The precipitous fall has offered a glimpse into what happens when a country quits smoking cigarettes. And the answer is a ton of good things. The dip has coincided with steep declines in the rate of lung cancer, the death rate associated with cardiovascular disease (it peaked in 1968) and a number of other negative health outcomes.

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But it has also, interestingly, coincided with another trend that isn't quite as encouraging: the rise of obesity. And it's not unreasonable to think that the two might be at least somewhat related.

"No one would recommend cigarette smoking as a way to combat obesity," said Charles Baum, a professor of economics at Middle Tennessee State University, and the author of a study published in the journal Review of Economics of the Household. "But realistically, the decline in smoking might very well have had some effect on the rise of obesity."

Baum, along with Shin-Yi Chou, a research associate at the National Bureau of Economic Research (NBER), wondered what, if any, changes in human behavior have played a role in the growing pervasiveness of obesity, which has doubled over the past quarter-century. So they tried to estimate how things beyond the obvious and more immediate shifts in things like diet and exercise might have contributed.

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In order to approximate the effect of various socio-environmental factors — things such as changes in food prices, physical demands at work, urban sprawl, racial composition, age distribution and cigarette consumption — Baum and Chou used almost 30 years’ worth of data from the National Longitudinal Survey of Youth. The survey gathered detailed information about the social and economic backgrounds, weight, height and many other characteristics of more than 12,000 youth. The researchers controlled for several variables, including age, education, income and work experience. And what they found is that nothing seemed to have had much of an effect at all. That is, aside from the changes in cigarette consumption.

"The decline in the prevalence of cigarette smoking didn't have a large effect, comparatively speaking, but it had an effect," said Baum. "It can explain about as much as 4 percent of the increase in obesity, according to our estimates."

The impact associated with the fall in cigarette consumption was the largest of all the factors the researchers tested — such as the rise of urbanization, the fall in national food stamp enrollment and the growth in the number of restaurants — and that held true in all three of the models they built to compare the various factors.

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The cigarette diet

The connection between the national shift away from smoking and growth in the collective American gut isn't as far-fetched as it might seem.

Cigarettes are a well-known appetite suppressant, not just within popular culture, but also among scientists who have studied tobacco's effect on the body. There is a plethora of research that suggests nicotine tends to decrease food intake (including this recent peer-reviewed study, which was published in the journal Science). The reasons, outlined in a slightly older, 1991 study, include resultant changes to the nervous system, physical activity and preferences in food consumption.

And it hasn't just been shown that smoking cigarettes can lead to weight loss — it's fairly well-established that people tend to gain weight after quitting, too.

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"We know smoking cessation leads to weight gain. That's pretty accepted," said Yoni Freedhoff, an obesity expert at the University of Ottawa. "Of course, the amount of weight gain varies, but it can be as high as 12 to 20 pounds."

The effect is so significant that a study conducted in the early 1990s, which found that "major weight gain is strongly related to smoking cessation," noted that it's likely part of what makes it so hard for people to quit.

So what?

Despite the collective eagerness to pin the obesity epidemic, which now affects more than a third of all U.S. adults, on any one particular thing, the truth is that it's more likely the result of a complex web of many and varied societal shifts. There are things, of course, that contribute more than others — the rise of processed food, for instance, or the uptick in the number of calories Americans consume. But there are also smaller, overlooked factors that slip through the cracks, evading the broader discussion.

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The fall of tobacco seems like it might very well be one of those things, as evidenced not only by Baum's study, but also by myriad studies undertaken over the years, which have built on our understanding of the relationship between smoking and weight.

"Obviously, it's hard to establish any causal relationship here, but I would definitely say it's plausible that the fall in smoking contributed to the rise in obesity," said Freedhoff, the obesity expert.

What exactly are we supposed to glean from the suggestion that the fall in smoking might have contributed to the rise in obesity? The answer is not that anyone should look back upon the days when more than half of the population smoked regularly with nostalgia. Rather, according to Baum, it's better to view the study's finding as more of a point of interest, a takeaway that allows us to look at how societal changes move like waves that ripple, touching other shifts, even if only slightly.