In trolling the headlines for a writing topic, I ran into this irresistible headline: Does pot prevent obesity? What new marijuana study says.
The headline is reasonably noncommittal, but that ends quickly:
Move over, fad diets. Scientists have identified something else that might keep off excess weight.
That’s right, marijuana users are less likely to be obese than their non-toking counterparts, according to a new study.
Buried much deeper in the piece is the cautionary, “The study didn’t actually prove smoking pot causes weight-loss.”
The reporting is typically sensationalist and deceptive, but the study itself was linked in the article, which is worth a bunch of Schwitzer points in my book. Let’s take a look.
The authors give us a good justification for looking into Obesity and Cannabis Use. As they say, cannabis and endocannabinoids have long been suspected of having significant effects on appetite and weight, so they took the easiest route to looking for an association: mining previously-done surveys for data. The two surveys provided self-reported data on cannabis use and on height and weight.
After crunching the data, the authors found that cannabis users were significantly less likely to be obese than non-users, but they quite explicitly acknowledged possible biases (emphasis mine):
Our results should be interpreted in the context of several limitations. Firstly, information on cannabis use, height, and weight was based on self-reports and was not confirmed by direct measurement. Self-reports tend to underestimate BMI but are unlikely to influence conclusions about associations, since they are unlikely to be related to cannabis use (WTF. –ed.)…Secondly, physical activity and diet are 2 major risk factors for obesity but were not taken into account in this study. Despite these limitations, this analysis showed that even if cannabis consumption increases appetite, people using cannabis are less likely to be obese than people who do not use cannabis.
Given their admission of the obvious problems with this study, it’s curious that they should conclude that ” people using cannabis are less likely to be obese than people who do not use cannabis.” This seems, from the data they’ve given us, a bit of a stretch given the limitations of the study.
This doesn’t make for sexy headlines, but it’s a lot closer to the truth: a single epidemiologic study has found some evidence that reported cannabis users have a lower reported obesity rate than reported non-users. There is no evidence of causality. At most, this could lead to a study better designed to answer the question. At most.
Le Strat, Y., & Le Foll, B. (2011). Obesity and Cannabis Use: Results From 2 Representative National Surveys American Journal of Epidemiology DOI: 10.1093/aje/kwr200