Being overweight or obese is associated with a higher risk of dying prematurely than being normal weight — and the risk increases with additional pounds, according to a large international collaborative study led by researchers at the Harvard T.H. Chan School of Public Health and the University of Cambridge, UK. The findings contradict recent reports that suggest a survival advantage to being overweight — the so-called “obesity paradox.”
The study was published online on July 13 in The Lancet.
The deleterious effects of excess body weight on chronic disease have been well documented. Recent studies suggesting otherwise have resulted in confusion among the public about what is a healthy weight. According to the authors of the new study, those prior studies had serious methodological limitations. One common problem is called reverse causation, in which a low body weight is the result of underlying or preclinical illness rather than the cause. Another problem is confounding by smoking because smokers tend to weigh less than nonsmokers but have much higher mortality rates.
“To obtain an unbiased relationship between BMI and mortality, it is essential to analyze individuals who never smoked and had no existing chronic diseases at the start of the study,” said Dr. Frank Hu, professor of nutrition and epidemiology at Harvard Chan School and a co-leader of the collaboration. Hu stressed that doctors should continue to counsel patients regarding the deleterious effects of excess body weight, which include a higher risk of diabetes, cardiovascular disease, and cancer.
In order to provide more definitive evidence for the association of excess body weight with premature mortality, researchers joined forces in 2013 to establish the Global BMI Mortality Collaboration, which involves over 500 investigators from over 300 global institutions.
“This international collaboration represents the largest and most rigorous effort so far to resolve the controversy regarding BMI and mortality,” said Shilpa Bhupathiraju, research scientist in the Department of Nutrition at Harvard Chan School and co-lead author of the study.
Other Harvard Chan School authors included Dr. Walter Willett, Fredrick John Stare Professor of Epidemiology and Nutrition and chair of the department of nutrition; and Dr. JoAnn Manson, professor of nutrition and epidemiology and chief of the Division of Preventive Medicine at Brigham and Women’s Hospital.