Within your body, bacteria outnumber your own cells by ten to one. Most of these bacteria—more than 1,000 species—are found inside your gut. And thanks to a remarkably complex neural network there that connects with the one in your brain, gut bacteria may influence your brain chemistry, moods, and stress responses, according to an article in the September issue of Monitor on Psychology.
It’s no surprise that we get “gut feelings.” The network of nerve cells lining the gut wall is so vast that it has been nicknamed the second brain. It contains some 100 million neurons—more than the spinal cord—which communicate not only among themselves, but also with the brain in your head.
In addition, the gut produces and responds to the same neurochemicals as the brain. For example, gut bacteria make about 95 percent of the body’s supply of serotonin, a chemical that helps regulate both intestinal activity and mood. Other neurochemicals that affect thought and mood in the brain—and that are made in the gut as well—include GABA, norepinephrine, dopamine, and acetylcholine.
Scientists are just starting to explore how the brain-gut connection may influence mental health. Most of the research so far has been done in animals.
In one new study, researchers at University College Cork in Ireland reared mice in a germ-free environment so that normal, health-promoting bacteria never had a chance to grow in their guts. In these mice, the absence of gut bacteria early in life affected levels of brain serotonin later on.
It’s a big leap from mice to humans. But if lack of normal gut bacteria throws off the brain’s serotonin supply in people, it could have major consequences for well-being. Serotonin is thought to play a key role in regulating mood, sleep, and appetite, and it may also be involved in learning and memory.
Other scientists are finding evidence for a brain-gut connection as well. For example, researchers at McMaster University in Canada discovered that they could make mice act less cautious by giving them antibiotics, which changed the mix of bacterial species in their guts.
According to John Cryan, PhD, one of the researchers at University College Cork, the brain-gut link could help explain why people with intestinal disorders such as irritable bowel syndrome are at high risk for anxiety disorders and depression. It could also be one reason why several small studies suggest that children with autism spectrum disorders have an abnormal mix of gut bacteria.
In a recent study from Columbia University, for example, researchers compared tissue samples taken from the guts of children with intestinal problems, some of whom also had autism. One particular type of bacteria, called Sutterella, was found in about half of the samples from children with autism, but in none of the samples from non-autistic children.
This raises the tantalizing prospect that doctors might one day treat anxiety and mood disorders with probiotics—beneficial bacteria. For now, it’s too early to say whether taking probiotic supplements or eating probiotic-enriched foods might have specific brain benefits.
But it’s certainly good for your whole body to follow a gut-healthy lifestyle: eating fiber, drinking plenty of water, exercising regularly, and reducing stress. In his book No Guts, No Glory, Steven Lamm, MD also recommends eating more raw or lightly cooked foods, avoiding high-glycemic carbs (such as white bread and pasta, pastries, and chips), and choosing locally grown, seasonal, organic foods when possible.
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