Are We Seeing An Upward Shift In Healthy Weights?Tuesday, May 17, 2016
Nevertheless, epidemiologists (and folks in health promotion) appear to like the notion that there is such a weight (at least at the population level), and often define it as the weight (or rather BMI level) where people have the longest life-expectancy.
Readers of this literature may have noticed that the BMI level associated with the lowest mortality has been creeping up.
Case in point, a new study by Shoaib Afzal and colleagues from Denmark, published in JAMA, that looks at the relationship between BMI and mortality in three distinct populations based cohorts.
The cohorts are from the same general population enrolled at different times: the Copenhagen City Heart Study in 1976-1978 (n = 13 704) and 1991-1994 (n = 9482) and the Copenhagen General Population Study in 2003-2013 (n = 97 362). All participants were followed up to November 2014, emigration, or death, whichever came first.
The key finding of this study is that over the various studies, there was a 3.3 unit increase in BMI associated with the lowest mortality when comparing the 1976-1978 cohort with that recruited in 2003-2013.
Thus, The BMI value that was associated with the lowest all-cause mortality was 23.7 in the 1976-1978 cohort, 24.6 in the 1991-1994 cohort, and 27.0 in the 2003-2013 cohort.
Similarly, the corresponding BMI estimates for cardiovascular mortality were 23.2, 24.0, and 26.4, respectively, and for other mortality, 24.1, 26.8, and 27.8, respectively.
At a population level, these shifts are anything but spectacular!
After all, a 3.3 unit increase in BMI for someone who is 5’7″ (1.7 m) is just over 20 lbs (~10 Kg).
In plain language, this means that to have the same life expectancy today, of someone back in the late 70s, you’d actually have to be about 20 lbs heavier.
While I am sure that these data will be welcomed by those who would argue that the whole obesity epidemic thing is overrated, I think that the data are indeed interesting for another reason.
Namely, they should prompt speculation about why heavier people are living longer today than before.
There are two general possible explanations for this:
For one these changes may be the result of a general improvement in health status of Danes related to decreased smoking, increased physical activity or changes in social determinants of health (e.g. work hours).
On the other hand, as the authors argue, this secular trend may be that improved treatment for cardiovascular risk factors or complicating diseases, which has indeed reduced mortality in all weight classes, may have had even greater beneficial effects in people with a higher BMI. Thus, obese individuals may have had a higher selective decrease in mortality.
There is in fact no doubt that medical management of problems directly linked to obesity including diabetes, hypertension and dyslipidemia have dramatically improved over the past decades.
Thus, it appears that the notion of “healthy” weight is a shifting target and that changes in lifestyle and medical management may have more than compensated for an almost 20 lb weight increase in the population.
This is all the more reason that the current BMI cutoffs and weight-centric management of obesity both at a population and individual level may need to be revisited or at least tempered with measures of health that go beyond just numbers on the scale.