Depression: A Gut Feeling?

In the scientific publication, “Influence of Tryptophan and Serotonin on Mood and Cognition with a Possible Role of the Gut-Brain Axis.” (2016) in the 8th volume and 56th issue of Nutrients, authors Jenkins, Nguyen, Polglaze and Bertrand explain how the amino acids Tryptophan and Serotonin affect depression, anxiety, sleep, memory and more through the gut-brain axis. This is a summary and interpretation of their findings.

The direct link between the intestines (gut) and the brain through the vagus nerve and dorsal root ganglia (DRG) is a hot topic in scientific circles right now. The idea that the food we eat and the microbes that live in our gut may actually impact our mood, behavior and social interactions is like the proverbial caveman’s discovery of fire! We can now say that the link exists between the gut and brain, flowing both ways with one affecting the other and vice-versa.

It has long been known that the neurotransmitter, serotonin, is involved in the “happy” mood or feeling that we experience. This is why low serotonin levels in the brain result in depression and so much more. Well, we don’t get serotonin from simply eating it. Our bodies have to make it. The amino acid, tryptophan, is actually the starting molecule for making serotonin. It is an amino acid that our body must have in order to make proteins in our cells.

Tryptophan is found in foods high in protein, like eggs, poultry, fish, meats, dairy, nuts, seeds and more. Tryptophan is broken down to 5-HTP and then again to 5-HT (serotonin). In the brain, serotonin is taken up by vesicles (bubbles) in our neurons that cover a large portion of our brain. Since serotonin is spread out over most of the brain, it affects many things like sleep, appetite control, body temperature, mood and awareness.

Depression affects around 20% of the World Population. The common therapy for depression is the use of antidepressant drugs. These drugs are called “serotonin reuptake inhibitors” or “serotonin/noradrenaline reuptake inhibitors.” They work partly to keep more serotonin in the gaps between neurons so that we feel happy longer. However, studies show that antidepressants are only partially effective for moderate to severe levels of depression. We must look beyond the brain to understand the path of serotonin from beginning to end.

In human studies, when dietary tryptophan (and the product, serotonin) is depleted patients show reduced verbal reasoning, episodic memory, working memory and emotional processing. When tryptophan is added to the diet, patients often see improved attention scores, visual memory, facial recognition memory, reaction times and a decrease in the startle response.

Dietary tryptophan (and the product, serotonin) has a clear impact on sleep. People suffering from depression often report “poor sleep quality.” Treatment with antidepressants can worsen these symptoms and decrease the total amount of sleep. This makes complete sense, since serotonin is the starting hormone for making melatonin in the pineal gland, which regulates sleep. Proper tryptophan levels are able to increase sleepiness and decrease wakefulness. Tryptophan also increases thequality of sleep and increases morning alertness and attention.

In the body, serotonin can be found in the wall of the gut or traveling along the blood “superhighway” attached to platelets. Did you know that a whopping90% of total body serotonin is made in the enterochromaffin cells of the gut wall!? So, the gut has a pretty big role when it comes to serotonin levels! Even better, we have now found that certain gut microbes produce short chain fatty acids (SCFAs) that cause the gut wall cells to make more serotonin. What if you’ve unknowingly killed off some or most of these good microbes through antibiotic use, diet, and environmental toxins? Are you able to make as much serotonin as a healthy individual? The answer is probably not. This may be why people who suffer from gut health issues like irritable bowel syndrome (IBS) often present with altered levels of serotonin in the gut andaltered cognitive function.

The great news is, studies show that mice given the probiotic, Lactobacillus rhamnosus, show reduced levels of the stress hormone, corticosteroid. This lead to lower levels of anxiety and depression. This effect was not seen when the vagus nerve was clipped, suggesting a direct connection between the gut and brain and the impact of gut microbes on mental health.

If you’re interested in improving your mental health, mood, memory, sleep, attention and more, then be purposeful about choosing a variety of foods rich in tryptophan. Foods rich in protein such as eggs, poultry, fish, meat, dairy, nuts, and seeds can provide the raw material of tryptophan. A daily probiotic, including L. rhamnosus and other important species, may provide the short chain fatty acids that increase serotonin production. Finally, it's important to protect your good gut bacteria by avoiding  unnecessary antibiotic use, processed foods containing high amounts of sugar or preservatives, and environmental toxins.

If you are interested in reading more, this article is based on the following publication:
Jenkins, T., Nguyen, J., Polglaze, K., and Bertrand, P. (2016) Influence of Tryptophan and Serotonin on Mood and Cognition with a Possible Role of the Gut-Brain Axis. Nutrients. 8(56). Doi:10.3390/nu8010056