For a while there, the scientific basis of the munchies was up in the air. It certainly seemed like a real thing, but despite mountains of anecdotal evidence that smoking cannabis leads to feeling hungry, science spent several decades either denying or ignoring its effects. This means that while just about any pot smoker in the world could tell you that hitting the bong leads directly to hitting the drive-thru, science is just now scrambling to figure out precisely why.
The main appetite-enhancing molecule in marijuana is tetrahydrocannabinol, or THC. The cannabis plant evolved this little wonder partially because our bodies (and the bodies of virtually all animals) use a natural cannabinoid system to control all sorts of important body functions. THC is basically meant to interfere with our psychology as much as possible (in reasonable doses) and we find its effects quite pleasurable. Not the least of these positive effects is hunger.
When you smoke, you put THC directly into the bloodstream via the lungs, where it can circulate to all three of the major sites of appetite control: the nose, the stomach, and the brain. All three of these key areas for regulating appetite contain one crucial target for THC: the cannabinoid receptor type 1, or CB1. How CB1 uniquely acts in each of these areas gives weed its power over hunger.
First, THC affects the nose, where the olfactory bulb straddles the line between the brain and the sinus cavity. This bunched collection of neurons is packed with CB1 receptors and when stimulated it does what the body normally does in response to hunger: sharpens your sense of smell. In mice, a heightened ability to smell food led them to seek out more food, more often. Makes sense, right? Sense of taste is inextricably linked to smell, so cannabis smokers usually find that a more sensitive nose leads food to taste better overall.
In the stomach, CB1 receptors are found mostly on cells that help control our feeling of fullness. When you’re high, it’s not just overall numbness that makes it tough to recognize when you're full; the feeling is also masked by adjustments in the levels of certain hormones, like the appetite-promoting ghrelin. On the other hand, research has shown that blocking CB1 receptors in the gut can have an anorexigenic effect, keeping mice from eating as much as normal. This is presumably because they feel more full.
THC’s effects on hunger and smell are really just an indirect consequence of the molecule’s affects on its real target: the brain. THC has the important and relatively rare ability to move naturally across the blood-brain barrier, which means that it can complement its indirect effects by getting right up next to the brain itself.
Most generally, cannabis has been proven to stimulate the brain to increase circulating levels of dopamine and endorphins—neurotransmitters that promote a sense of euphoria. This generally helps people relax enough to enjoy food, but more importantly, dopamine can increase sensory enjoyment of food while eating. This is probably why some longtime smokers feel that eating while sober has become quite literally, bland.
Marijuana’s most powerful effect on the brain, though, comes from the CB1 receptors on POMC neurons in one of the brain’s most ancient regions, the hypothalamus. POMC neurons slow the urge to continue eating by making you feel full, but research published just last year found that activating the cells’ CB1 receptors actually flipped POMC function entirely. According to the researchers, this means that our caloric brake-pedal becomes confused with the gas, leading the very signals that usually reduce eating to increase it.
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Scientists may be uptight about the nature of this evidence, but they're not blind; it's been a long time since anybody really doubted the idea that marijuana sends both mice and men sprinting for the feed bowl. The munchies are very real, and science is even on the way to understanding precisely why.