A Quarter Of U.S. Adults Will Be 'Severely Obese' By 2030, Harvard Study Projects

In case you thought the obesity epidemic was petering out, think again. A New England Journal of Medicine study led by Harvard Chan School of Public Health researchers projects that roughly half the population will be obese and one-quarter severely obese 10 years from now.

Based on a national survey of more than 6 million adults and analysis of other health data, they estimate that 40% of American adults have obesity now, and 18% currently reach the severe level.

WBUR's Jonathan Cain spoke with lead author Zachary Ward from Harvard Chan’s Center for Health Decision Science. Their conversation has been edited.

Q: Could you summarize what you found?

We found that obesity is increasing everywhere in the U.S. No state is projected to be below 35% by 2030, and some states are approaching 60%. So we're finding overall that about one in two American adults will have obesity and about one in four will have severe obesity. That's a BMI of 35 or above. So we're looking at large increases in the number of people who have obesity and the degree or the severity of obesity.

How do those number compare to today?

We're at about 40% obesity nationally in the U.S. right now, and 35% for a state is sort of on the high end. And so we're finding that the best states, the states with the lowest obesity prevalence in 2030 are going to be at about the same level as some of the worst states now.

What's the impact and significance?

Well, obesity — and especially severe obesity — is associated with increased risk of diseases like diabetes, cancers, heart disease and has negative consequences for life expectancy. So we're looking at increased risk for morbidity, for increased risk of death and higher health care costs, especially for low-income adults who are at higher risk for severe obesity.

Could you explain your methodology?

The CDC has been collecting data on adult BMI for many years now — since the '90s — but it's based on a telephone survey. So they will call people and ask them, among other questions, how tall are you? How much do you weigh? And people will tend to overestimate their height — people want to be a bit taller — and people tend to underestimate their weight. And we found that this degree of bias really underestimates people's BMI, especially for people who are at a higher BMI. So we have a much larger degree of underestimation for people with severe obesity.

So we corrected for that by comparing the distributions of BMI from the self-reported data, telephone-based data, to nationally representative measured data where people actually go and measure people's height and weight. We applied this new method that we developed to adjust for this bias.

How big a correction was it?

We're finding that states are actually five to eight percentage points higher than reported just based on the self-reported data.

Is the line increasing steadily? What allows you to confidently predict what will happen by 2030?

We find that severe obesity, for example, is rising pretty rapidly. It's not really a straight line. And if you look at the trends, they're remarkably stable over the past 20 years. So we have a lot of data, data from over 6 million adults. And so we can fit trends with that with pretty good precision.

We also tested the accuracy of our approach. So we pretended we only had data up through 2010 and then made projections for 2016 and compared those to the actual data that was there. And we found that we were about 95% accurate. Now, 2030 is a longer timeframe than how we tested our approach. But the trends are pretty stable, and so we feel pretty good about the stability of these estimates.

What do you hope leaders do with this information?

We've been working with different state and local health agencies and health departments to model out interventions that they might be interested in, looking at the cost effectiveness of different policies and programs. So we hope that people will look at this and get a better sense of where their state is going, which groups are most at risk. And that the policymakers will decide to act on this now, while we still have a chance to change these trends.

Any other key points?

One is that the rise of severe obesity is especially concerning. We're finding that severe obesity, while it used to be a pretty rare condition, we're projecting that it will become the most common BMI category nationwide for women, for non-Hispanic black adults and for low-income adults or adults who make less than $50,000 per year. So we're finding large disparities by sex, race, ethnicity and income for severe obesity.

What has been proven effective to help address such disparities?

In previous work, we've looked at the potential impact of a sugar-sweetened beverage excise tax. And this is something that some states are considering now. Some cities have already implemented it. So it seems like sort of a low-hanging fruit — an obvious intervention that you might want to implement if you're serious about trying to address this obesity epidemic.