Monday, December 6, 2010

Weight and Mortality: Fear Not, It's Not That Bad

The Disease Management Care Blog is surrounded by death and destruction. Sea water will soon be lapping on its property line, radiation is in its airports, toxic mercury is in its compact fluorescent bulbs, killer E. coli is in its ground meat, invading Staph is on its skin and now the Grim Reaper is lurking in it's chubby abdominal paunch. At least that's the apparent message from this article published December 2 in the New England Journal. Mainstream media is spreading that alarm. Fat can shorten life. Fat is risky. Fat leads to higher mortality. Yikes!

But there's nothing like looking at the original article and underlying data to quell its panic. The authors pooled data from 19 observational population studies taken from the National Cancer Institutes's "Cohort Consortium," resulting in a database of 1.46 million adults. The median age was 58 years and the median body mass index (BMI) was slightly elevated 26.2 with a range at went from 15 to 50. The tricky part in the study was to statistically neutralize or exclude the impact of other determinants of mortality, like physical activity, past or current tobacco use, alcohol consumption, educational achievement and marital status. The authors then calculated hazard ratios for different categories of BMI. Hazard ratios can be thought of as measures of relative risk compared to a reference group.

In this study, persons with a BMI between 22.5 to 24.9 had the lowest mortality, so they were considered "one." Among non-smokers, the hazard increased as BMI increased: it was 1.03 if the BMI ranged between 25 to just over 27, 1.17 if the BMI was 27.5 to 29.9, 1.39 if it was 30-34.9, 1.98 (in other words, it almost doubled) if it was 35-39.9 and 2.92 if 40-49.9.

Sounds awful, right?

But while the hazard ratio from being fat can increase by "17%," or "39%" or "double," the DMCB asks: what does that mean, exactly? If in relative terms, if the death rate at baseline is one in a thousand and it "doubles" to two in a thousand, it that bad? On the other hand, if the death rate is one in ten and it doubles to two in ten, that sounds a lot worse. Which is it?

In the DMCB's read of the Journal article, it couldn't get a fix on this absolute hazard or risk of death. However, it took comfort in this graph using the same sort of data that appeared in a Lancet Letter to the Editor. As weight increases, yearly mortality "steeply" increases from about 5 per thousand (that's less than 0.5%) to about 8 to 9 per thousand (as in less than 1%). For smokers, the numbers are worse, but still in the range of 1-2%.

In other words, death rates can double with weight gain, but in absolute terms, the personal risk to the overfed DMCB seems to be pretty small. The "hazard ratio" increases 30 to 100%, but the absolute odds that DMCB will survive the year along with the rest of its skinny colleagues is very very good.

Better, says the DMCB, to relax. That's why, when it goes local mall for some holiday gift shopping and stops at the food court this season, it will fear not. Better, it says, to enjoy its shake, pizza and fries and celebrate.

1 comment:

Rambling Rachel said...

"You're all fat and you're going to die!" makes for good headlines. It also funds the swelling wellness industry. Please don't kill everyone's fun with your logic and refined analysis.

By the way, lots of people agree with your conclusion. Google "Kate Harding" and "Fat Acceptance." Antedotically, Grandma survived her 2009 sickness and its accompanying loss of 30 pounds because she was fat. She probably wouldn't have survived if she had been a skinny Grandma.