Mental health and the microbiome: Can fiber help ward off depression?

In recent decades, the gut to brain communication in the language of hormones and neurotransmitters has been of great interest in psychiatric research. It is well known that microbiome composition in the gut creates the chemical products and neurotransmitters that affect behavior and emotion [1]. The  neurotransmitters we associate with the brain that affect psychiatric disorders are found at high levels in the gut and these molecules even exist in larger populations in the colon than the brain. For instance, 90% of serotonin is produced in the gut and 50% of dopamine [2]. These important neurotransmitters primarily communicate through the vagus nerve that connects signals from the brain to much of the periphery, including the enteric nervous system that controls gut reactions [3]. 

In fact, in a more recent SSRI study in mice, it was shown that when the vagal nerve was cut, SSRIs ceased having a positive therapeutic effect. Meaning, increased stimulation of the vagal nerve by serotonin in the gut is likely the true mechanism for antidepressant effects and therefore more attention paid to how to improve vagal nerve function is an open new path to antidepressant and mood related pathologies [4]  (There is a lot of research related to the functioning of the vagus nerve and mental health that is beyond the scope of this specific article. Stay tuned for future posts.)

In short, the makeup of the microbiome in your gut and the ability of your vagus nerve to transport signals has significant impacts on your emotions and behavior. And with an improved microbiome, comes a helpful route out of negative emotions and depression. 

So how do we keep the gut microbiome diverse and healthy? 

One main factor, is fiber, especially resistant starches. Resistant starch does not get digested in the small intestine, but rather gets fermented by microbiota in the colon [5].  We have made carbohydrates the enemy recently, but there are ones that are an exception to this demonization. Resistant starches do not raise blood sugar and insulin the way simple sugars and starches do. They are fibers that feed your healthy gut microbes and lead to diversity that no probiotic can provide. 

There have been specific studies, primarily in vitro, attesting to the effectiveness of prebiotics on anxiety and depression. In mice, treatment with FOS and GOS prebiotics was shown to reduce stress hormone, increase short chain fatty acids,( which have been shown to improve behavior), and decrease some pro-inflammatory proteins [6].

This evidence and ongoing research has proven the importance of fiber to our mental health. 

Some of the best sources include: Sweet potato with the skin, green peas, oats, beans and lentils and green bananas. 

(Please note: Those with SIBO (small intestinal bacterial overgrowth) may experience gas and bloating due to the fermentation of these starches. So if you have these side effects from increasing your fiber and resistant starch consumption, make an appointment with your naturopathic doctor right away to get to the bottom of it.)