Fewer Americans smoke. More are obese. Is there a connection?

Gut Check is a periodic look at health claims made by studies, newsmakers, or conventional wisdom. We ask: Should you believe this?

The Claim: Because quitting smoking causes many people to gain weight, the decline in smoking since the 1960s helped fuel the rise in the rate of obesity, argues a paper by three economists.

The Backstory: Gaining weight after quitting smoking is so common you almost don’t need research to “discover” it. But since Gut Check believes in data and not anecdotes, we recommend this 1991 study, which found an average 6-pound gain for men and an 8-pound increase for women; this one from 1998, which found average gains of 11 to 13 pounds; and this 2012 meta-analysis of 62 studies, which concluded that quitting adds 10 pounds. The 15 studies reviewed by the US Surgeon General in 1990 found that four-fifths of people who quit smoking gained weight (average: 5 pounds). Similarly, there is no question that Americans have been packing on the pounds in recent decades: in the early 1960s, 13 percent of US adults were obese; now, 35 percent are. Over roughly that period, the percentage of adults who smoke fell from 42 percent to 19 percent.

First Take: The new study, published online by the National Bureau of Economic Research and submitted to an economics journal, finds a higher weight gain from smoking cessation than much other research has — 11 to 12 pounds for an average-height adult, or 1.5 to 1.7 body mass index units. Younger adults, women, the less educated, and smokers who had a lower BMI when they quit gained the most. Contrary to earlier studies, the economists find that the weight gain lasts for five years or more rather than melting away in a year or two.

They used a ton of equations to calculate how much the drop in smoking contributed to the rise in obesity rates. (If you’re uncomfortable with concepts such as “heteroskedasticity-robust standard errors,” we cannot in good conscience recommend reading the whole paper.) Their answer: 14 percent. This is the first study to quantify the effect of less smoking on the nation’s obesity rate. The economists, led by Charles Courtemanche of Georgia State University, call 14 percent substantial: in a study last year, he calculated that the growth of restaurants and big box grocers like Walmart (more eating out + cheaper food = more calories consumed) explain 17 percent and 12 percent, respectively, of the rise in obesity.

Second Take: Gut Check won’t scold anyone this time for confounding correlation with causation; the epidemiological evidence that quitting smoking can cause weight gain is solid, and the biological mechanism is plausible: nicotine suppresses appetite and accelerates metabolism, said psychologist Peter Hendricks of the University of Alabama, Birmingham, who studies substance abuse.

But that doesn’t mean quitters who gain weight get bumped up from a healthy BMI to overweight, let alone obese. Few of those who stop smoking pack on significant pounds. The 2012 meta-analysis, for instance, found that 53 percent either gained less than 11 pounds or even lost weight, while only 13 percent gained more than 22 pounds. Although the new paper finds a larger average weight gain, it also found that thinner quitters gained more weight than heavier ones. Given that pattern, said obesity expert David Allison of UAB, thin quitters — even with some extra pounds — might not become overweight, let alone obese. “In that case the decrease in smoking rates wouldn’t have much effect on the prevalence of obesity and overweight,” he said.

“There are definitely assumptions in this paper, including that an average weight gain shifts the whole distribution” of the population’s weight in a way that bumps many people into the higher-BMI categories, Courtemanche acknowledged in an interview. “But we don’t have any way” of knowing if that’s so.

Another reason for caution about blaming less smoking for America’s obesity epidemic: in 2009, Courtemanche concluded that the rise in cigarette prices (which reduce smoking) “is actually associated with a long-run reduction” in BMI and obesity. That’s the opposite of the new paper. “I’m trying to reconcile these [new] findings with my past research,” he said, but the discrepancy underlines how much econometrics, or the use of mathematical modeling in economics, depends on assumptions and the choice of statistical tools.

The larger issue is, “even if less smoking nudged the US obesity rate up, so what?” The economists don’t recommend getting more people to light up as a way of combatting obesity, of course. Although obesity-related diseases kill 112,000 people per year in the United States, tobacco kills at least 480,000.

On the other hand, if weight gain from quitting smoking is indeed larger and longer-lasting than earlier studies found, doctors should take seriously would-be quitters’ concerns about weight gain and counsel them on ways to reduce calories and increase physical activity, said UAB’s Hendricks.

There’s one possibly policy-relevant tidbit in the economists’ analysis: it seems that intense smoking cessation programs are more likely to lead to weight gain than higher cigarette prices are. That might be because price increases, which tend to be gradual, cause smokers to cut back slowly and thereby control their weight better than if they quit cold turkey. Still, that’s hardly a reason to steer smokers away from cessation programs.

The Takeaway: Given all the epidemiological evidence that quitting smoking causes weight gain, and the solid biological explanation of it, this paper’s finding of an 11- to 12-pound average gain is plausible. But it doesn’t prove that the decline in smoking was a significant contributor to America’s obesity epidemic.