Though a commonly known side-effect, it seems a tad odd for a plant, when either smoked or ingested, to produce a feeling of hunger along with the disorienting effects it’s often intended for. But as a growing number of states loosen their marijuana laws, research into the plant’s effects on the brain grow in equal measure — now offering a conclusive answer to many a user’s query: What causes the munchies?

Learning what causes the munchies — more formally known as the intense hunger pangs felt when the brain’s in the presence of tetrahydrocannabinol, or THC — confers a far wider set of implications for marijuana research than simply knowledge for knowledge’s sake. Growing acceptance of medical marijuana has spurred doctors to use the drug for anti-inflammatory purposes, pain relief, and, among others, jumpstarting a person’s appetite, particularly in people undergoing chemotherapy and sufferers of eating disorders.

How Can I Be So Hungry?

Hunger and satiety are two feelings regulated primarily by the hormones ghrelin and leptin. They’re secreted in peripheral parts of the body, primarily in fat cells, then sent to the brain’s hypothalamus, where they’re processed as “hungry” and “full” signals that either tell you to keep eating or stop. Just recently, scientists from Beth Israel Deaconess Medical Center found the neural pathways for both sensations reside in the same patches of neurons within the brain.

But that still leaves the fundamental question unanswered, and it’s one that a team of European neuroscientists led by Giovanni Marsicano of the University of Bordeaux has found in a recent study an answer to. The cause of the munchies, it would seem, stems from THC binding to the brain’s olfactory bulb — the locus for the entire spectrum of smell sensation, from every buttery whiff of popcorn to every putrid stench of waste.

As for why marijuana, specifically — and not some other thing — makes you hungry, it turns out the brain already has a built-in set of receptors, called endocannabinoids, which mediate, among other sensations, appetite. It’s here, the team found, that THC intensifies the endocannabinoid response. It fits into the normal pathway of our bodies’ own mechanism for initiating hunger pangs, and revs them up tenfold. And since it hits directly at the source of our sensations, it doesn’t matter how much food is in our bellies — we’re going to eat.

Mice Munchies

Marsicano and his team discovered these pathways through tests on mice models — frequently used because they share a number of cognitive similarities with humans. Mice were given banana and almond oils to sniff at their hearts’ desire, until their brains habituated to the scents and the mice got bored. Then the researchers introduced THC to some of the mice, which compelled them to keep sniffing. When given the chance, many of these mice also ate in greater amounts — suggesting their own version of mouse munchies.

Later, the team performed a second experiment in hopes of pinpointing olfaction’s exact role in the sensation. Researchers compared two sets of mice: One set was normal, with otherwise correct cognitive anatomy; the other was genetically modified to lack a type of cannabinoid receptor in their olfactory bulbs. When they were both exposed to the oils again, only the genetically engineered group kept sniffing. Interestingly, their appetites stayed normal, suggesting the sense of smell as critical for so-called munchies-production.

Finally, the team analyzed the behavior of mice lacking cannabinoid receptors during starvation. A group of mice fasted for 24 hours, some with their receptors intact, some without. Those that retained their receptors predictably demonstrated greater sensitivity once they were introduced to food, responding more quickly and eating more of it. Meanwhile, the other group showed no increased sensitivity and didn’t display any gluttonous behavior.

In the end, this told Marsicano and his colleagues found that THC mediates hunger through the same neural pathway, telling smokers and consumers more broadly that the body is starving. Food is present. It’s time to eat.

Source: Soria-Gomez E, Bellocchio L, Reguero L. The endocannabinoid system controls food intake via olfactory processes. Nature Neuroscience. 2014.