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October 19, 2020

How to calculate your 'Body Shape Index'—and why it might matter more than your BMI

Daily Briefing

    Plenty of research has shown that being overweight or obese is tied to a higher risk of premature death, but a new research review suggests that where a person carries excess fat "could make a big difference"—and even may lower a person's risk of an early death, Nicholas Baker writes for the New York Times' "Well."

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

    Risk of premature death may depend on where you carry your weight

    For the review, which was recently published in BMJ, researchers examined 72 prospective studies with data on mortality and body fat among more than 2.5 million participants.

    The researchers found that central adiposity, or having a large waist, "was consistently associated with a higher risk of all-cause mortality," Bakalar writes. Further, when the researchers looked at pooled data from 50 of the studies, they found that every increase of four inches in waist size was linked to an 11% increase in a person's relative risk of premature death, Bakalar notes—and the researchers determined that association was statistically significant after adjusting for participants' alcohol consumption, physical activity levels, and smoking status.

    However, the researchers found that excess fat in participants' thighs and hips were associated with a lower risk of death. According to the researchers, three studies show that every two-inch rise in participants' thigh circumference was linked to an 18% decline in all-cause mortality risk. In addition, nine studies that involved nearly 300,000 participants showed that a four-inch increase in a woman's hip circumference was linked to a 10% reduction in mortality risk.

    And when the researchers looked at waist size in comparison to other parts of the body, they found "still more information about the risk for premature death," Bakalar writes. When looking at two people who have the same hip size, the researchers found that the person with the larger waist had a higher risk of premature death.

    "For example," Bakalar writes, "consider one man with a 34-inch waist and 37-inch hips, and another with the same hip size but a 41-inch waist." According to the research, the latter man's "relative risk of death … was almost 50% higher," Bakalar explains.

    The researchers also discovered that just minute changes in waist-to-hip ratio were associated with drastic differences in risk. "In 31 studies that reported the ratio, each 0.1 unit increase in waist-to-hip ratio was associated with a 20% higher relative risk of death, with a stronger association in women than in men," Bakalar writes.

    Why is carrying fat in some places less risky than others?

    As Bakalar explains, having a larger waist indicates that a person is carrying higher levels of visceral fat, which is fat that's "stored in the abdomen around the internal organs." Research has shown links between visceral fat and an increased risk for developing certain life-threatening diseases, including Alzheimer's disease, cancer, heart disease, and Type 2 diabetes.

    In contrast, Tauseef Ahmad Khan, a postdoctoral fellow at the University of Toronto and a co-author of the research review, told Bakalar, "Thigh size is an indicator of the amount of muscle, which is protective … [a]nd hip fat is not visceral fat, but subcutaneous fat, which is considered beneficial."

    Unfortunately, however, it's likely not possible for people to change where their bodies carry fat, Bakalar writes.

    "It doesn't work that way," Khan explained. "You have to reduce overall weight, and that also reduces central fatness."

    Further, it's not clear whether "having too small a waist" carries risk, Bakalar writes.

    Kahn told Bakalar, "There is a range in these measures, … a range in which these numbers are beneficial. Above that range, there is higher risk, but more research has to be done about lower ranges."

    Want to know your risk? There's a calculator for that.

    It is possible to know your individual risk, however, by "using a formula called A.B.S.I., or a body shape index," which "put[s] many of these various factors together in a single measurement," Bakalar writes. He notes that, while the A.B.S.I. formula typically is used "as a research tool," anyone can use the tool to calculate their risk.

    According to Bakalar, the tool calculates a person's risk based on his or her age, height, sex, waist circumference, and weight—and it may provide "a more accurate estimate of risk" than a person's body mass index (BMI), which only accounts for a person's height and weight. He explains, "Each 0.005 unit increase in A.B.S.I. [is] associated with a 15% higher risk of all-cause mortality."

    Overall, Khan told Bakalar that "[t]he takeaway message" for people should be to "watch [their] waist size." Khan said, when it comes to fat and a person's risk of premature death, "[i]t's more important than a simple measure of weight. You can have a normal weight and BMI, but if your waist is large, that puts you at high risk" (Bakalar, "Well," New York Times, 10/1).

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