It's not looking good for the "obesity paradox," those comforting (to many) recent findings that being somewhat overweight may actually help people live longer.
A huge new four-continent study led by Harvard and University of Cambridge researchers looks at data on over 10 million people and concludes that no, if you control correctly for factors like smoking and chronic illness, heavier people do not live longer. Quite the opposite: The more overweight you are, it found, the higher your risk for premature death. (Being underweight also heightens risk.)
WBUR intern Annika Leybold spoke with Shilpa Bhupathiraju, research scientist in the Department of Nutrition at the Harvard Chan School of Public Health and co-lead author of the study. Excerpts, lightly edited:
What is the biggest takeaway from this study?
I think that this study really does inform the public that there is no protection from being overweight, and that the general population really needs to aim for a healthy body weight. You know, there was some confusion earlier, with reports stating that overweight might be protective, but what we see in our analyses is that once you take care of the biases and the other methodological limitations, that’s really not the case. From a public health standpoint, I think the public needs to be aware of the effects of excess body weight, which include higher risk of Type 2 diabetes, cancer, and cardiovascular disease.
Can you walk us through your study?
This is a major international collaboration with over 500 investigators from over 300 institutions in over 32 countries. The goal was to look at the relationship between body mass index and mortality across major global regions. We also wanted to specifically test to see if being overweight is associated with a higher risk of mortality.
What we found is that across all major geographical regions, being overweight and upwards was associated with a higher risk of death. And we found that this effect was greater in men than women, and the effects were greater in younger people than older people.
This study tries to reduce the methodological problems that we’ve seen in similar studies in the past. Can you explain that?
If you look at the BMI categories, the category from 18.5 to less than 25 is a pretty heterogeneous group. In that group we have heavy smokers, and we know that heavy smokers have a lower body weight, and it also includes people who have existing chronic diseases, and those with existing chronic diseases like cancer also lose body weight.
So, it’s a mix of really healthy people and those who are smokers and those who have existing chronic diseases. This is called reverse causation, where low body weight is actually the result of an underlying illness like cancer rather than the actual cause. The smoking issue is called confounding, where you know smokers tend to weigh less than non-smokers but they actually have higher mortality rates.
So, in our primary analysis we restricted our analysis to those who didn’t have any chronic diseases at baseline, and we limited the analysis to never-smokers, and we also excluded deaths in the first five years of follow up. If there were any deaths that happened in the first five years, we excluded those because those would more likely be due to the existing chronic diseases.
That’s how we tried to address these methodological issues. It's important to address them because you need to get an unbiased relationship between BMI and mortality. Once you have these really sick people and these heavy smokers thrown in with these healthy people, you don’t get valid estimates between BMI and mortality.
It’s common knowledge that overweight people have a higher risk of heart attacks and other health problems like diabetes that might put them at a higher risk for mortality — so what’s new about this study?
I think what we really confirmed is that the entire spectrum of excess adiposity, including overweight, is associated with higher risk of premature death. We also looked at specific causes of death, and again, we saw a higher risk for cardiovascular mortality, for cancer mortality, and for respiratory disease mortality.
The unique aspect of the study is that we looked at it globally. So we had studies from Europe, North America, Australia, New Zealand, and East Asia and South Asia as well. It really gave us a good global look at the relation between BMI and mortality across these major regions.
Details from the Harvard Chan School of Public Health press release:
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.