Every January, fat’s in the crosshairs of health columnists, fitness magazines, and desperate Americans. This year, PopSci looks at the macronutrient beyond its most negative associations. What’s fat good for? How do we get it to go where we want it to? Where does it wander when it’s lost? This, my friends, is Fat Month.
Like a nice mutton, your body gets more fatty as it gets older. This is just the unfortunate reality of aging: lean muscle slowly degrades, some of it turning into fat(and some people stop thinking you look tasty).
This is not a huge problem for sheep, who lack the mental capacity to understand their body composition might affect their health, but it is a problem for human medicine. The primary way we judge a person’s obesity level is a metric called body mass index, or BMI. It’s calculated by dividing a person’s weight by their height squared, which means it’s really only an index of how a person’s height compares to their weight.
You may notice that the word “fat” did not appear in any part of that explanation. That’s because BMI is not actually a good indicator of how much body fat a person has, even though it’s the primary indicator we use to determine body fat percentage. Athletes who carry lots of lean muscle can end up with an “overweight” BMI because muscle weighs more than fat, and we know that small amounts of belly fat can put your health at risk even when your BMI says you’re within healthy range. BMI may generally correlate with how much body fat a person has, but because it only takes height and weight into account, it’s easy for this metric to miss the mark.
One study found that 8 percent of men and 7 percent of women are incorrectly told they’re obese despite having normal body fat content, while 41 percent of men and 32 percent of women are told they’re not obese even though they are. That error rate is … not great. Especially when obesity can lead to dire health problems like diabetes, heart disease, and cancer.
BMI gets a pass, though, because it has something that none of the other, more accurate, tests can offer: it’s cheap and easy. Every other method requires some kind of technical equipment and training to administer. To calculate your BMI, someone just needs to weigh and measure you. But despite its ease-of-use, there are other options doctors are exploring.