What better time than the first week of the new year for the Journal of the American Medical Association to announce that carrying a little excess weight might not kill you after all. There's still ample evidence that those who are medically obese are at risk of type 2 diabetes and a host of other potentially deadly problems. But according to a new mega-study that encompassed 100 other studies, those people who were merely overweight by the current charts actually lived a little bit longer than those whose weight fell into the "healthy" range.

Healthy weight obviously varies by height, so doctors have been using what they call the body mass index - a calculation based on weight and height. According to the CDC, 33.3% of American adults have a body mass index that qualifies them as  overweight and 36% as obese. If the new study turn out to have any merit, those categories might have to be revised and doctors could need to make a New Year's resolution to cut back on the nagging.

I wrote about the coverage of this story for my other blog, the Knight Science Journalism Tracker, since I'm fascinated by cases where scientists find contradictory results and the press swings the health advice pendulum back and forth.

Scientists are engaged in a heated disagreement on the fat study. The lead author is a highly credible researcher at the Centers for Disease Control and Prevention, but an equally credible scientist from the Harvard School of Public Health has been publicly calling her latest paper "rubbish".

When I read what these dueling experts were saying, however, I realized they agree that science has yet to firmly establish the weight range for ideal human health. We know obesity is bad, but there may be more room in the healthy range than the current charts indicate and a lot of individual variation in how much bulk we can carry for our respective heights.

The New York Times story on this new study used an interesting anecdote about a woman who was deemed by the medical examiner in 1917 to be the picture of "perfect" health. She was 5'7" and weighed 171 pounds. Today, it seems politically incorrect for a doctor to be commenting on the way a woman looks. But we all know some men and women have healthier looking proportions than others.

Even Walter Willett, the Harvard doctor who called the new study rubbish, admitted that relying on body mass index alone isn't enough to judge a patient's health. He was quoted in a couple of stories suggesting that unless you were obese or anorexic as a young person, you might be better off to use your own weight at 20 as your ideal.

Many commentators have noticed that the body mass index doesn't distinguish between fat and muscle, and therefore our current criteria would classify many body builders and athletes as overweight. On average, women don't carry as much of their weight in the form of muscle, but other studies have shown that it's abdominal fat that's bad, while some fat in the hips and butt is harmless and in your older years might give you some protective padding if you fall.

One intriguing aspect of this story is that for all the talk about "personalized" medicine, the obsession with scales and body mass index gives doctors and easy one-size-fits-all way to judge patients' health without having to really look at them. Someday, we may have personal DNA scans, but until then, some doctor could rely more on their eyes and less on the scales and charts.

The Times story didn't say how long their anecdotal "perfect" woman lived, but the online version linked to a follow-up that revealed she got to 90 and remained healthy until the very end. That's pretty impressive, though my great grandmother was also heavy and lived independently right into her second century. According to family lore she died at 105 only because she broke her hip after falling off a stepladder. I can't imagine her doctors nagging her to lose weight near the end of her life. If they had, she could have told them where to put their weight charts.