It was always something of a groovy scientific mystery: why does using marijuana make people hungry, even after eating a big meal? That question has finally been answered, and scientists say they were surprised to learn what happens inside the brain to give marijuana-users the "munchies."

In a study published Wednesday (18.02.2015) in the journal Nature, a group of researchers at Yale University used mice to examine how marijuana affects certain receptors in the brain that are in charge of telling us when we are full.

It turns out marijuana can have some interesting effects on the brain besides making us feel "high." The team discovered that the active ingredients in marijuana, cannabinoids, interact with neurons in the brain that normally tell the body it has had enough to eat. The marijuana molecules increased the amount of neurochemicals in the brain that under normal conditions would send out a signal to stop eating. But strangely, the cannabinoids reversed the effect of these neurons, which instead emitted neurochemical signals to do the opposite: continue eating.

Marijuana's surprising effects

"We were very surprised," said Dr. Tamas Horvath, the study's lead researcher and the Professor of Neurobiology and Comparative Medicine at Yale University.

"The question was, how could it be that a neuron that is a brake, all of a sudden becomes the accelerator? What’s happening inside these cells that makes them flip 180 degrees?"

In order to study the munchies effect, Horvath’s team injected mice with a cannabis-like substance just after the mice had eaten a meal. Normally, the mice would take a nap, but according to the study’s results, about 45 minutes later, they got "the munchies" and began eating more food.

While studying the effects inside the mice’s brains, Horvath and his team discovered the surprising neurological effects and they believe the same holds true for both mice and well, stoners.

"We hope it does explain how it is that after you have a dinner and you smoke marijuana, all of a sudden you go after food again," Horvath told DW in an interview.

The study could have some important implications, as the medicinal and recreational use of marijuana continues to become more acceptable - and more legal - in many countries. For example, it could help in the treatment of patients dealing with a loss of appetite due to complications with chemotherapy, cancer or HIV. Further studies might be able to isolate the cannabinoids' effect on appetite; helping patients who need to take in more calories but don't want to get high, according to Horvath.

'It makes no sense, evolutionarily'

The results of the study could also help answer another pot paradox: why don’t more stoners become obese from all that snacking? The study doesn't address this directly, but Horvath has a hunch.

While under the influence of marijuana, the cannabinoids act as a hunger stimulant, releasing molecules that normally suppress hunger but flipping their effect to cause food cravings. But once the marijuana wears off, Horvath says the "stop eating" chemicals are still hanging out in your brain, and their messaging goes back to normal.

"So you can have a rebound of not eating for a long period of time after you have the munchies."

In other words, after an initial bout of the munchies, marijuana-users often go a long time without eating, perhaps keeping their weight under control. In fact, Horvath says there is no association between marijuana use and obesity in the existing scientific literature.

Why don't we crave carrots?

So, now that scientists understand why the munchies happen, is it possible to make marijuana-users crave healthy food instead of the stereotypical stoner snacks like chips, cookies, or pizza? The short answer: no.

"It makes no sense, evolutionarily," says Horvath.

He believes our brains are hard-wired to seek out high-calorie food when hungry. It could be an evolutionary response left over from a time when humans were constantly searching for their next meal.

"Food-scarcity was the driver. So if you have the ability to shove-in and store as much food as you can, that assured that your next meal – it doesn't matter as much when it comes," said Horvath.

"You're not going to go for the carrots; you're going to go for something that is the densest in calories."

So it turns out, despite the latest advancements, science still can't make stoners eat their veggies.