By Dr. Mercola 
Researchers examined the performance of germ-free mice, who lack gut bacteria, on a kind of maze used to test anxiety-like behaviors. The maze is in the shape of a plus with two open and two closed arms; normally, mice will avoid open spaces to minimize the risk of being seen by predators.
Normal mice, as expected, spent far more time in the closed arms when placed in the maze. The germ-free mice, however, entered the open arms far more often, spending significantly more time there than in the closed arms.
According to the study in Neurogastroenterology & Motility, when they examined the animals’ brains, they found that:
“these differences in behavior were accompanied by alterations in the expression levels of several genes in the germ-free mice. … Bacteria colonize the gut in the days following birth, during a sensitive period of brain development, and apparently influence behavior by inducing changes in the expression of certain genes.”
Neurogastroenterology & Motility March 2011; 23(3); 255–e119
Dr. Mercola’s Comments:
Most people fail to realize that your gut is quite literally your second brain, and actually has the ability to significantly influence your:
So while modern psychiatry still falsely claims that psychological problems such as depression are caused by a chemical imbalance in your brain , researchers keep finding that depression and a variety of behavioral problems actually appear to be linked to an imbalance of bacteria in your gut!
Germ-Free Mice Engage in High-Risk Behavior
In the featured study published last month in Neurogastroenterology & Motility, mice that lack gut bacteria were found to behave differently from normal mice, engaging in what would be referred to as “high-risk behavior.” This altered behavior was accompanied by neurochemical changes in the mouse brain.
According to the authors, microbiota (your gut flora) may play a role in the communication between your gut and your brain, and:
“Acquisition of intestinal microbiota in the immediate postnatal period has a defining impact on the development and function of the gastrointestinal, immune, neuroendocrine and metabolic systems. For example, the presence of gut microbiota regulates the set point for hypothalamic-pituitary-adrenal (HPA) axis activity.”
The neurotransmitter serotonin activates your hypothalamic-pituitary-adrenal axis  by stimulating certain serotonin receptors in your brain. Additionally, neurotransmitters like serotonin can also be found in your gut. In fact, the greatest concentration of serotonin, which is involved in mood control, depression and aggression, is found in your intestines, not your brain!
So it actually makes perfect sense to nourish your gut flora for optimal serotonin function as it can have a profound impact on your mood, psychological health, and behavior.
The authors concluded that:
“[T]he presence or absence of conventional intestinal microbiota influences the development of behavior…”
This conclusion adds support to another recent animal study , which also found that gut bacteria may influence mammalian early brain development and behavior. But that’s not all. They also discovered that the absence or presence of gut microorganisms during infancypermanently alters gene expression.
Through gene profiling, they were able to discern that absence of gut bacteria altered genes and signaling pathways involved in learning, memory, and motor control. This suggests that gut bacteria is closely tied to early brain development and subsequent behavior. These behavioral changes could be reversed as long as the mice were exposed to normal microorganisms early in life. But once the germ-free mice had reached adulthood, colonizing them with bacteria did not influence their behavior.
According to Dr. Rochellys Diaz Heijtz, lead author of the study:
“The data suggests that there is a critical period early in life when gut microorganisms affect the brain and change the behavior in later life.”
In a similar way, probiotics have also been found to influence the activity of hundreds of your genes , helping them to express in a positive, disease-fighting manner.
The Gut-Brain Connection
When you consider the fact that the gut-brain connection is recognized as a basic tenet of physiology and medicine, and that there’s no shortage of evidence of gastrointestinal involvement in a variety of neurological diseases, it’s easy to see how the balance of gut bacteria can play a significant role in your psychology and behavior as well.
With this in mind, it should also be crystal clear that nourishing your gut flora is extremely important, from cradle to grave, because in a very real sense you have two brains, one inside your skull and one in your gut, and each needs its own vital nourishment.
Interestingly, these two organs are actually created out of the same type of tissue. During fetal development, one part turns into your central nervous system while the other develops into your enteric nervous system. These two systems are connected via the vagus nerve, the tenth cranial nerve that runs from your brain stem down to your abdomen. This is what connects your two brains together, and explains such phenomena as getting butterflies in your stomach when you’re nervous, for example. (For an interesting and well-written layman’s explanation of this connection, read through Sandra Blakeslee’s 1996 New York Times article Complex and Hidden Brain in Gut Makes Stomachaches and Butterflies .)
Your gut and brain work in tandem, each influencing the other. This is why your intestinal health can have such a profound influence on your mental health, and vice versa.
As a result, it should be obvious that your diet is closely linked to your mental health. Furthermore, it’s requires almost no stretch of the imagination to see how lack of nutrition can have an adverse effect on your mood and subsequently your behavior.
Have We Become Too Sanitized for Our Own Sanity?
Another study published last year in the Archives of General Psychiatry  reviewed the evidence for signs that psychiatric problems might be caused by lack of natural microorganisms  in soil, food, and the gut. And it did find such a link.
Rates of depression in younger people have steadily grown to outnumber rates of depression in the older populations, and one reason for this could be the lack of exposure to bacteria, both outside and inside your body.
Quite simply, modern society may have gotten too sanitized and pasteurized for our own good.
Fermented foods have been traditional staples in most cultures, but modern food manufacturing, with its focus on killing ALL bacteria in the name of food safety, has eliminated most of these foods. You can still find traditionally fermented foods  like natto or kefir, but they’re not the dietary staples they once used to be, and many people don’t like them when trying them out for the first time in adulthood.
When you deprive your child of all this bacteria, her immune system—which is her primary defense system against inflammation—actually gets weaker, not stronger. And higher levels of inflammation are not only a hallmark of heart disease and diabetes, but also of depression.
The authors explain  it as follows:
“Significant data suggest that a variety of microorganisms (frequently referred to as the “old friends”) were tasked by coevolutionary processes with training the human immune system to tolerate a wide array of non-threatening but potentially proinflammatory stimuli. Lacking such immune training, vulnerable individuals in the modern world are at significantly increased risk of mounting inappropriate inflammatory attacks on harmless environmental antigens (leading to asthma), benign food contents and commensals in the gut (leading to inflammatory bowel disease), or self-antigens (leading to any of a host of autoimmune diseases).
Loss of exposure to the old friends may promote major depression by increasing background levels of depressogenic cytokines and may predispose vulnerable individuals in industrialized societies to mount inappropriately aggressive inflammatory responses to psychosocial stressors, again leading to increased rates of depression.
… Measured exposure to the old friends or their antigens may offer promise for the prevention and treatment of major depression in modern industrialized societies.”
Researchers around the World have Linked Gut Problems to Brain Disorders
Brain disorders can take many forms, one of which is autism. In this particular area you can again find compelling evidence of the link between brain and gut health. For example, gluten intolerance is frequently a feature of autism, and many autistic children will improve when following a strict gluten-free diet. Many autistic children also tend to improve when given probiotics, either in the form of fermented foods or probiotic supplements.
Dr. Andrew Wakefield is just one of many who have investigated the connection between developmental disorders and bowel disease. He has published about 130-140 peer-reviewed papers looking at the mechanism and cause of inflammatory bowel disease, and has extensively investigated the brain-bowel connection in the context of children with developmental disorders such as autism.
A large number of replication studies have also been performed around the world, by other researchers, confirming the curious link between brain disorders such as autism and gastrointestinal dysfunction. For a list of more than 25 of those studies, please see this previous article .
Other Health Benefits of Probiotics
Your body contains about 100 trillion bacteria — more than 10 TIMES the number of cells you have in your entire body. Ideally, the ratio between the bacteria in your gut is 85 percent “good” and 15 percent “bad.”
In addition to the psychological implications discussed above, a healthy ratio of good to bad gut bacteria is essential for:
- Protection against over-growth of other microorganisms that could cause disease
- Digestion of food and absorption of nutrients
- Digesting and absorbing certain carbohydrates
- Producing vitamins, absorbing minerals and eliminating toxins
- Preventing allergies
Signs of having an excess of unhealthy bacteria in your gut include gas and bloating, fatigue, sugar cravings, nausea, headaches, constipation or diarrhea.
What Interferes With Healthy Gut Bacteria?
Your gut bacteria do not live in a bubble; rather, they are an active and integrated part of your body, and as such are vulnerable to your lifestyle. If you eat a lot of processed foods, for instance, your gut bacteria are going to be compromised because processed foods in general will destroy healthy microflora and feed bad bacteria and yeast.
Your gut bacteria are also very sensitive to:
- Chlorinated water
- Antibacterial soap
- Agricultural chemicals
Because of these latter items, to which virtually all of us are exposed at least occasionally, it’s generally a good idea to “reseed” the good bacteria in your gut by taking a high-quality probiotic supplement or eating fermented foods.
Tips for Optimizing Your Gut Bacteria
Getting back to the issue of inflammation for a moment, it’s important to realize that an estimated 80 percent of your immune system is actually located in your gut, which is why you need to regularly reseed your gut with good bacteria.
Additionally, when you consider that your gut is your second brain AND the seat of your immune system, it becomes easy to see how your gut health can impact your brain function, psyche, and behavior, as they are interconnected and interdependent in a number of different ways—several of which are discussed above.
In light of this, here are my recommendations for optimizing your gut bacteria.
- Fermented foods are still the best route to optimal digestive health, as long as you eat the traditionally made, unpasteurized versions. Healthy choices include lassi (an Indian yoghurt drink, traditionally enjoyed before dinner), fermented milk such as kefir , various pickled fermentations of cabbage, turnips, eggplant, cucumbers, onions, squash and carrots, and natto  (fermented soy).If you regularly eat fermented foods such as these that, again, have not been pasteurized (pasteurization kills the naturally occurring probiotics), your healthy gut bacteria will thrive.
- Probiotic supplement. Although I’m not a major proponent of taking many supplements (as I believe the majority of your nutrients need to come from food), probiotics are definitely an exception. I have used many different brands over the past 15 years and there are many good ones out there. I also spent a long time researching and developing my own, called Complete Probiotics, in which I incorporated everything I have learned about this important tool over the years.
If you do not eat fermented foods, taking a high quality probiotic supplement is definitely recommended.
Copyright Dr. Joseph Mercola, 2011. All Rights Reserved.