Though marijuana may give you the munchies, it doesn't actually seem to result in higher levels of obesity or a higher chance of diabetes. In fact, it seems that marijuana does exactly the opposite. This 2013 study surveyed 4657 adults abouts their current and past marijuana use and measured the respondents fasting insulin levels as well. The researchers found that not only was marijuana use associated with lower fasting insulin levels, but with a smaller waist circumference as well.
