A new study proposes that people who smoke marijuana daily have a lower percentage of body fat. The study observed a sample of adults 18-26 years old and observed them again six years later. In these observations, the researchers measured BMIs and amount of marijuana used. The study concludes women who smoked marijuana everyday had a 3.1% lower BMI and men who smoked everyday had a 2.7% lower BMI in contrast to those who did not smoke everyday.
In class we learned that correlation is not causation. Just because the study shows a correlation between smoking marijuana everyday and a lower body fat percentage- does not mean that they are related. The study does not cite any mechanism that result in marijuana causing a lower body fat percentage. However, the study does control for confounding/third variables that would effect the correlation. According to the study, they controlled for diet, exercise, and alcohol drinking habits.
The study took place over a period of time, six years. As we learned in class this helps reduce the factor of random chance in the study. However, since the study was observational random chance cannot be entirely ruled out. Also, the study shows correlation between daily marijuana smoking and lower BMI’s but this can be explained different ways. Direct causality, a person smokes marijuana everyday this then causes them to have a lower BMI. Reverse causality, there is something about have a lower BMI that causes a person to smoke marijuana everyday. Confounding or third variables, smoking marijuana daily and lower BMI are not causally related but effected by something else. For example income: a person smokes a lot of marijuana this then causes them to have little extra money which in turns causes them to eat less and have a lower BMI. Another scenario is socioeconomic factor: people from lower income areas have lower BMIs as they have less food because lower income areas are more at risk for gang activity they have a larger likeliness of being exposed to marijuana and smoking everyday. Another third/confounding variable is culture: people who grow up in a hippie lifestyle home are more likely to smoke as it is apart of the culture, they are also more likely to have lower BMI’s because they are more likely to be vegetarians as both are norms of the culture. Since there is no mechanism found in the study- any of these correlations could be feasible to explain the relationship between lower BMIs and smoking marijuana daily.
This study involved a lot of the concepts that we have spoken about in class. It was a longitudinal study. We talked about how this reduces random chance- but does not rule it out. The study’s outcome shows a link between marijuana and BMI but there is not evidence for a mechanism that would explain how marijuana effects BMI therefor we cannot be sure it is marijuana causing the lower BMI, the lower BMI causing daily marijuana smoking, or another confounding variable causing both. The study controlled for factors that may effect the outcome- this increases validity of the results. However, because this is a correlation (as we learned in class) there are still options for reverse causality and random chance to explain why there is a link. Overall, I would not recommend smoking marijuana everyday as a weight loss tactic as there is not found mechanism.