Hippocrates, the father of modern medicine, once said, “All disease begins in the gut,” and many contemporary doctors believe he would certainly include mental illnesses within that statement. Recent research, including an article published in Scientific American, establishes the link between brain and gut. Yet other researchers go so far as to propose that changes in diet can lead to changes in mental health if such dietary changes alter the composition of the bacteria living in our gut.


'Gut microbiota' encompasses all the microorganisms that inhabit our intestinal tract. Infants are born with a nearly sterile gut, yet within days of birth, their intestines become colonized with microbiota encompassing a variety of organisms, including bacteria, fungi, archaea, and other species that have not yet been identified. It is now known that the human body contains about 100 trillion or so gut microbiota; this teeming ecosystem is so significant, in fact, that some scientists conceptualize it as an entirely separate 'organ' acquired after birth.

Surprisingly, a network of neurons — about 100 million nerve cells, to be precise — line the stomach and gut. Within this enteric nervous system, the neurotransmitter serotonin does most of the communication work. Yet, psychiatrists have long associated levels of serotonin in the brain with mood; a class of antidepressants known as SSRIs (selective serotonin reuptake inhibitors), in fact, blocks the reabsorption of serotonin in the brain and modulates mood.

What is interesting to note, though, is that about 80 percent of the body's total serotonin is in the gut, where it regulates intestinal movements. Meanwhile, the vagus nerve, which travels from the head to the abdomen, is busy carrying messages back and forth between the viscera and the brain; yet a full 90 percent of the fibers in the vagus nerve are conducting information from the bottom up — messages (and with it, serotonin) run more commonly from the gut to the brain.

Gut Brain Axis

Because of this clear connection between the two organs, scientists are now racing to understand and identify the individual components and complexities of gut microbiota. Even with an incomplete understanding, though, researchers believe that diet, which alters the composition of the flora in your gut, can also affect a person’s behavior. The Gut and Psychology Syndrome, or GAPS Diet, developed by Dr. Natasha Campbell-McBride, is based on this connection.

After working with both children and adults with neurological and psychiatric conditions, including autism, attention-deficit hyperactivity disorder (ADHD), and depression, Campbell-McBride found that a ‘typical’ portrait emerged. Often, her patients suffered with digestive disorders, malnourishment, allergies, asthma, eczema, chronic cystitis, and fussy eating habits. Because these patients have abnormal gut microbiota, they have deficiencies in important minerals, vitamins, essential fats, many amino-acids, and other nutrients, including important nutrients for normal development and function of the brain. Their poorly functioning digestive system means toxic substances have accumulated in their bodies, which in turn causes mental disorders as well as behavioral abnormalities and other problems.

Norwegian scientists, whose work is published in the journal Antonie van Leeuwenhoek, similarly discuss the composition of gut microbiota and its physiological effects. “Diet control can be used to favor the growth of some selected gut inhabitants,” the authors stated in their article, which also includes some thoughts on the beneficial effects of eating fermented foods.


The GAPS diet begins and ends with rebuilding, refeeding, and reseeding the gut with healthy bacteria through proper eating and probiotics. Unlike other diets, it does not emphasize raw food so much as cooked and even fermented vegetables, the unusual mainstay of this nutritional plan. (Anyone considering adopting this diet, should first read Campbell-McBride's book and discuss with their doctor.)

Campbell-McBride suggests that there are two natural food groups: plants (fruits, veggies, grains, beans, nuts, and seeds) and animal products (meat, fish, dairy products, eggs). And of these, plant foods generally cleanse our bodies by helping us eliminate and detoxify, while meat, fish, dairy, and eggs nourish our bones, muscles, brain, immune system, and other organs. A person with abnormal gut microbiota does not require the fiber that is provided by raw plant foods so much as easily digested cooked and fermented vegetables.

“The number and mixture of toxins can be very individual, causing different neurological and psychiatric symptoms," Campbell-McBride stated in her carefully researched paper, Gut and Psychology Syndrome (GAP Syndrome or GAPS), which is posted on her website. “Due to the absence or greatly reduced numbers of beneficial bacteria in the gut flora, the person’s digestive system instead of being a source of nourishment becomes a major source of toxicity in the body.”

Source: Umu OCO, Oostindjer M, Pope PB, et al. Potential applications of gut microbiota to control human physiology. Antonie van Leeuwenhoek. 2013.