Usually when we hear “gut health” we think of digestion and regular elimination; when we hear “mental health” we think of freedom from depression and psychological disorders. But each go much deeper than that and they overlap. The gut and the brain are so intimately associated that an entire field called neurogastroenterology exists!  

Have you ever experienced mental stress resulting in gastrointestinal symptoms? — or observed that eating goes hand in hand with your emotions? These links are backed up by biochemistry. Your brain’s networks in charge of feeding are tied to those that process feelings, reward and cognition.1 

It’s known that messages travel the opposite way as well. Your gut impacts mood and thinking. The complicated two-way messaging system is referred to as a Gut Brain Axis. This bidirectional communication between the gut and central nervous system (CNS) plays a key role in shaping cognition, emotional behavior and pain response. 


 

Here’s what is going on in your gut – the natural bacteria of the large intestine (collectively called flora, microbiota, or microbiome) affect the brain in several ways. The many pathways include:   

  • Enteric Nervous System – Encompassing the whole GI tract,2 signals derived from the enteric nervous system can affect mood.1 Its vagus nerve acts as the “communication superhighway,” sending signals to brain1,3,4 including the presence of molecules in the gut.2 Gut microbes can activate this vagus nerve.2 The FDA even recognized its influence by approving vagal nerve stimulation as a treatment of chronic resistant depression.1  
  • Gut Barrier Function – The intestinal lining is the gateway between external substances and our system,4 and “gut permeability” refers to its strength. Microbes interact locally with intestinal cells,2 impact gut permeability3,4 and thus influence what gets into circulation and can reach the brain.3  Nutritional insults (poor diet, allergens, antibiotics, environmental toxins) can disrupt this intestinal barrier function.5
  • Immune System – Housing most of the body’s immune cells,2 the gut can talk to the brain this way.3 Gut microbes act on the immune system’s communication,2,3 affecting molecules that promote or suppress inflammation.2,5 Preventing chronic low-level inflammation protects against mental health disorders, cognitive decline, and the development of neurological diseases.2  
  • Hormones and Neurotransmitters – Ghrelin (from the stomach) influences emotions and cognitive processes while insulin (from the pancreas) alters synaptic activity and cognitive processing.1 Microbiota interact directly with the CNS through this endocrine pathway as well,2 actively secreting hormones and neuropeptides4 such as gamma-aminobutyric acid5 and serotonin, the “feel-good” neurotransmitter.  
  • Metabolites – Gut microbes produce short-chain fatty acids such as the neuroprotective butyrate which benefits the brain,5 and there may be other chemicals that gut bacteria produce which trigger signals to the brain. Imbalance of gut flora can produce noxious components that impact mental well-being.5 
  • Neuronal Integrity– The brain and nervous system require a relatively high amount of energy, thus neurons rely on food.1 The nutrient availability for those neurons is influenced by gut microbiota by way of supplying several vitamins.5 The gut can also stimulate learning and adaptive changes in the junctions between neurons.1  


 

Changing gut microbiota can affect the brain, altering the way it responds to the environment. Even short insults such as a course of antibiotics or bout of gastroenteritis can damage microbes. How to bring gut flora back into balance and promote mental wellness? Feed it right! 

Different types and sources of dietary fiber will support a diverse gut microbiome.2 Fiber is found in plant foods and acts as a prebiotic, feeding the microbiota. High fiber foods include beans, bran, vegetables, corn, oats, barley, dried fruit, and seeds. 

Yogurt cultures help cultivate the digestive system’s microbiota, which is a complicated process. In a very small study published in Gastroenterology, researchers at UCLA discovered that regular probiotic intake through yogurt cultured milk showed altered brain function in women, with greater brain network activity.6  

Evidence shows that probiotic supplementation can have a positive benefit on mood, reducing anxiety, signs of distress and depression.2,3 It’s possible that certain bacteria strains affect neurotransmitters, alter pathways modulating neural signaling and inhibit neuroinflammation.3 Externally introduced probiotics can change gut bacteria composition within weeks, ‘talking’ to current gut microbes rather than colonizing and taking up residence.3 Don’t overdo it! Rarely, too much of a good thing can cause problems, such as bacterial overgrowth in the small intestine!7   

Dietary components beyond probiotics, namely vitamins A and D, can maintain and improve gut barrier function.4 These fat-soluble vitamins can be found in egg yolks, fatty fish, cheese, liver, and fortified foods. 

In a nutshell, gut microbiota composition influences cognition and mood. Gut flora can also influence the body’s inflammatory stress response and may impact its susceptibility to neurological disease. A diet rich in whole plant foods, plus probiotic sources such as fermented foods, is essential to support mental health through a healthier gut microbiome. 

References: 

  1. Brain Foods: The Effects of Nutrients on Brain Function. F Gomez-Pinilla. Nature Reviews Neuroscience. 2008 Jul; 9(7): 568-578. 
  2. Exploring the Microbiome-Gut-Brain Axis. C Dennett. Today’s Dietitian. 2017 Feb; 19(2): 14-15. 
  3. Brain Health: Probiotics – Big Brain Boost or Just Hype? J Santa Cruz. Today’s Dietitian. 2018 Dec; 20 (12): 8. 
  4. Exploring the Gut-Brain Axis. T Wolfram. Food & Nutrition. December 15, 2017. 
  5. Integrative Nutrition Therapy for Mood Disorders. KM Swift. Today’s Dietitian, 2017 Oct; 19 (10): 36-40. 
  6. Consumption of Fermented Milk Product with Probiotic Modulates Brain Activity. K Tillisch, et al. Gastroenterology. 2013 Jun; 144(7): 10 
  7. Brain fogginess, gas and bloating: a link between SIBO, probiotics and metabolic acidosis. SSC Rao, et al. Clinical and Translational Gastroenterology. 2018 June 19; 9(6); 162.