It's well known that smoking weed can give you the munchies. But exactly how cannabis stimulates appetite, and why that's useful knowledge, has been revealed by new research in the United States.
The researchers at Washington State University exposed rats and mice to cannabis vapor to stimulate specific brain regions associated with appetite.
They also observed the rodents' feeding behavior, such as how often they ate.
Independent experts that DW contacted, such as Donald Abrams, an oncologist at University of California San Francisco, suggested the findings were a useful addition to existing research into the medicinal application of cannabis.
"Rats are not people," said Abrams. "But as one who went to college in the 60s, [I'm] aware that cannabis stimulates appetite."
And that can be useful to help people undergoing treatment for cancer, who lack appetite but need to eat to keep up their strength.
Cannabis activates specific neurons for appetite
In their study, the researchers exposed rats and mice to passive amounts of cannabis vapor, similar to the amounts people smoke on average.
First, they observed the feeding behavior of the rats and mice and found that they looked for food more often after they had been exposed to that cannabis vapor.
Then, they looked at the neural activity in the mice and found that the cannabis activated a small group of specific neurons in the mediobasal hypothalamus.
The hypothalamus is known to control appetite, as well as other functions such as body temperature and mood.
But when those specific neurons are activated, it creates a cascade of neural signals linked to motivation and movement. In humans, that's what gets you off the sofa, rummaging in kitchen cupboards for biscuits and candy.
And the rats and mice in this study were no different — they went looking for food as well.
How the chemicals in cannabis influence appetite
The researchers investigated the interaction between chemicals in cannabis and known brain activity linked to appetite and feeding.
Cannabis releases chemicals known as cannabinoids: delta-9-tetrahydrocannabinol (THC) and cannabidiol (CBD).
THC and CBD trigger neurons in the hypothalamus that express a protein called cannabinoid-1 receptor (CB1 receptor). The CB1 receptor is known to increase appetite and stimulate feeding.
But the new research found that as soon as the mice in their study saw food, the hypothalamus activated significantly more cells with the CB1 receptor.
They tested this by switching off the relevant neurons in some mice and observed that the cannabis stimulated appetite significantly less.
Medicinal cannabis to help stimulate appetite
Scientists have been studying the appetite-stimulating qualities of cannabis for some time. The hope is to use medical cannabis to help stimulate appetite in people undergoing chemotherapy or those experiencing anorexia.
Synthetic drugs have been developed to mimic the effect of cannabis. However, in some studies — for example on the treatment of anorexia — the drugs did not work reliably.
Michelle Sexton, a researcher at the University California San Diego, US, said that may have been because the drugs were taken orally, which may not be as effective as smoking cannabis. But Sexton told DW by email that "the evidence for vaporized cannabis for effects on appetite are under-studied."
Cannabis remains a banned substance in the US and other countries, including Germany. It is not accepted for medical use, even in US states such as Colorado and California, where cannabis is available to buy from dispensaries.
"Cannabis is the only anti-nausea therapy that increases appetite. It is also good for pain, insomnia, anxiety and depression, so something I frequently recommend to people living with and beyond cancer," said Abrams, who told DW he had been recommending it to his cancer patients for 40 years, but that he was not allowed to prescribe it.
