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The best body mass indexes for low mortality rates

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ideal bmi healthy adultsWant to pencil in a healthy target weight for that New Year's resolution? A new analysis of data from 1.46 million adults has zeroed in on the body mass index (BMI) with the lowest risk of death from any cause—and they aren't the ones that most tilt the scales downward.


For healthy white adults who don't smoke, those with a BMI (based on height and weight but not body composition) between 20 and 24.9 had the lowest risk of death after a median follow up period of 10 years, according to the new report, which was published online December 1 in The New England Journal of Medicine. For a 179-centimeter (five-foot nine-inch) tall individual, that would be a weight between about 61 and 76 kilograms (135 to 168 pounds).


The researchers analyzed data from 19 studies that collected information on height, weight, lifestyle and demographics from non-Hispanic white adults ages 19 to 84.


With the abundance of data, "we were able to evaluate a wide range of BMI levels and other characteristics that may influence the relationship between excess weight and risk of death," Amy Berrington de Gonzalez, of the National Cancer Institute and co-author of the new study, said in a prepared statement.


Berrington de Gonzalez and her colleagues controlled for age, alcohol consumption, education, marital status, physical activity as well as variation across studies. (Most of the information collected in the various studies was self-reported by the subjects, but the researchers developed an algorithm to adjust calculated BMIs based on common self-reporting errors.)


Two thirds of U.S. adults are currently considered overweight or obese (having BMIs 25 and over). The new range of lowest mortality risk for healthy non-smokers falls within the current span of "normal" BMIs for men and women (which is 18.5 to 24.9).


When the whole subject population was assessed, however, those with higher BMIs (22.5 to 24.9) had the lowest mortality rates. But when the researchers omitted current and former smokers as well as those who reported cancer or heart disease, the optimal BMI range was 20 to 24.9. (Those with the lowest BMIs were the most likely to be current smokers, but those with the highest BMIs were the most likely to be former smokers.)


As the report authors note, underweight individuals (those with BMIs 18.4 and under) are a tricky bunch to assess and might still be skewing some of the results. Some might be slim due to undiagnosed illness that could increase their risk of an early death, and others might be leading a healthy, active lifestyle that could help them live longer. And the longer the follow-up period, the lower the mortality rate for people with lower BMIs, the researchers found. (Because the new study was a meta analysis of previously collected data, the researchers were not able to track the impact of weight loss or lifestyle changes on mortality risk.)


Those at the upper reaches of the BMI scale had the highest mortality risks. Obese women (those with BMIs of 30 to 34.9) who had never smoked and didn't report any disease at the outset of the study period were about 44 percent more likely to die during the follow-up period than those with BMIs of 20 to 24.9. That likelihood doubled to 88 percent for otherwise healthy women with BMIs of 35 to 39.9, the researchers found. As of 2008, some 17 percent of non-Hispanic women in the U.S. had BMIs 35 and above, the researchers noted in the paper.


In addition to the high rates of obesity in U.S. adults, the pattern might be especially worrying with the growing number of overweight young people. The researchers found that the overall increased risk of death for those with BMIs 25 and above was strongest in individuals who had topped that number before the age of 50.


Image courtesy of iStockphoto/GOSPHOTODESIGN

The views expressed are those of the author and are not necessarily those of Scientific American.

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