BMI is essentially a ratio between your height and your weight:
BMI is calculated by dividing an adult’s weight by the square of their height.
Why square the height? Because volume increases exponentially while height increases linearly. If you think of yourself as being basically a cylinder shape (as opposed to, say, a sphere — I hope you don’t look like a big fat ball), the amount of meat inside you doesn’t increase in a linear manner. Basically, as you get taller, you also get wider.
So, BMI sounds all sciency and it has math in it too, so it must be right? Wrong. Your BMI doesn’t tell you a damn thing about your body composition.
The BBC lays it out in their recent article, Can we trust BMI to measure obesity? (the answer is, of course, no):
There are some people who carry a lot of muscle and little fat, like bodybuilders, boxers and rugby players.
Muscle is much denser than fat so they may end up with a BMI that classes them as obese, despite the fact they may be fit and healthy.
Ah, the Brits! Notice the shout-out to boxers and rugby players. In the USA, we’d be talking about football players. No, not that football, the other one. With all the big dudes in pads.
But, on the other hand, very few of us are professional athletes. I hope you are, in fact, doing some kind of resistance training, even if it’s not at the same level or intensity as a body builder. But this kind of exercise will lead to an increase in muscle mass, and therefore your weight may increase even as the amount of fat you’re lugging around may drop.
If you aren’t doing any resistance training, and you’re getting older (like everyone else) and you have a great BMI, it might be because you are “skinny-fat” — you are losing muscle mass and replacing it with fat. From the BBC article:
As people age, they lose muscle and may be classed in the “healthy weight” range even though they may be carrying excess fat. This is particularly true of smokers.
In fact, this scenario is much more likely than you suddenly discovering that you are a bodybuilder.
The point is this: a healthy body is best measured by tracking your body composition, or the percentage of your body weight that is made up of fat. If it’s too high or too low (that’s right, I said it, you can have too little fat) then you are at greater risk for health problems.