Don’t Rely on BMI (Body Mass Index)

A few years back my BMI, or “Body Mass Index,” was 26.9 – well into the “overweight” category. It was right there on my medical chart, and every time I visited the doctor they’d be sure to remind me that I was “overweight.” My weight tracking and food logging apps displayed it ominously in red to alert me that I was in trouble. Too heavy! Lose weight! I got nagged about it from several directions.

The only problem is BMI can’t be correctly used to determine whether a person needs to lose weight or not. In fact, “losing weight” is the wrong issue to be thinking about entirely. Instead, if we need to be thinking about losing anything, we should be thinking about fat, not weight. We need to think in terms of body composition — our relative percentages of muscle, fat, and bone.

During this same period I did a DEXA (or DXA) body composition scan – the most accurate kind of body composition test available. My results showed I had a perfectly healthy body fat percentage, excellent fat distribution (less belly, more hips/legs), and great Relative Skeletal Muscle Index (RSMI). My bone density was  right at the top of the chart — and not just “good considering your age,” it was outright excellent, and way out of range for my age group. As the gentleman explaining the charts and graphs on my printout said, “You got all A’s!”

I was training hard in Aikido six days a week, and working with a strength coach, lifting weights. I had no lifestyle-related health problems, and was in the best shape of my life.

So the BMI can go suck an egg.

People who have lots of muscle and sturdy bones will often have a high BMI, while they are actually quite healthy and fit. This is common among athletes. Remember, muscle weighs more than fat. So strong, athletic, healthy people are incorrectly being told they need to lose weight.

The opposite situation, and probably a more widespread problem, is that many people have low body weight, and a “healthy” BMI, but they have very little muscle. That’s called being “skinny-fat.” They are not healthy, fit, and lean, just small. Their doctors won’t pester them to lose weight, instead they incorrectly congratulate them for doing great because they “aren’t fat.” But in addition to making us stronger and more capable, muscle mass raises our metabolism and helps moderate blood sugar levels. It’s healthier to have more muscle.

Either way, don’t trust the BMI to tell you how you’re doing. Sure, it can indicate something that needs further investigation, but it cannot definitively determine whether you are “fat” or not.

That’s not even what it’s for – it’s being applied incorrectly. Here’s more about why you should not be relying on the BMI as a measure of your health or fitness:

Read or listen to “Top 10 Reasons Why The BMI Is Bogus,” from NPR’s Weekend Edition Saturday, 4 July, 2009.