It's fair to say that we're more microbe than human. For every cell in your body, there are about 10 microorganisms that reside along the lining of the gut. And, as scientists have learned, what transpires within this delicate ecosystem otherwise known as the “microbiome” can have a profound impact on our well-being.
A recent study, for instance, suggests that eating even lean red meat may increase the risk of heart disease because its reaction with certain species of bacteria converts it into a harmful substance that hastens the buildup of clogged arteries. Meanwhile, other studies have shown that ensuring a proper balance of microflora can help ward off heart disease, obesity and a host of life-threatening conditions.
Now, researchers believe they may have even figured out a way to use these little buggers to boost learning.
Thus far, much of the evidence for the influence of microbiota on the mind have come from studies done on mice. One influential experiment by Japanese investigators in 2004 observed that mice that lacked gut bacteria released more stress hormones than those with it when put in difficult situations. Simply restoring some of those germs, it turned out, was all that was needed to bring back a normal response. Last year, a Canadian team was able to produce a similar change using fecal transplants by transferring microflora from fearless mice to timid onese, enabling the timid ones to be, for all intents and purposes, mightier mice.
But perhaps even more intriguing is the claim made last month by leading British researcher John Cryan. Previously, the University College Cork neurocientist found, in a study detailed in the journal Proceedings of the National Academy of Sciences, that Lactobacillus rhamnosus, a pro
-biotic present in yogurt, exerted a measurable relaxing effect on overanxious mice. In an interview for the BBC, Cryan hints he's gone a step further, claiming his team has now identified a probiotic strain that may actually improve cognitive function.
“We have unpublished data showing that probiotics can enhance learning in animal models,” he told BBC Future.
According to the report, mice were fed two types of bacteria, but only the newly-discoverd probiotic translated to better mental performance. Encouraged by the results, Cryan is currently planning similar trials with healthy human subjects to see if he has the same results.
“It's a remarkable claim," Dr. Jon Lieff, a neuropsychiatist and former president of the American Association for Geriatric Psychiatry. “I'd still have see [the data] to believe it. But I have no doubt that, based on what we know, that the notion that microbes can help enhance cognition is at least plausible.”
For Lieff, the possibility that probiotics can make people (in essence) "smarter" isn't too much of a stretch, considering that gut bacteria plays a crucial role in regulating a lot of what goes on in the body. At any given moment, colonies in our guts are swapping DNA with cells, hijacking immune cells and secreting various hormones like dopamine and serotonin.
One potential explanation for why Cryan's discovery could hold up, Lieff says, is that certain strains of bacteria have the ability to stimulate the production of brain-derived neurotrophic factor (BDNF), a protein that supports memory and learning by facilitating the formation of new neural connections.
"Hormones and growth factors that are secreted also can make their way into the brain via the blood stream," Lieff says.
Also of particular interest to researchers is the vagus nerve, a stretch of long, wiry fibers that serves as a sort of conduit for brain-gut communication. While scientists once assumed the nerve only enabled signals to be transmitted from the brain to the gut, the reality, they now believe, is that information travels in both directions. And, as Lieff point out, microbes that pry their way into immune cells, which have a direct route to the brain, can also modify signals that are sent out.
Dr. Emeran Mayer, a gut-brain researcher who isn't involved in Cryan's research, thinks the relaying of signals through the vagus nerve is the most likely mechanism for some of the brain and behavior-altering effects of microflora. The most convincing evidence for this comes from an experiment in which Cryan discovered that those same probiotic-induced benefits can essentially be erased simply by snipping the nerve.
"Evidence for hormonal signaling have not been reported so far," Mayer told Smithsonian.com. "But we do know that the vagal pathway is activated by serotonin released in the gut from cells that respond to signals from microbiota."
Still, the UCLA-based gastroenterologist remains highly skeptical of Cryan’s claims.
"There is currently no evidence whatsoever, in humans or in animals, that gut bacteria can enhance brain function," Mayer says. "I know [Cryan] well, but I feel he's gone way overboard with his speculation based on a few mice studies."
Lieff, too, thinks this may be a case where the hype has gotten way too far ahead of the actual science. Effects found in mice studies, he says, often don't carry over to people, since human brains operate very differently.
So in that case, until a magic pill arrives, what's the best way to boost one's brain power?
"Diet definitely can improve the brain's ability to function, though the problem is we haven't really figured out the exact optimal formula," Lieff says. "For now, the best proven ways to boost your brain are to exercise and making sure you get enough sleep. Basically, the kind of old-fashioned advice your mother would tell you."
Editor's note, March 18, 2014: An earlier version spelled Dr. Lieff's name incorrectly. It has since been fixed. We regret the error.