Study: Smoking Pot Not Associated with Weight Gain in Later Life

Cannabis is commonly prescribed to increase appetite, but evidence conflicts over its long-term effect on BMI and obesity. A new longitudinal study adds a crucial piece of evidence for toppling the long-standing myth that smoking weed causes weight gain.

The study surveyed adolescents between the ages of 15 and 19, and then followed up again with the same people over 20 years later. They asked if they had ever smoked pot, their cigarette and alcohol intake, as well as their height, weight, physical activity and other potentially confounding factors.

Researchers found those who smoked pot were not more likely to gain weight later in life than those who didn’t, and those who started smoking as adolescents had the smallest increase in BMI than any other group, even after adjusting other factors like tobacco, alcohol intake and exercise.

This latest piece of research from Denmark, though promising, has some drawbacks that may have skewed its results. Thousands of participants dropped out between the initial and follow up surveys. This may not have been a problem if not for the fact that a high number of dropouts were obese or had a high BMI, whereas a slightly lower proportion of those who started with a high BMI followed up 20 years later for the second survey. The surveys also relied on self-reported data from the participants that might not be entirely truthful.

Despite the inherent errors in the study, one experimental uncertainty may actually point to the conclusion that cannabis intake causes weight loss and a lower BMI. The authors noted that drug survey takers tend to under-report the amount of cannabis they smoke, which may have masked an inverse relationship between smoking cannabis and BMI.

Of the seven previous pieces of research on this topic, five are cross-sectional (meaning they look at a given population at one point in time) and two are longitudinal (a long-term, follow-up study). Four of those five cross-sectional show that cannabis consumption is associated with lower BMI, while the fifth one had the opposite conclusion. One longitudinal study published in 2013 observed weight gain among pot smokers, and another long-term study from 2006 found no association whatsoever.

Other research on mice has shown some of the fascinating effects of tetrahydrocannabinol (THC, the active component of cannabis) has on the symbiotic bacteria in our gut. Canadian scientists fed several groups of mice a specialized high-fat diet that has been shown to influence gut bacteria and induce weight gain. The mice that also enjoyed daily doses of THC displayed a different distribution of gut bacteria all together and didn’t put on any weight despite the high-fat diet.

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