Converging Evidence

This study reported in Time is yet another one finding overweight as measured by BMI to be protective against the risk of death among elderly adults. (I was unable to find the study in PubMed or google scholar to link to an ungated copy. Readers with library rights can find it in the current issue of the Journal of the American Geriatrics Society.)

Australian men and women ages 70-75 self-reported height, weight, demographics, lifestyle (including smoking, drinking, and exercise), and health status characteristics. Participants were followed for ten years or until death, if sooner. Cause of death data was collected from the Australian Bureau of Statistics and classified as cardiovascular disease, cancer, or chronic respiratory disease.

The researchers found that both men and women with BMI in the overweight range had a lower risk of death in the following ten years compared to participants with normal weight BMIs. Their model finds the minimum mortality risk to occur when BMI is about 26.

I liked this about their analysis:

"A potential source of bias arises if, for some participants, illness has caused weight loss and this illness also increases the risk of mortality. To determine whether the presence of preexisting illness modified the relationship with BMI, men and women were categorized as healthy if they reported no prior history of diabetes mellitus, heart disease, stroke, hypertension, or chronic respiratory illness and if they were not current smokers. Regression models were also fitted conditional on 1-, 2-, and 3-year survival. This removed the influence of early mortality from the hazard ratio estimates, and these were compared with those obtained from the full cohort."

However, a significant problem with the cross-sectional or short-term longitudinal studies of the elderly is that all those people (fat or not) who died before they reached old age are not represented. To me, this makes these results sound potentially like, "People who are fat but still manage to reach the age of 70 despite the higher risk of so many diseases due to overweight, perhaps because they won the genetic jackpot for health and longevity, live longer than thinner people in their 70s who may not be as genetically blessed." I think we need longer-term longitudinal studies, starting when participants are younger, that include measurement of key variables of interests at regular intervals (unlike this study, which only measured weight, etc., at time 1) in order to better understand the influence of weight on mortality.

I think there is some common sense appeal to the idea that having a bit of a weight buffer when you're old might help if you get sick, can't eat, and start losing weight. Of course, too often results from these studies are interpreted to mean that it's healthier to be overweight, full stop. I have lost track of the number of times that study a few years ago in Obesity was linked to by young overweight bloggers to justify the healthiness of their weight even though the research only looked at people over age 65. Many of those robust oldsters may have been quite lean back in the day.

I still wonder how "normal weight obesity" (having a BMI in the normal range but a high amount of body fat and a relatively low amount of muscle mass), which has to be common among older people, affects the results of these studies. Would using a better measure of body fat give different results?

But there does appear to be converging evidence that overweight old people may not benefit in terms of mortality risk from weight loss. (The effects on general health and quality of life may be another story.) Interesting.