July 12, 2019

What your BMI fails to capture—and 3 easy-to-use metrics that might be better

Daily Briefing
    Editor's note: This popular story from the Daily Briefing's archives was republished on Feb. 12, 2021.

    Body mass index (BMI) is often used as a catch-all marker of a person's overall health—but several other simple measurements might offer a better glimpse into fitness and mortality risk, James Hamblin writes for The Atlantic.

    Why BMI is a flawed metric of health

    BMI is quick and easy to calculate: It's simply defined as a person's weight in kilograms divided by their height in meters squared. People with a BMI of between 25 and 30 are typically classified as "overweight," and those with a BMI above 30 are typically considered "obese."

    But while evidence shows a clear correlation between extremely high BMIs and higher mortality, the picture is fuzzier at lower levels of BMI. Hamblin cites a 2013 JAMA study that found individuals with stage 1 obesity—that is, whose BMI was at least 30 but under 35—had no elevated mortality risk relative to normal-weight individuals, while individuals who were classified as "overweight" experienced "significantly lower all-cause mortality."

    And that's not the only limitation with using BMI as a catch-all health metric. For example, based on his BMI alone, body builder Dwayne "The Rock" Johnson is classified as obese, Hamblin notes. Further, the metric ignores the health problems faced by people who have normal BMI despite having "excessive body fat."

    In fact, there's a stronger correlation between overall health and body-fat percentage and distribution than there is with overall weight, Hamblin writes—but getting an accurate measurement of someone's muscle-to-fat ratio is much harder than simply measuring their height and weight.

    3 metrics that might be better than BMI

    Still, there are several other easy-to-capture metrics that can be used to gauge fitness—and according to Hamblin, while they're all flawed in their own ways, they might nonetheless be a better predictor of overall health than BMI.

    1. How fast someone walks

    Walking speed has been shown in research to be predictive of an individual's health status, Hamblin writes.

    In a study published in JAMA, researchers looked at almost 35,000 people ages 65 and older and found that those who maintained a walking speed of about 2.6 feet per second over a short distance—equivalent to a 33-minute mile—were likely to hit their average life expectancy. Further, with every increase in speed of roughly four inches per second, the risk of dying in the next decade dropped by about 12%.

    2. Grip strength

    Several studies have found grip strength to be predictive of a person's health, Hamblin writes. In a 2018 study published in The BMJ, researchers looked at about 500,000 middle-aged people and found that the strength of a person's grip could predict risk of lung cancer, heart disease, and all-cause mortality fairly well.

    Another study, published in 2016 in the Journal of Epidemiology and Community Health, found that grip strength among people in their 80s was predictive of how likely they were to live past 100. And in a 2012 BMJ study, researchers found that grip strength among 18-year-olds in the Swedish military was relatively predictive of cardiovascular death 25 years later.

    3. Your ability to do a push-up

    Other research has found that a person's ability to do push-ups can be predictive of heart disease, Hamblin writes.

    Stefanos Kales, a professor at Harvard Medical School, noticed that the leading cause of death among firefighters on duty was sudden cardiac death, rather than smoke inhalation, burns, or trauma. These findings inspired his lab to look at the firefighters' push-up ability. They found that push-ups were a better predictor of cardiovascular disease than a submaximal treadmill test.

    "The results show a strong association between push-up capacity and decreased risk of subsequent cardiovascular disease," Kales said.

     What to make of these metrics

    While there's "nothing magic about grip strength or walking speed" or push-ups, "these abilities tend to tell us a lot," Hamblin writes.

    "Essentially, these quick metrics serve as surrogates that correlate with all kinds of factors that determine a person's overall health—which can otherwise be totally impractical, invasive, and expensive to measure directly," he writes (Hamblin, The Atlantic, 6/27; Männistö et al., British Journal of Nutrition, March 2014).

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