A nonsmoking man aged 40 years with obesity can expect to live on average 4.2 years less than a similar man of healthy weight, whereas a nonsmoking woman of the same age with obesity will live on average 3.5 years less than a similar woman of healthy weight, according to an analysis of more than 3.6 million adults in the United Kingdom published in The Lancet Diabetes & Endocrinology.
“BMI had a J-shaped association with overall mortality, and BMI outside the healthy range was associated with up to several years of lost life span, with most of the absolute mortality burden driven by obesity,” Krishnan Bhaskaran, PhD, of the department of noncommunicable disease epidemiology at the London School of Hygiene and Tropical Medicine, U.K., and colleagues wrote. “However, the overall association between BMI and mortality was driven by varying associations with individual, cause-specific mortality outcomes, including predominantly inverse associations for mental and behavioral, neurological and external causes.”
Bhaskaran and colleagues analyzed prospectively collected data from 3,632,674 adults in the U.K. Clinical Practice Research Datalink (CRPD), linked to national death registration data. Exposure was assigned as the earliest BMI recorded during CPRD research-standard follow-up or Jan. 1, 1998. To minimize reverse causality, researchers excluded the first 5 years of follow-up after the initial BMI record, with follow-up ending at death or on March 8, 2016. Researchers used Cox regression models to examine the associations between BMI and all-cause mortality and between BMI and cause-specific mortality outcomes, stratified by sex and adjusted for baseline age, smoking, alcohol use, diabetes status, socioeconomic status and calendar period. Researchers also used a Poisson model that included BMI, age and sex to estimate the expected age of death for men and women aged 40 years at baseline, by BMI category.
Within the cohort, 1,969,648 people were never smokers, of whom 188,057 died during follow-up.
The researchers observed a J-shaped association between BMI and overall mortality. For each 5-kg/m² increase in BMI, estimated HR for all-cause death was 0.81 (95% CI, 0.8-0.82) for those with BMI less than 25 kg/m², whereas the HR increased to 1.21 (95% CI, 1.2-1.22) for those with BMI at least 25 kg/m².
The researchers observed the same J-shaped association between BMI and most specific causes of death, including cancer and cardiovascular and respiratory diseases, with lowest risk for death in the BMI range of 21 kg/m² to 25 kg/m². For mental, behavioral, neurologic and accidental causes of death, BMI was inversely associated with mortality up to a BMI range of 24 kg/m² to 27 kg/m², with little association at higher BMI, according to the researchers.
The expected age of death for a never smoker aged 40 years with healthy weight was 82.2 years for men and 84.3 years for women, according to the researchers, with underweight, overweight and obesity all associated with reductions in life expectancy. Overall obesity, they wrote, defined as any BMI above 30 kg/m², was associated with a 4.2-year reduction in life expectancy for men and a 3.5-year reduction for women. Obesity class III, defined as BMI at least 40 kg/m², was associated with a 9.1-year reduction in life expectancy for men and a 7.7-year reduction in life expectancy for women.
“Assuming causality, we estimated that 4.3% of all deaths might be attributable to obesity and 5.5% to overweight including obesity,” the researchers wrote.
In commentary accompanying the study, Deirdre K. Tobias, DSc, assistant professor of medicine at Brigham and Women’s Hospital and the Harvard T.H. Chan School of Public Health, and Frank B. Hu, MD, PhD, the Fredrick J. Stare Professor of Nutrition and Epidemiology and chair of the department of nutrition at the Harvard T.H. Chan School of Public Health, wrote that the findings further support the strong association between increasing BMI and excess mortality at a BMI 25 kg/m² or higher.
“Although the excess mortality associated with overweight is relatively small, it is important for overweight individuals to prevent further weight gain,” Tobias and Hu wrote. “Even among older populations, a plateau or decline in body weight often masks a trajectory of fat mass gain, offset by losses in lean body mass (ie, decreases in muscle tissue density). Therefore, it is important for older individuals to prevent an increase in waist size, a marker of abdominal obesity, while minimizing loss of muscle mass.” – by Regina Schaffer
Disclosures: Bhaskaran reports he has received grants from the British Heart Foundation, the Medical Research Council, the Royal Society and the Wellcome Trust. Please see the full study for the other authors’ relevant financial disclosures. Hu reports he has received grants from the California Walnut Commission and personal fees from Diet Quality Photo Navigation, Metagenics and Standard Process.