Measures of Maximum BMI, Weight History Linked to All-Cause Mortality
Incorporating weight history into studies on obesity and mortality may reduce confounding by illness in research results, which occurs when a preexisting disease affects weight and mortality risk.
A monotonic link may exist between maximum body mass index (BMI) and all-cause mortality, according to study results published in JAMA Network Open.
Researchers relied on data from the Framingham Heart Study original and offspring cohorts. Researchers noted maximum BMI over 24 years of weight history prior to follow-up for all-cause mortality and cause-specific mortality such as cardiovascular disease, cancer, or other causes. In total, there were 6197 participants (mean age, 62.8 at baseline; 55% female) with 3478 deaths throughout a mean follow-up of 17 years. Body weight in pounds and height in inches were recorded at every follow-up, and BMI was calculated at each examination as opposed to self-reported data.
BMI was categorized as underweight (<18.5 kg/m2), normal weight (18.5 to <25 kg/m2), overweight (25 to <30 kg/m2), obese class 1 (30 to <35 kg/m2), and obese class 2 (35 to <40 kg/m2). Normal weight was treated as the reference category. Using maximum BMI measurements, 77.3% of participants were classified as overweight or obese, while only 66.5% were overweight or obese using baseline BMI.
A monotonic relationship was noted between maximum BMI and mortality, with increasing risks recorded for class 1 obesity (hazard ratio [HR], 1.27; 95% CI, 1.14-1.41) and class 2 obesity (HR, 1.93; 95% CI, 1.68-2.20). No significant association with mortality was reported in the overweight category (HR, 1.08; 95% CI, 0.99-1.18). Among participants who never smoked, risk for mortality was increased for the overweight (HR, 1.31; 95% CI, 1.13-1.51), obese class 1 (HR, 1.57; 95% CI, 1.34-1.85), and obese class 2 categories (HR, 2.38; 95% CI, 1.95-2.90) compared with the full study cohort. For normal-weight individuals who were formerly overweight or obese, the mortality rates were 47.48 and 66.67/1000 person-years, respectively; individuals who never exceeded normal body weight had a mortality rate of 27.93/1000 person-years.
For cause-specific mortality, cardiovascular disease had larger mortality risks associated with maximum BMI compared with other outcomes.
The investigators suggested incorporating weight history into studies on obesity and mortality, which could reduce the poor outcomes of reverse causation due to weight loss from illness. “Regardless of the underlying mechanisms of weight loss in the present study, the fact that those who lost weight exhibited higher mortality risks in the present study reinforces the need to treat them separately from those who maintained normal weight across time, accomplished only by incorporating weight history,” concluded the researchers.
Xu H, Cupples LA, Stokes A, Liu CT. Association of obesity with mortality over 24 years of weight history findings from the Framingham heart study [published online November 16, 2018]. JAMA Netw Open. doi:10.1001/jamanetworkopen.2018.4587