Why you can’t bank on living longer if you’re overweight

By Deborah Kotz, Globe Staff

I’ve watched my weight creep up with growing angst as I near the threshold for becoming overweight, so I was tempted to rejoice after reading the new study finding that moderately overweight folks live the longest. “Yay. I’m not that fat after all,” wrote one of my Facebook friends in her status update -- as if describing my thoughts exactly.

But there’s a potential danger in allowing the latest research to free us from the scale and abolish our “absurb fear of fat,” as a New York Times op-ed writer declared, adding that “our current definition of ‘normal weight’ makes absolutely no sense.”

Since the writer, Paul Campos, is a law professor, I figured it was probably wise to read through the details of the study, which was published on Tuesday in the Journal of the American Medical Association, and consult researchers in the obesity field to get their take on the new finding.

First off, the study -- which analyzed mortality data from nearly 3 million participants who took part in previous obesity studies in the U.S. and other countries -- didn’t find a huge difference in death rates among those who were normal weight, overweight, or even obese.

People who were overweight or mildly obese -- weighing between 146 and 203 pounds for a 5’4” person -- had a 5 to 6 percent lower risk of dying compared to those who were at a healthy weight, between 108 and 145 pounds. Those who were extremely obese had nearly a 30 percent greater risk of dying over the duration of the studies, which varied in length from five years to a few decades.

“What we found suggests that over a broad range of body mass index levels, there’s not much variation in mortality,” said study author Katherine Flegal, a senior scientist at the Centers for Disease Control and Prevention. “How much body weight affects health is a different issue, and we would tell individuals to consult their doctors about their own individual risks.”

For example, it’s pretty well accepted among public health experts that our nation’s increasing girth has contributed dramatically to a near tripling in US diabetes rates over the past 30 years -- from 2.5 percent to nearly 7 percent of the population.

“It’s important for doctors to keep doing what they’re doing as far as using body mass index as an indicator for health along with blood pressure, cholesterol levels, and blood sugar measurements,” Harvard Medical School nutrition professor Dr. George Blackburn said. He added that the new study had flaws in its methodology.

For example, older folks in the lower end of the healthy weight spectrum may have been more likely to have been former smokers with health problems that kept them slim. While the new research took into account current smokers, who tend to be thinner, it didn’t distinguish between those who never smoked and those who quit.

A 2010 National Cancer Institute study published in the New England Journal of Medicine performed a similar analysis of death rates among nearly 1.5 million people who participated in multiple studies -- but only analyzed deaths in healthy participants who never smoked. Those researchers found a clear relationship between increasing body mass index and death rates with a 13 percent greater death risk in moderately overweight individuals, which rose to a nearly 90 percent greater risk in those who were obese.

One of the study co-authors, Dr. Walter Willet, chair of the nutrition department at the Harvard School of Public Health, called the latest finding a “pile of rubbish” and said it was “absurd” to think that gaining 60 to 90 pounds wouldn’t have an impact on shortening our lifespan.

But other obesity researchers supported the latest findings. In an editorial that accompanied the study, researchers from the Pennington Biomedical Research Center pointed out that body mass index is an imperfect predictor of disease risk and must be considered in the context of other factors like a waist measurement -- which indicates whether there’s an excess of unhealthy abdominal fat -- physical fitness, and heart disease risk factors like high blood pressure.

Determining how much weight gain over time is too much probably depends on a person’s genes, diet, and level of exercise along with other lifestyle factors. But the evidence is clear from the vast body of research that getting to the extreme end of obesity will likely result in poor health and an earlier death. The challenge, as Willet pointed out, is to keep ourselves from reaching that point, which means taking action as the scale starts to creep upwards.

“If you put on 5 or 6 pounds, that’s the time when you need to build in a little more activity and be a little more careful about what you eat,” he said.

Deborah Kotz can be reached at dkotz@globe.com. Follow her on Twitter @debkotz2.