Why You Shouldn’t use BMI to Gauge Weight Loss

“Hit the gym, bro! You’ve got to knock down that BMI!” For over 100 years governments and health and fitness professionals have been saying things like this (sans the “bro” which is a recent addition), using BMI or Body Mass Index to gauge weight loss and measure people’s health. It’s popular because it reduces a complex system down to one simple number, which makes getting healthy seem comfortable. Hit the gym, reduce your BMI and get healthier…bro.

However new research is calling this conventional wisdom into question. It turns out, surprise surprise, that complex systems are…complex! While it may be attractive and convenient to imagine that all the various factors that determine our health can be summed by one measurement, it’s just not true. There just isn’t one single weight that’s healthy for everyone.

A recent UCLA study found that tens of millions of people classified as overweight or obese using BMI alone is actually healthy. More dangerous than this is the finding that roughly 20 million Americans rated as “healthy” by their BMI are, in fact, not healthy at all based on other measures.

So how can this be? How can so many professionals be so misguided?

BMI is a ratio of your weight to your height. The higher your weight relative to your height the worse your BMI which, it’s assumed, puts you at a higher risk for weight-related health issues. Problem is this ratio can’t tell the difference between fat and muscle. Per pound, muscle is about 18% denser than fat. This means a given volume of muscle weighs more than the same volume of fat.

As a result, many athletic, muscular people can appear overweight or obese if judged solely by their BMI. It’s common for two people, one fit and athletic, the other sedentary and overweight to have the exact same BMI. It shouldn’t take a rocket scientist or, say, a nutritionist or fitness expert to recognize this fatal flaw. A measurement is only as good as its predictive power and BMI, at least for individuals, shows little value in that area.

The other issue that BMI glosses over is fat distribution. Two overweight people with similar BMIs may have drastically different health trajectories because BMI doesn’t consider where the fat is stored. Someone that carries all their extra weight around their midsection is at a much higher risk for heart disease compared to a person of the same weight that tends to distribute their weight throughout their body or in less problematic areas.

If you’re concerned about weight loss, what can you do? How do you gauge a healthy weight if BMI is about as good at predicting health as shoe size is at predicting movie preferences? Most health and fitness professionals today agree that measuring your hip to waist ratio is far more accurate. According to the WHO, this ratio should be no more than 0.85 for women and 0.9 for men.
The same UCLA study described a man with a healthy BMI but a poor hip to waist ratio. Because he held most of his weight around his midsection, his mortality risk was 87% higher than another man with a similar BMI but a better hip to waist ratio.

Being concerned with your health is excellent, and there’s nothing wrong with concentrating on weight loss. Just be sure you’re doing it for the right reasons. And the next time you see someone stressing out about their health, say to them, “Hit the gym, sister! You’ve got to knock down that hip to waist ratio!”