Easier isn’t always better. Never has this been more true than with body mass index, or BMI.
A measure that takes into account one’s height and weight, body mass index has been used since the 1800s as an easy way to estimate body composition and, by extension, health. After all, coming across a scale and tape measure isn’t too difficult. To calculate your BMI, that’s all you need.
BMI has been strongly linked with America’s most common diseases, conditions and causes of death, including heart disease, Type 2 diabetes and even cancer. That means, as BMI goes up, so does your risk of serious disease. For instance, an adult male who stands 5 feet, 10 inches and weighs 170 pounds would have a BMI of 24.4, which is the upper end of what is considered healthy. (A BMI of 25 is considered overweight.) That same BMI number would apply to a woman who is 5 feet, 4 inches tall and weighs 142 pounds.
BMIs of 40 or higher are linked with a significantly shorter lifespan, with the average person with a BMI of 55 to 59.9 living 13.7 fewer years compared to the average person with a normal BMI of 18.5 to 24.9, according to a 2014 review published in PLOS ONE.
“When studying large populations, we need an easy way of accessing body composition,” explains Dr. Lisa Neff, an endocrinologist who specializes in diabetes and metabolism at Northwestern Memorial Hospital in Chicago. “That’s why BMI is the marker most often used in health outcomes research. However, when we are studying BMI, what we are really hoping to get an assessment of is body fat. That’s the primary detrimental aspect of body composition. When we talk about risk, it’s body fat, or excess body fat, with which we are really concerned.”
But unfortunately, BMI and body fat are anything but the same thing. BMI is all about quantity and body fat looks at quality. For instance, if you have a high BMI, you could either be overweight (aka overfat) or, on the flip side, you could just really be muscular and have a perfectly healthy or extremely low body fat percentage, explains San Diego bariatric surgeon Dr. Julie Ellner. And, in fact, 2016 research published in the International Journal of Obesity shows that BMI incorrectly labels more than 54 million Americans as “unhealthy.” That’s nearly half of all overweight men and women – and 29 and 16 percent of those classified as obese or severely obese, respectively.
Meanwhile, if you have a normal BMI, you could be healthy – or you could be silently suffering from normal weight obesity, often referred to as being “skinny fat.” With normal weight obesity, weight and BMI check out fine, but body fat levels are just as high as they are in those typically considered obese. The same 2016 study found that more than 30 percent of people with so-called “healthy” BMIs have poor cardiometabolic health – that includes hypertension, high cholesterol, excess inflammation and/or insulin resistance. That’s exactly what BMI is supposed to help identify, and why your doctor probably has a BMI chart hanging in the exam room.
While athletes, bodybuilders and anyone who regularly strength trains is at risk of being mislabeled as overweight or obese, BMI is most likely to miss normal weight obesity in women as well as older people who may be suffering from age-related muscle loss known as sarcopenia, according to Neff. In many such older individuals, weight can actually be quite stable, she notes. In many men and women, as muscle levels wane, metabolic rate dips and any potential weight loss is mediated by an increase in fat. Year after year, weight stays about the same, although body fat percentage gradually grows, increasing the risk of obesity-related illnesses.
Better Than BMI: Body Fat, Waist Circumference and Waist-to-Hip Ratio
Since BMI is really just an estimation of body fat, it makes the most sense to stop tracking your BMI and start evaluating your body fat percentage, Ellner says.
“My advice to patients is to get a good body composition scale for home use,” she says. “It will calculate body fat percentage and will give credit to the patients who work out and are above-average healthy, while putting those who are at a ‘good weight’ but are unhealthy on notice that they need to change their program.”
These scales aren’t 100 percent accurate – technology still has a ways to go before consumer scales are on par with expensive dual-energy X-ray absorptiometry (DXA) scans, underwater weighing and caliper skin-fold measurements at determining exact body fat levels, but they are as precise as anyone vying for good health or healthy weight loss needs. While there is no one mutually agreed upon healthy body fat range, body fat percentages greater than roughly 21 in men and 31 in women point to obesity. Men and women need to maintain levels of body fat of at least 3 and 12 percent, respectively, for optimal physiological function.
While waist circumference and waist-to-hip ratio, like BMI, estimate body fat indirectly, they focus on where you store the fat that you have. That’s because visceral fat, which lies in and around the vital organs, is more highly correlated with chronic disease compared to fat stored elsewhere in the body, Ellner explains. “Abdominal obesity” is strongly linked to an increased risk of cardiovascular disease, regardless of BMI, according to the Journal of Cardiovascular Disease Research.
To track your waist circumference, wrap a flexible measuring tape or string around your waist, marked by the top of your hipbones. Divide that number by the circumference of your hips at their largest point, and you’ll have your waist-to-hip ratio.