BMI and all-cause mortality: a review

From the Lancet, “Body-mass index and all-cause mortality: individual-participant-data meta-analysis of 239 prospective studies in four continents“:

Background: Overweight and obesity are increasing worldwide. To help assess their relevance to mortality in different populations we conducted individual-participant data meta-analyses of prospective studies of body-mass index (BMI), limiting confounding and reverse causality by restricting analyses to never-smokers and excluding pre-existing disease and the first 5 years of follow-up.

Methods: Of 10 625 411 participants in Asia, Australia and New Zealand, Europe, and North America from 239 prospective studies (median follow-up 13·7 years, IQR 11·4–14·7), 3 951 455 people in 189 studies were never-smokers without chronic diseases at recruitment who survived 5 years, of whom 385 879 died. The primary analyses are of these deaths, and study, age, and sex adjusted hazard ratios (HRs), relative to BMI 22·5–<25·0 kg/m2.

You know there’s a big obesity problem, right? Unfortunately, much of the research looking at the relationship between obesity and health outcomes is flawed. Some of it leads to surprising conclusions. For instance, a number of studies exist that show a somewhat protective effect of obesity on mortality.

Of course, this could be reverse causality. Often, poor health leads to weight loss. Therefore, low BMI is sometimes a marker of poor health, not the cause. But observational studies can’t always tease that out.

This study used individual data on more than 10.6 million participants from 239 prospective studies from all over the world. Moreover, almost 4 million of the people were from 189 studies that looked at never-smokers without chronic disease who lived at least five years.

This allowed them to limit confounding and reverse causality. They looked at otherwise healthy people who weren’t near death, and who didn’t smoke (which is also related to both obesity and illness in different ways). And since they had individual-participant data, they could follow people prospectively over time.

Let’s start with a review of BMI. The Body Mass Index is a measure of body fat that’s calculated from height and weight. Normal weight is considered 18.5 to 25. Less than 18.5 is underweight. 25 up to 30 is considered overweight. 30 or over is considered obese.

BMI isn’t perfect. People are built differently, and this doesn’t account for additional muscle mass. It’s a rough estimate of fat, and we should treat it as such. That said, it’s a widely used and validated marker in general.

For people with a BMI from 20-25, all-cause mortality was minimal. Below that range, mortality jumped 13% for those with a BMI of 18.5-20 and 51% for those with a BMI of 15-18.5.

Overweight was associated with increased mortality as well. Those with a BMI of 25-27.5 (the low end of overweight) had a 7% increase. Those with a BMI of 27.5-30 (the high end of overweight) had an 20% increase.

And so did obesity. Grade 1 obesity (BMI 30-35) had an 45% increased mortality rate.  Grade 2 obesity (BMI 35-40) had an 94% increased mortality rate.  Grade 3 obesity (BMI 40-60) had an 176% increased mortality rate.

These relationships were seen all over the world. The association between increased BMI and increased mortality also appeared to be greater in younger people than older people, and greater in men than in women.

If you search the media, you can find articles that say that being overweight isn’t bad for you, and that “controversy” exists over whether being overweight is better for you for some health metrics. This meta-analysis is pretty good evidence against that argument.

The association between increased BMI (and underweight) and increased mortality is pretty robust in many populations living on four different continents. Having a high (or low) BMI is not good for you.


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