Why does cannabis keep some people skinny? Experts explain how weed and metabolism are connected

"Chronic cannabis users tend to be less overweight than non-cannabis users."

Although it's hard to make the case that cannabis makes anyone more slothful — that's simply an old drug war myth — there is plenty of evidence that marijuana stimulates appetite. Some patients may need to gorge themselves, such as people with HIV or cancer who sometimes have trouble eating. Getting the munchies can be a good thing in this case, explains Dr. Peter Grinspoon, a primary care physician at Harvard Medical School who specializes in medical marijuana. Some research indicates that cannabis users also tend to have lower body mass index (BMI), even if this is an imperfect metric for weight and bodily health.

"Any of us who have used cannabis can attest to the fact that it can make you very hungry," Grinspoon told Salon. "But contrary to stereotypes, chronic cannabis users in several studies have been shown to have a lower BMI. There's sort of a paradox there. And it's not entirely understood why chronic cannabis users tend to be less overweight than non-cannabis users."

For example, a study published last year in the journal Cannabis and Cannabinoid Research reviewed 16 studies examining this relationship, identifying many of the different ways that cannabis seems to regulate metabolism.

"Based on the data presented, the hypothesis arises that Cannabis sativa and its derivatives can be potentially effective in treating and reversing the damage caused by inflammation in obesity," the authors conclude. "It is clear that phytocannabinoids [drugs] derived from Cannabis sativa have therapeutic potential due to their anti-inflammatory, antioxidant, and neuroprotective properties, making the plant a study option for reducing and reversing inflammation and comorbidities associated with obesity."

"The hypothesis arises that Cannabis sativa and its derivatives can be potentially effective in treating and reversing the damage caused by inflammation in obesity."

Inflammation is a natural process in the body, a way for the immune system to fight infections or injuries. But too much of it can be a bad thing. Chronic inflammation can contribute to the development of various diseases such as cancer, heart disease and obesity. In fact, many experts believe that obesity is linked to low-grade inflammation, which can contribute to the development of various metabolic disorders.

It could be that many of the chemicals in cannabis, called cannabinoids, can be anti-inflammatory. CBD (cannabidiol), for example, is a drug found in marijuana that has shown broad antioxidative and anti-inflammatory behavior across a multitude of different cell receptors. That means it seems to act on numerous systems in the body, including those related to pain, memory, mood . . . and appetite.

One of the most obvious ways that cannabis can potentially help people maintain a lower weight is by replacing alcohol. Quitting booze has been shown to help with weight, among other health benefits. So the "California sober" crowd, who have ditched all drugs except cannabis, may experience some weight loss.

Furthermore, cannabinoids like CBD and THC (tetrahydrocannabinol), the drug best known for marijuana's trademark euphoria, act on the endocannabinoid system (ECS), a network in the body actually named after the plant. Cannabinoid receptors have a lot more functions than getting one stoned, of course. The ECS largely maintains homeostasis, balancing various physiological processes including immune function, sleep and reproductive function. It's a complex system that isn't well-studied.

"The relationship of cannabis to body weight is complex and a bit counterintuitive," Dr. Ethan Russo, a neurologist and psychopharmacology researcher who served as a study physician for numerous clinical trials involving cannabinoids, told Salon in an email. "The answer lies at least partially in the gut microbiome. A 2015 study in mice genetically prone towards obesity demonstrated that THC altered the ratios of gut bacteria statistically significantly in a manner that prevented weight gain despite intake of a high fat diet. Likely the same is true in humans."

One of the most interesting molecules in cannabis for regulating weight is THCV (tetrahydrocannabivarin), which is an analog of THC. Like its cousin, THCV can get someone stoned, but it requires much higher quantities. Some research indicates that even if you took a lot of THCV, the intoxicating effect would still be pretty mild. THCV is more medicinal in that sense, and Russo and others believe it has therapeutic promise for obesity, diabetes and even the possible treatment of addiction.

"[THCV] has therapeutic promise for obesity, diabetes and even the possible treatment of addiction."

"There was a human study where it [THCV] not only lowered blood sugar, but also facilitated improved markers of pancreatic function," Grinspoon said. "Insulin comes from a part of the pancreas called the beta cells, and those are what eventually give out, which gives you Type 2 diabetes. And it appeared that [in the THCV study] the beta cells were working less hard, which, in theory could postpone diabetes – because the mechanism is that they will lose their ability to produce enough insulin."

Russo pointed to several animal studies that demonstrated THCV produced weight loss, decreased body fat and serum leptin concentrations with increased energy expenditure in obese mice.

"Subsequently, an fMRI study in humans showed significantly altered reward and aversion patterns in the brain in a manner that suggests therapeutic efficacy in obesity," Russo says. "And without triggering depression that is commonly encountered with other weight loss drugs."

However, THCV typically appears in very low concentrations in cannabis plants. That means your typical joint probably won't have enough THCV to do much for anyone, though it will likely have plenty of THC. Yes, just that little V makes a huge difference in how the molecule interacts with our bodies.

While it is possible to selectively breed cannabis plants to produce more THCV, some variants of the plant have more than others. They're still relatively rare, but more and more companies are selling highly concentrated THCV products online or in states where weed isn't totally legal yet. This is through a loophole in the law that allows these products if they're derived from hemp plants. It's all very complicated thanks to archaic laws prohibiting cannabis on a federal level, and that in turn makes these products difficult to regulate.

Some of these gummy or vape products are making a lot of wild health claims, such as THCV being "natural" weight loss that's "safe." But many hemp-derived marijuana products are manufactured in sloppy agricultural labs, which can make them tainted with byproducts of backyard chemistry, unlike the edibles or vape pens sold at state-licensed dispensaries.

"They are synthetics with inevitable contamination with chemical byproducts and even solvent residues," Russo says. "This is another counterproductive result of prohibition."

Given all the hype around cannabis for weight, especially THCV, it might be tempting for people to run out and try these products, whether they have decent quality control or not. But despite all the evidence pointing in this direction, we still need a lot more research into what cannabis does to the endocannabinoid system, including ruling out potential side effects. In the meantime, we need better quality control for gray market products potentially containing leftovers from crude extraction processes, while making health claims that aren't entirely based on evidence.

"With all weight loss, there's no silver bullet," Grinspoon says. But despite cannabis seeming to have this effect, people shouldn't expect toking up to equal shedding pounds. "That doesn't necessarily translate into, 'I'm gonna use cannabis to treat your obesity.'"