The brain’s powers are a little overrated. To keep your body going, you don’t need a functioning brain, but you do need something to provide energy. Enter the gut.
We may not give it much thought—because, literally, it happens without conscious thought—but the process of extracting energy from food is an intricate one. It involves hundreds of millions of neurons that aren’t in your brain. Those neurons are found in the outer layers of your gut, and the enteric nervous system they form is so powerful that it can work without any direct input from the brain.
The actions this nervous system performs include ensuring food passes at regulated speed, getting the right juices secreted to make digestion easier, and managing the mucus of the intestinal lining. These are crucial functions. And in the past decade, we have learned just how much of an impact the gut can have on the rest of the body and the mind.
Command and control
For instance, the processes the enteric nervous system performs also gives it some control over the trillions of microbes that sit in your gut. Many of them are essential for our health, because they help us extract nutrients that we wouldn’t otherwise be able to, and some even fend off infections.
One way enteric neurons control these microbes is by changing the thickness of the mucus lining. Justin and Erica Sonnenburg, researchers at Stanford University and authors of , say this process is similar to how “creatures adapted to a moist rain forest would struggle in the desert.” Depending on what kinds of microbes are best suited for a job, the mucus lining can determine their population in the gut.
And there’s more. It had been suspected that what happens in the gut could have an impact on the brain. Now we have found too many correlations to ignore the gut-to-brain connection.
A 2011 study 00607-X/abstract) split a group of mice based on their personality: timid vs adventurous. Then the researchers took another set of mice with microbe-free guts. In half, they installed the microbiome of timid mice, and in the other half they placed the microbiome of adventurous mice. Lo and behold those germ-free mice took on the personality traits of the microbiome-owner.
In a 2013 study, using another mice model, researchers at the California Institute of Technology found that mice with autistic features—such as stress, anti-social nature, and troubling gastrointestinal symptoms—had much lower levels of Bacteroides fragilis than normal mice did. Worse still, when injected a chemical (4-ethylphenylsulphate) found in the guts of autistic mice in to normal mice, they developed autistic symptoms too.
In a 2014 study, researchers at University College Cork found that mice born via C-sections had a greater risk of suffering from depression than mice born vaginally. Turns out, the C-section mice had far less diverse species of microbes in their gut, most likely because they couldn’t pick up the beneficial microbes found in their mother’s vagina.
Although mice are easier to manipulate, such connections are not limited to mice alone. In a 2013 study, researchers at Arizona State University found that humans with behavioral conditions, such as autism, had significant differences in their gut microbiome as compared to more normal humans.
Until now, however, these gut-brain connections have been mere correlations. With some help from tapeworms, a new study changes that.
Tapeworms to the rescue
One of the connecting factors between the brain and the gut has been the immune system. Neurological diseases, such as Alzheimer’s and multiple sclerosis, are linked to changes in the immune system, and auto-immune diseases of the gut, such as Crohn’s disease, are linked to mental illnesses.
Now a new study published in published in Brain, Behavior and Immunity has made use of this immune-system connection to show how the gut can have an impact on the brain. To trigger this connection, Staci Bilbo, a neuroscientist at Duke University, and her colleagues used tapeworms and showed how these nasty creatures can stop memory loss.
She split a group of 30 rats in two: those infected with the Hymenolepis diminuta worm and those without. Then, in both groups, she induced a second infection aimed at increasing the production of a brain signaling chemical called IL-1β. The chemical is usually beneficial, but in excess it can cause damage and has been associated with brain disease.
To test their memories, the rats were put in a room and were allowed to become familiar with it. Then Bilbo gave them a shock so that they would connect the room with bad memories. The next day she re-introduced both wormed and un-wormed rats in the room.
She found that mice with tapeworms were twice as likely to recoil from the room as rats that did not have worms. So the tapeworm infection seems to have protected the mice from memory loss, as compared to infection-free mice.
Good gut, good brain
The reason was that mice with tapeworm infection had already had an immune response, which kept the levels of IL-1β low when a second infection came along. Lower levels of IL-1β in the brain ensured the formation and retention of memories, more than in rats without the worms. Those who hadn’t had the infection produced far greater levels of IL-1β.
This kind of effect is called “biome depletion,” where a lack of exposure to infections causes immune systems to overreact to infections later in life. Thus exposure to some microbes can help avoid such a response, and, in the case of the rats, help prevent memory loss.
To be sure, tapeworm infections can be nasty, and nobody is recommending that you ingest some to protect against memory loss. The circuitous route taken was to come to a definitive conclusion that the changes in the gut can trigger changes in the brain.
The evidence from Bilbo’s study is among the first to draw a causal link. And more is sure to come. In 2014, the US National Institutes for Mental Health spent about $1 million on research looking at the microbiome-brain connection.
In comparison to the many billions of neurons in the brain, the gut’s hundreds of millions might not seem like much. And, yet, it’s quite clear “gut feelings” are no longer just a metaphor.