Speaking from experience, when you're working hard to get fit, it's fairly easy to fall into the numbers trap—the digit on the scale might seem unfavorable on a certain morning, for example, but in reality, anything from regular bloating to muscle gain might be at play.
To that point, experts have been saying for years that our BMI, or body mass index, is just plain inaccurate. And after it was a government-issued standard for years, even the CDC now says that it's not a good marker for weight or fitness. Under its jurisdiction, which goes by weight in proportion to height, someone who is extremely fit might be deemed "overweight" or even "obese." That's because muscle is denser than fat, which often results in a higher weight—even if, technically, you have a smaller waistline. (Again, see aforementioned blogger for proof.)
New research released today not only underlines all this, but it also offers a more accurate way of measuring physical fitness. In the study, scientists observed more than 42,000 people over the course of 10 years, monitoring both their BMIs and their waist-to-hip ratios (a measure of how much weight they were carrying in their midsections). More than 5000 of the participants died over the course of the study—and the researchers found that, ultimately, body composition did play a factor into survival rate. It just had nothing to do with BMI.
The scientists concluded that those with normal BMIs who had a high waist-to-hip ratio were 22% more likely to die than those with normal BMIs who had lower waist-to-hip ratios. But what's even more telling is this: Those who had high BMIs but lower waist-to-hip ratios were less likely to die than those with normal BMIs with higher waist-to-hip ratios. In other words, those who were technically categorized as overweight or obese were still more physically fit than those in the "normal" weight range with a higher amount of belly fat. This categorically proves how little BMI matters in the scheme of physical fitness.
If you're curious about your waist-to-hip ratio, calculating it is easy: You just divide your waist circumference by the widest part of your hips and butt. By the World Health Organization's standard, any ratio that falls under .85 is considered healthy. But our additional two cents is to avoid making yourself crazy with all the numbers and just focus on how your clothes are fitting—and most importantly, how you're feeling. That's the metric that really matters, no?
Hamer M, O'donovan G, Stensel D, Stamatakis E. Normal-Weight Central Obesity and Risk for Mortality. Ann Intern Med. 2017;166(12):917-918. doi:10.7326/L17-0022
World Health Organization. Waist Circumference and Waist-Hip Ratio. Report of a WHO Expert Consultation, Geneva, 8-11 December 2008.