New Light on Those Gut Feelings

If you were asked, “What are humans primarily made of?” would you answer: cells? Many people would, but we are made mostly of bacteria, and the majority of that bacteria resides in the intestines — what we refer to as our gut.[1] Current research is proving that gut bacteria is far more important than previously thought. The microbiome, the term for the collective gut bacteria, may be a vitally important connection to the mind, particularly the mood and certain behaviors such as obsessive compulsive disorder (OCD), anxiety, and depression. By tweaking that bacteria, these conditions can be treated and, in some instances, eradicated.

It’s not as strange as it sounds. We already know that some people refer to their stomachs as their second brain, implying they can “feel” stress in that area. It’s accepted that being under a lot of stress lowers immunity and there’s a reason: even moderate stress can lower the level of beneficial bacteria in our gut and, when this occurs, harmful bacteria multiply, creating inflammation. Once inflammation occurs, infections take hold easily and we get sick.

But that same gut/brain connection can be reversed, so it works in ways that may make you well. Take the case of Mary, a teenager who arrived in the office of Boston-based psychiatrist, Dr. James Greenblatt. Mary had several digestive complaints, OCD, and attention deficit hyperactivity disorder (ADHD). Dr. Greenblatt noted, “Mary’s parents had been running around for many years and she’d had a poor response to medicine.” He then noted, “When a patient doesn’t respond, that’s a red flag.”[2]

So Greenblatt tested Mary’s urine for the metabolite HPHPA, which is a byproduct of clostridia, a bacteria that lives in the human intestinal tract. It was elevated. Greenblatt then placed Mary on a regimen of powerful probiotics to increase her good bacteria while prescribing antibiotics. Her metabolite levels decreased significantly, and so did her symptoms.

Within six months, Mary’s symptoms were subsiding and by high school graduation, Mary did not exhibit any symptom of either OCD or ADHD. Today, three years later, Mary is a senior in high school and still symptomless. Cases like these are causing science to take another look at the human gut.

Human intestines possess amazing attributes. With 100 million neurons entrenched in the wall of the gut, they boast an independent neural network so strong that it does not stop working even when the vagus nerve, the most vital nerve connection between the gut and the brain, is cut. And it needs to be a strong organ: over 100 trillion microbes call the gut their home.[1] Metabolic processes and digestion are programmed by the gut; even the body’s immune system is regulated through its power. But the key to the brain/gut axis may lie in the hundreds of neurochemicals that influence both mood and memory. One neurotransmitter in particular, serotonin, is responsible for our feelings of well-being and happiness; gut bacteria manufactures 95% of the body’s serotonin.[3] By manipulating the levels of gut bacteria, behavior can ultimately be influenced.

Illustrating this is a 2011 study performed by Dr. Premysl Bercik. He fed a particular breed of shy, timorous mice an antibiotic that measurably altered the make up of their gut bacteria. The result? “Their behavior completely changed,” Bercik says. “They became bold and adventurous.”[1]

In another study, William Shaw, Ph.D collected urine samples from autistic and schizophrenic patients. What he found was an increased level of a metabolite of clostridia (HPHPA), a gut bacteria, could be linked to behavior. He studied one child who was undergoing an acute psychotic episode. The concentration of the metabolite remained high until the episode was over.[3]

How might this work? Since neurotransmitters, the brain’s chemicals that communicate throughout our bodies, are created from raw materials supplied by intestinal microorganisms, their chemical make up can predict behavior. Shaw explains, “Patients with values of HPHPA greater than 500 mmol/mol creatinine in the urine almost always have severe neurological, psychiatric, or gastrointestinal disorders such as autism, severe depression, psychotic behavior or schizophrenia, muscle paralysis, or colitis or sometimes a combination of these disorders.”

But researchers caution that this discovery is not a magical cure for patients. Not every patient exhibiting symptoms fits the profile of having elevated levels of metabolites. And pathways still need to be untangled. What this does mean for patients and doctors is that there is new hope in treating common mental disorders using less prescription medication. With increasing knowledge of how the body functions, science can work with the body to create healthier lives.

Image from [4].


  1. S. Carpenter, That gut feeling, APA 33, p. 50, 2012.
  2. S. D. James, Anxiety in your head could come from your gut, 2013.
  3. W. Shaw, Increased urinary excretion of a 3-(3-hydroxyphenyl)-3-hydroxypropionic acid (HPHPA), an abnormal phenylalanine metabolite of Clostridia spp. in the gastrointestinal tract, in urine samples from patients with autism and schizophrenia, Nutr. Neurosci. 13, pp. 135–143, 2010.
  4. Wikimedia Commons, 12 Minutes to Heaven Teaser.