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If cannabis causes the munchies, how is it possible that pot smokers are thinner than nonsmokers?

A new study published in the American Journal of Epidemiology finds an intriguing connection between marijuana use and body weight, showing that rates of obesity are lower by roughly a third in people who smoke pot at least three times a week, compared with those who don’t use marijuana at all.

Researchers analyzed data from two large national surveys of the American population, which together included some 52,000 participants. In the first survey, they found that 22% of those who did not smoke marijuana were obese, compared with just 14% of the regular marijuana smokers. The second survey found that 25% of nonsmokers were obese, compared with 17% of regular cannabis users.

The association between pot smoking and lower risk of obesity remained strong even after adjusting for other factors that could influence body fat and health, such as cigarette smoking, age and gender. But the correlation between weed and weight doesn’t mean that marijuana smoking actually causes weight loss.

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Many other factors could account for the connection. For example, some research finds that highly religious people are less likely to take drugs, but more likely to be obese — perhaps because they’re substituting one compulsive behavior (overeating) for the other (smoking marijuana). So, some of the obese people in the national surveys may be religious folk, who might otherwise be heavy marijuana smokers, but are eating too much instead. That could make it look like marijuana is slimming.

Also consider that one of the most popular uses of medical marijuana is to stimulate appetite in people with cancer, AIDS or other diseases. Such patients are significantly less likely to be obese than the general population — so in this case, weight loss would precede or prompt the marijuana smoking.

Whatever the explanation for the link between marijuana and less obesity, it’s unlikely that cannabis could serve as an effective diet aid. For one, smoking pot has been shown to increase appetite in multiple studies, at least in the short-term, so it likely wouldn’t help dieters resist temptation.

Secondly, a drug that has the opposite effect of THC, marijuana’s main psychoactive ingredient, has itself been shown to aid dieting. Called rimonabant, at high doses the drug nearly tripled the weight loss achieved by people taking placebo. It also frequently caused severe depression and suicidal thoughts, however, so while it was briefly approved by European authorities, it was ultimately pulled from the market.

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Of course, none of this explains why marijuana smokers in the national survey samples didn’t get fatter by taking a drug that can clearly stimulate appetite. One factor may be tolerance: many of marijuana’s effects are reduced in frequent users, as the body adjusts to it.

Another may be substitution — the smokers could be seeking comfort by smoking more marijuana, rather than eating more. Or, perhaps other ingredients in cannabis like cannabidiol (CBD) could reduce the appetite-increasing effects of THC in the same way that they reduce its paranoia-inducing properties.

Whatever the case, marijuana research never lacks for surprises!

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Maia Szalavitz is a health writer at TIME.com. Find her on Twitter at @maiasz. You can also continue the discussion on TIME Healthland’s Facebook page and on Twitter at @TIMEHealthland.