New study provides strong evidence on dangers of excess weight
For immediate release: July 13, 2016
Boston, MA — 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, 2016 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 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.
For the new study, consortium researchers looked at data from more than 10.6 million participants from 239 large studies, conducted between 1970 and 2015, in 32 countries. A combined 1.6 million deaths were recorded across these studies, in which participants were followed for an average of 14 years. For the primary analyses, to address potential biases caused by smoking and preexisting diseases, the researchers excluded participants who were current or former smokers, those who had chronic diseases at the beginning of the study, and any who died in the first five years of follow-up, so that the group they analyzed included 4 million adults. They looked at participants’ body mass index (BMI)—an indicator of body fat calculated by dividing a person’s weight in kilograms by their height in meters squared (kg/m2).
The results showed that participants with BMI of 22.5-<25 kg/m2 (considered a healthy weight range) had the lowest mortality risk during the time they were followed. The risk of mortality increased significantly throughout the overweight range: a BMI of 25-<27.5 kg/m2 was associated with a 7% higher risk of mortality; a BMI of 27.5-<30 kg/m2 was associated with a 20% higher risk; a BMI of 30.0-<35.0 kg/m2 was associated with a 45% higher risk; a BMI of 35.0-<40.0 kg/m2 was associated with a 94% higher risk; and a BMI of 40.0-<60.0 kg/m2 was associated with a nearly three-fold risk. Every 5 units higher BMI above 25 kg/m2 was associated with about 31% higher risk of premature death. Participants who were underweight also had a higher mortality risk.
Looking at specific causes of death, the study found that, for each 5-unit increase in BMI above 25 kg/m2, the corresponding increases in risk were 49% for cardiovascular mortality, 38% for respiratory disease mortality, and 19% for cancer mortality. Researchers also found that the hazards of excess body weight were greater in younger than in older people and in men than in women.
Other Harvard Chan School authors included Walter Willett, Fredrick John Stare Professor of epidemiology and nutrition and chair of the Department of Nutrition; and JoAnn Manson, professor of nutrition and epidemiology and chief of the Division of Preventive Medicine at Brigham and Women’s Hospital.
Funding for the work of the coordinating center at the Harvard Chan School came from the National Institutes of Health (research grants PO1 CA87969, UM1 CA176726, UM1 CA167552, DK58845, P30 DK046200, and U54CA155626). The coordinating center at the University of Cambridge was funded by the UK Medical Research Council (G0800270), British Heart Foundation (SP/09/002), British Heart Foundation Cambridge Cardiovascular Centre of Excellence, and UK National Institute for Health Research Cambridge Biomedical Research Centre.
“Body-mass index and all-cause mortality: Individual-participant-data meta-analysis of 239 prospective studies in four continents,” Emanuele Di Angelantonio, Shilpa N. Bhupathiraju, David Wormser, Pei Gao, Stephen Kaptoge, Amy Berrington de Gonzalez, Benjamin J. Cairns, Rachel Huxley, Chandra L. Jackson, Grace Joshy, Sarah Lewington, JoAnn E. Manson, Neil Murphy, Alpa V. Patel, Jonathan M. Samet, Mark Woodward, Wei Zheng, Maigen Zhou, Narinder Bansal, Aurelio Barricarte, Brian Carter, James R. Cerhan, Rory Collins, George Davey Smith, Xianghua Fang, Oscar H. Franco, Jane Green, Jim Halsey, Janet S. Hildebrand, Keum Ji Jung, Rosemary J. Korda, Dale F. McLerran, Steven C. Moore, Linda M. O’Keeffe, Ellie Paige, Anna Ramond, Gillian K. Reeves, Betsy Rolland, Carlotta Sacerdote, Naveed Sattar, Eleni Sofianopoulou, June Stevens, Michael Thun, Hirotsugu Ueshima, Ling Yang, Young Duk Yun, Peter Willeit, Emily Banks, Valerie Beral, Zhengming Chen, Susan M. Gapstur, Marc J. Gunter, Patricia Hartge, Sun Ha Jee, Tai-Hing Lam, Richard Peto, John D. Potter, Walter C. Willett, Simon G. Thompson, John Danesh, Frank B. Hu, The Lancet, July 13, 2016, doi: 10.1016/ S0140-6736(16)30175-1
Harvard T.H. Chan School of Public Health
Dr. Emanuele Di Angelantonio
University of Cambridge
+44 754 083 9317
Harvard T.H. Chan School of Public Health brings together dedicated experts from many disciplines to educate new generations of global health leaders and produce powerful ideas that improve the lives and health of people everywhere. As a community of leading scientists, educators, and students, we work together to take innovative ideas from the laboratory to people’s lives—not only making scientific breakthroughs, but also working to change individual behaviors, public policies, and health care practices. Each year, more than 400 faculty members at Harvard Chan School teach 1,000-plus full-time students from around the world and train thousands more through online and executive education courses. Founded in 1913 as the Harvard-MIT School of Health Officers, the School is recognized as America’s oldest professional training program in public health.