How the microbiome influences anxiety and depression:
Microbiotas are bacteria and enzymes that colonize the body shortly after birth, within the first few days, and are present until we expire. This colonization of microbiota is known as a microbiome, and all mammals have them. The microbiomes help us have a normal balanced body that is able to fight off disease. The types of bacteria are too many to name, with up to 1,000 different species in one microbiome. Age, geography, sex, diet, genes, stress, and antibiotic treatments all have an influence on what types of microbiota are present in the body. Gut microbiota are fundamental to how the central nervous system, or CNS, works. Scientist are finding out just how fundamental microbiota are because a lot of recent animal studies are pointing to the relationship between the gut health and CNS and hypothalamic-pituitary-adrenocortical, or HPA axis, functions.
The microbiomes in the gut develop alongside the HPA axis during our early childhood and ultimately govern our stress response and our immune system throughout life. If a major stressor is present during early childhood it may disrupt the homeostasis of these systems. One study on rats showed a major difference in the composition of the microbiomes present in the rats separated from their mothers at birth compared to the rats that were allowed to remain with the mother. The rats that were separated from their mother showed signs of increased stress reactivity and cortisol levels later in life. Probiotics were then administered to the rats showing signs of stress related behavior and the probiotics regulated the cortisol levels in the rats. The probiotic used was Lactobacillus farciminis, and it reduced the intestinal permeability which is related to HPA hyper-activity. This is evedent in another study on mice that were fed the rodent equivalent of e-coli called Citrobacter rodentium. The Citrobacter rodentium created stress on the gut and the microbiomes in the gut which then set off a reaction that traveled to the brain through the vagus nerve, resulting in CNS dysfunction in the mice.
The vagus nerve is the longest nerve in the human body and travels from the brain stem down to the abdomen (2). The nerve travels along the body interacting with many organs and systems along the way, including the medulla of the brainstem, the larynx, the heart, esophagus, stomach, colon, and the gastro-intestinal tract (2). The vagus nerve sends a distress signal from the gut and the brain will react according to the signal. The good news is that the majority of studies show an increase in functionality of the HPA axis, the CNS, and the immune system when probiotics are included in the diet.
Behavior is also impacted when the balance of the gut is altered. In multiple rodent studies the introduction of inflammation or infection of the gut resulted in increased levels of anxiety, but when they were treated with probiotics the anxiety levels decreased, along with the inflammation or infection. This is more evidence that infection and gut inflammation can increase anxiety and depression, while probiotics can decrease anxiety and depression.
So what about probiotic treatment and humans? While there haven’t been a significant number of studies on humans, the studies that have taken place show great promise. In one study healthy subjects were either given a probiotic mix of Lactobacillus helveticus R0052 and B. longum R0175 or a placebo. They were tested at the end of 30 days on anxiety, depression, and stress. The group taking the probiotic mix was much better off and had experienced less psychological stress than the control. Another group of healthy subjects were fed probiotic drinks or a placebo control for three weeks. This group was tested on cognition and mood when they returned and they too showed significant improvement in both. Yet another study on people suffering with chronic fatigue syndrome, a disorder that caused anxiety and GI dysfunction, who took Lacuobacillus casei for 2 months had less anxiety and less GI issues. More studies are being done to decode the exact nature of the gut-brain axis, but the reoccurring theme is that probiotics may be a powerful tool that can help heal the damage stress does to our brain and guts. In fact the hippocampus, the CNS, the immune system, and the HPA axis all perform better when we include probiotics in our diet.