Louis Aronne, MD, professor of medicine, Weill Cornell Medical College, director, Comprehensive Weight Control Center at Weill Cornell Medicine and New York-Presbyterian, both in New York City.
Bottom Line: One of life’s great mysteries…explained!
I haven’t gained weight since college, but I have lost some height as I’ve aged. Since BMI is calculated based on height vs. weight, does that mean I’m now fatter?
The answer to that is a little complicated. Losing height as we age is inevitable, but even health professionals don’t agree whether body mass index (BMI) for older people should be calculated using current height…or height before inches were lost. BMI is one way to estimate body fat and provide a clue to overall health and risk for chronic health problems such as heart disease and diabetes. However, BMI is not the only way to do that—or even necessarily the best way. For one thing, BMI doesn’t take into account muscle mass. Muscle weighs more than fat by volume, which means that a fit, muscular athlete can have a BMI score within the “overweight” zone while actually having low body fat and low risk for fat-related health conditions. Losing muscle mass occurs naturally with aging and needs to be taken into account when deciding whether you are too heavy. And even if you are overweight, losing weight that is mostly muscle is not good for you. If you had to choose between the two—losing muscle to lose weight or staying a bit overweight to retain muscle—keeping the muscle (and the weight) through exercise and good nutrition would be the healthier choice. You need the strength that muscles provide! BMI also doesn’t take into account different body builds. Take two hypothetical people with the same BMI, for example. If one of them has a slight build (less muscular) and the other a heavy build (more muscular), the slight one will have more body fat—and yet the same BMI. We also lose bone mass as we age, so our bones weigh less, and whatever we weigh is proportionately more fat (because of less bone and muscle) than when we were younger. There are two other measurements that can help give you an idea whether your weight is just right or needs paring down—one is your waist circumference and the other is your waist-to-hip ratio. A waist of more than 40 inches for men and more than 35 inches for women is considered a general marker of being overweight—but depending on your height and build, your healthful waist circumference might be far less. For example, it’s recently been shown that a waist measurement more than 50% of your height is an indicator of health risks from being overweight. To get your waist-to-hip ratio, divide your waist measurement by your hip measurement. For men, a healthy ratio is 0.95 or less, and for women it’s 0.85 or less. As for how to measure BMI when you’ve lost height, as a practical matter, we use patients’ current, lower heights. You may end up with an uptick in your BMI, but take that with a grain of salt. Consider the rest of your overall health—and concentrate more on maintaining muscle than on losing weight.