New research published Monday adds fuel to an ongoing debate in the public health community over whether a few extra pounds are good, or bad, for you.
Earlier research found that being somewhat overweight, but not obese, may result in a longer life.
But today's study in the Annals of Internal Medicine suggests that being slightly overweight may actually decrease a person's life span, which is more in line with conventional wisdom about weight.
So who's right? It's all about study design and statistical analysis.
Let's start with the newest study, headed by demographer Andrew Stokes at Boston University School of Public Health. His group found a 6 percent increased risk of dying from any cause among individuals with a history of being overweight.
Although Stokes says that 6 percent "is only a modest increase," it's still "extremely worrisome" because so many Americans are overweight.
"Our findings confirm that there is no benefit of being overweight on risk of death, and indicate that [being] overweight is actually associated with an increased risk of dying," he says.
These findings apply only to those who are overweight, not to obese people. There is little debate that people who are obese are at increased risk for diabetes, heart disease, certain cancers and even premature death.
Overweight is defined as a body mass index, or BMI, of 25 to 29.9, which is about ten to 30 pounds overweight depending on your height. Obese is defined as anyone with a BMI of 30 or above. You can calculate your BMI here.
About 38 percent of Americans over 20 years old are considered overweight, according to the Centers for Disease Control and Prevention. Another 30 percent or so fall into the obese category.
For their study, Stokes and colleagues gathered data on more than 225,000 adults over the age of 50 to determine whether being overweight affected life span. They included people participating in three major studies that lasted between eight and 20 years.
Stokes focused on each person's maximum BMI over a 16-year period, which he says makes the findings more reliable than earlier studies that have used a single BMI without regard to whether someone is gaining or losing weight at the time of the measurement.
Stokes's study was also more likely to exclude people who had temporarily lost weight due to illness.
His approach is different from that taken by the authors of the 2013 study that startled many in the public health community by suggesting that being overweight could lead to a longer life.
In that study, CDC epidemiologist Katherine Flegal and her colleagues analyzed results from 97 studies of obesity, covering nearly 3 million people, and found that the combined effect showed "a slight decrease in mortality" among overweight individuals, when compared to those of normal weight.
It was surprising news.
"Our article got called rubbish and ludicrous," Flegal told NPR in 2013, "so it really opens you to lots of criticism. I discovered, much to my sorrow, that this is kind of a flashpoint for people."
Flegal continues to stand by her results, and she criticized the new study for relying on people's memories of their own weight.
"We know people don't report their weight and height very accurately. Women tend to under report weight and men tend to over report height," Flegal, now of Stanford University, says.
She also says that her findings were more accurate than the new study's findings because her work was not based on participants' maximum weight, which might make them appear less healthy.
But Stokes says Flegal's study is "seriously flawed" because it fails to separate the effects of illness on weight from the effect of weight on risk of disease. "This new analysis provides a novel way of addressing this issue by using weight history to distinguish between people who were slim over time from those who were formerly heavy and lost weight after developing an illness," he says.
Meanwhile, it's not clear why being overweight would be protective or even life-extending. Some researchers speculate it could be that a little extra fat protects people if they fall, or that it offers an energy reserve during illness.
Faced with these conflicting findings, Dr. Steven Heymsfield says people who are overweight should check with their doctor to see if they have other weight-related health problems such as high blood pressure, blood sugar or cholesterol. Heymsfield is an obesity researcher at the Pennington Biomedical Research Center at Louisiana State University in Baton Rouge, La.
For people who do have those problems, there are generally effective treatments available for the conditions. At the same time, he says, people should work to prevent future weight gain and, if possible, lose weight and try to bring their BMI down to a healthier level.
Stokes says future research should look at whether overweight people who diet, exercise and lose weight can turn back their risk of disease to that of an individual who never gained weight in the first place.
"Individuals should try as hard as possible to maintain weight in a normal range for as large a portion of their adult life as possible," he says.
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