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Obesity may pose lower mortality risk than previously thought
The BMI value linked to the lowest all-cause mortality among Danish adults has increased in the past 40 years, indicating a possible need for a revision of the definition of BMI categories, according to recent study findings.
“Compared to the 1970s, today’s overweight individuals have lower mortality than so-called normal-weight individuals,” Børge G. Nordestgaard, MD, DMSc, professor of genetic epidemiology at the University of Copenhagen and Copenhagen University Hospital in Denmark, said in a press release. “The reason for this change is unknown.”
Nordestgaard and colleagues evaluated data from the Copenhagen City Heart Study collected in 1976-1978 (n = 13,704) and 1991-1994 (n = 9,482) and from the Copenhagen General Population Study collected in 2003-2013 (n = 97,362) to determine all-cause mortality rates associated with BMI levels. Follow-up was conducted from inclusion in the studies to November 2014, emigration or death, whichever came first.
BMI categories were defined as normal weight (18.5-24.9 kg/m2), overweight (25-29.9 kg/m2) or obese ( 30 kg/m2).
In 1976-1978, the BMI value linked to the lowest all-cause mortality rate was 23.7 kg/m2. That value rose to 24.6 kg/m2 in 1991-1994 and to 27 kg/m2 in 2003-2013. Similarly, the BMI values associated with the lowest cardiovascular mortality rate were 23.2 kg/m2 in 1976-1978, 24 kg/m2 in 1991-1994 and 26.4 kg/m2 in 2003-2013. For the “other” mortality category, BMI values associated with the lowest rate were 24.1 kg/m2 in 1976-1978, 26.8 kg/m2 in 1991-1994 and 27.8 kg/m2 in 2003-2013.
Mortality was greater in the overweight category (35/1,000 person-years) compared with the normal-weight category (28/1,000 person-years; HR = 1.04; 95% CI, 0.99-1.08) in 1976-1978 and in 1991-1994 (overweight, 22/1,000 person-years; normal weight, 15/1,000 person-years; HR = 0.97; 95% CI, 0.91-1.04); mortality was the same in the two categories in 2003-2013 (4/1,000 person-years).
Mortality was greater in the obese category compared with the normal-weight category in all time periods: 46/1,000 person-years vs. 28/1,000 person-years (HR = 1.31; 95% CI, 1.23-1.39) in 1976-1978; 28/1,000 person-years vs. 15/1,000 person-years (HR = 1.13; 95% CI, 1.04-1.22) in 1991-1994; and 5/1,000 person-years vs. 4/1,000 person-years (HR = 0.99; 95% CI, 0.92-1.07) in 2003-2013.
“Importantly, our results should not be interpreted as suggesting that now people can eat as much as they like, or that so-called normal-weight individuals should eat more to become overweight,” Nordestgaard said in the release. “That said, maybe overweight people need not be quite as worried about their weight as before.” – by Amber Cox
Disclosure: The researchers report no relevant financial disclosures.
Perspective
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The JAMA publication by Afzal and colleagues on BMI associated with the lowest mortality in Denmark highlights a new perspective on this question because it compares cohorts from three different time periods. The data show that the BMI associated with the lowest mortality increased by 3.3 kg/m2 from cohorts enrolled from 1976 to 1978 through 2003 to 2013. A possible explanation was put forth by the authors suggesting that while improved treatments for cardiovascular disease and other comorbidities has reduced mortality in all weight classes, the effects may be in higher BMI classes than lower BMI classes. This is plausible since obesity or high BMI, is a risk factor for diseases such as CVD and type 2 diabetes, and those with higher BMI would suffer from these at a higher rate than those with lower BMI. Thus, treatments for these comorbidities would be used in those with higher BMIs more often than lower BMIs.
The usual controversy in studying BMI and mortality revolves around how the studies control for confounding factors such as age, sex, smoking status, and occult disease causing weight loss before diagnosis. This study, having focused on three cohorts divided by time, brings in an additional factor, which is secular trends in disease treatment and shows that this may have affected the BMI associated with lowest mortality. In a sense, this study suggests that it is important to address quality of life and comorbidities and that they need to be factored in to any conclusion made from associations of BMI and mortality.