It sounds like a stupid dieting infomercial – eat more and lose weight! – but studies continue to show that marijuana users, despite eating an additional 600 calories per day, are less likely to be obese or suffer from diabetes.

Skeptical? You should be. (I mean… come on, munchies and weight loss?)

Well, here’s the evidence.

Exhibit A: A 2011 study in the American Journal of Epidemiology compared obesity rates between marijuana users and non-users in two nationally representative surveys. In one, 22% of Americans who did not use marijuana were obese, compared to 14% of marijuana users. In the other, 25.3% of non-users were obese, compared to 15.2% of users. The association between marijuana usage and a lower body mass index remained even after controlling for age, sex, and tobacco use. Furthermore, an experiment in which rats were injected with cannabis showed that marijuana caused significant weight loss compared to the control group.

But wait, there’s more. A study of 11,000 Americans published in the British Medical Journal also found that, after controlling for age, sex, and socioeconomic status, marijuana users had significantly lower rates of diabetes.

What’s the secret behind this magic? Well, a 2013 study in the American Journal of Medicine attempted to answer that question. The researchers looked at about 5,000 American adults and measured the relationship between their use of cannabis, waist-circumference, insulin levels, blood sugar, and insulin resistance. They found that, after controlling for age, sex, race, education level, income, marital status, tobacco use, physical activity level, and alcohol use, regular cannabis smokers both had thinner waists and better metabolic indicators. They conclude by suggesting that the biochemical effects of THC and Cannabidiol, the two main psychoactive components in marijuana, may explain their findings.

Studies such as these gave rise to “the seemingly paradoxical hypothesis that THC or a THC/cannabidiol combination drug may produce weight loss and may be a useful therapeutic for the treatment of obesity and its complications.” 

For now, though, we can’t say that marijuana use causes weight loss in humans. (In fact, the only way to do that for sure would be to run an experiment where some people are randomly assigned to smoke weed, which, of course, would be illegal).

Maia Szalavitz, a columnist for Time magazine, identifies some alternative explanations:

1)      The religiosity hypothesis: people who frequently attend religious services are more likely to obese. They also are much less likely to smoke marijuana.

2)      The medical marijuana hypothesis: people who suffer from HIV or are undergoing chemotherapy are often prescribed cannabis in states which permit medical marijuana to bring back their appetite. Consequently, the observation that marijuana smokers are less likely to be obese may have causation backwards.

3)      The “pick-your-poison” hypothesis: Some people comfort eat when they’re sad. Some people smoke weed when they’re sad. Maybe people who smoke weed when they’re sad don’t comfort eat. (This one is definitely wrong since we know marijuana users eat more than normal).

4)      The rimonabant dilemma: Rimonabant, a drug that has the opposite effect of THC, the chemical known to make marijuana users hungry and high, has been shown to cause significant weight loss, though because it also frequently leads to depression and suicide, it’s no longer used for this purpose.

Another one that comes to mind is that people who use other illicit drugs, like cocaine or meth, often smoke weed as well (though not the other way around), and use of these drugs might explain the lower rates of obesity in the marijuana-using population. Studies so far haven’t looked at this, though they have controlled for tobacco and alcohol usage.

However, these hypotheses remain untested and some have obvious problems. Moreover, these hypotheses cannot explain why, according to the American Journal of Medicine study I described earlier, marijuana users have reduced levels of blood sugar and insulin and have higher levels of “good” cholesterol. (Intriguingly, the response was not “dose-dependent”; in other words, how much marijuana people smoked did not affect these outcomes.) What’s more, it’s not just that marijuana smokers are less likely to be obese; they are less likely to be obese despite eating more food.

In any case, none of this is to say that marijuana is good for you – there are also a variety of negative consequences associated with use. Side effects may include increased risks of stroke, heart attacks, blood vessel constriction, testicular cancer (but not lung cancer?), and the drug, at least in some people, carries some risk of dependence.

But compared to some weight loss methods, marijuana use could be relatively benign.

[image credit: http://upload.wikimedia.org/wikipedia/commons/f/ff/Potleaf.jpg]