BMI scale (30 and over), while marathon-runners register as "Underweight" (BMI 16 - 18.5).

In addition, the BMI classification difference between Overweight and Normal doesn't seem to be supported by epidemiological data. There's no difference in life expectancy between those two groups.

The list goes on. BMI has its uses, but it's not really meant to help you track your own, personal health. It sucks at that.

So Then?

Essentially, if you're interested in tracking your body fat percentage as a metric contributing to your own health, it's probably best to steer clear of using BMI. Rather go with direct body fat measurements instead.

Better yet, talk to your doctor about what steps you can take to manage your health. Ask how much exercise you should be be doing, and whether or not you should see a Dietitian (NOT a Nutritionist!) about managing your diet. No website or magazine can give you a reliable estimate of how healthy you are.

And please remember that personal trainers, however muscly they may be, are not medically trained. If they were, they would be working in medical practice, not in your gym. That usually doesn't stop them from offering all kinds of advice on nutrition, supplementation and even medical interventions. You should take their advice on how many reps to do on the leg-press machine, but take everything else with a grain of salt. And when in doubt, check with your doctor.