Why Muscle Mass Trumps BMI

muscle mass vs bmi
Why BMI isn’t all-that after all. Photo credit: Heather Neal

One of the first things you learn in the series of courses that leads to a nutrition degree is how to calculate BMI — body mass index. It’s one of the things you determine during every nutrition assessment no matter the cause for the visit. It becomes ingrained in your head. Hi, what’s your name? How much do you weigh? How tall are you? Oh, ok your BMI is___. Even the worst of mathematicians earns the ability to at least remotely estimate BMI upon hearing height and weight. It’s a mathematical equation that becomes second nature.

You might be wondering what a mathematical equation has to do with nutrition and health. Though I can calculate a BMI easier than I can divide 35 by 7, I can’t tell you its importance. Actually it’s the exact opposite — dietetics professionals have been wondering for years why we continue to use this seemingly arbitrary equation as such a solid predictor of health.

BMI is a measure of body mass. Essentially it’s another way to look at weight. At first glance, it’s a better measure than weight because it takes into account a person’s height. A 200-pound man that’s 6-and-a-half feet tall is completely different from a 200-pound 5-footer. BMI helps identify whether someone is at a healthy weight vs. overweight, obese, underweight, etc. The problem is that it doesn’t take into account body composition. That means it doesn’t matter whether that 6.5-foot man is 7% body fat or 47% body fat.

While this has been a long-known conundrum, BMI still gets used for many important things — insurance guidelines, medication use, disease predictors, etc. But maybe things can finally start to change. Finally. Research in The American Journal of Medicine examined thousands of individuals in a longterm study. They found that muscle mass was a better predictor of life span than BMI. Paring it down quite a bit, it basically means that you have a better chance of living longer if you have more muscle, regardless of your weight-to-height ratio.

That doesn’t mean we’re going to see BMI go out the window. BMI often tends to correlate with muscle mass correctly, as in most people with a high BMI have more fat. Those that have a high BMI because they have so much muscle are typically an exception to the rule. But what it does mean is that muscle mass is important. While we all know it’s important to keep our weight in check, maybe a better way of looking at the whole picture of health is to make sure our muscle is in check. Focus on strength training and other muscle-building activities instead of just rote cardio and dieting. (There’s good news hidden in that statement — you have to eat to build muscle!) This is especially true for women and the elderly, as women tend to have less muscle mass compared to men, and we lose muscle mass each year starting at a rather early age.