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The new field of microbiome research studies the microbes within the gut and the effects of these microbes on the host’s well-being. Microbes influence metabolism, immunity and behaviour. One mechanism appears to involve hormones because specific changes in hormone levels correlate with the presence of the gut microbiota. The microbiota produce and secrete hormones, respond to host hormones and regulate expression levels of host hormones (Neuman et al., 2015). Increasing evidence links both hormones and the microbiome to immune responses under both healthy conditions and autoimmune disease. There are many interconnections and the microbiome and hormones may work through shared pathways to affect the immune response (Neuman et al., 2015).

Organisms within the gut play a role in the early programming and later responsivity of the stress system. The gut is inhabited by 10¹³–10¹⁴ micro-organisms, which is ten times the number of cells in the human body, and contains 150 times as many genes as our genome (Dinan and Cryan, 2012) or, according to Verdino (2017), 360 times. When pathogens such as Escherichia coli enter the gut, the HPA can be activated. Stress can induce an increased permeability of the gut, allowing bacteria and bacterial antigens to cross the epithelial barrier and activate a mucosal immune response, which in turn alters the composition of the microbiome and leads to an enhanced HPA drive. Research indicates that patients with irritable bowel syndrome and major depression show alterations of the HPA which are induced by increased gut permeability. In the case of irritable bowel syndrome, the increased permeability can respond to probiotic therapy. The gut microbiota play a role in regulating the HPA. In a double-blind, placebo-controlled trial, participants were given a fruit bar that contained either the probiotic formula or a similarly tasting placebo bar for 30 days (Messaoudi et al., 2011). The experimental fruit bar group reported significantly lower levels of anxiety, anger, depression and somatization, on a number of self-report measures. Lower levels of cortisol were also evident in the fruit bar condition compared to the control group. Verdino (2017) cautiously concludes his review of the growing connection between gut health and emotional well-being as follows: “… it is crucial not to oversimplify the idea that nutritional intervention and a healthy gut will be the panacea for profound psychological difficulties. Severe mood and paralyzing anxiety disorders are not going to be cured with probiotic yogurt and prebiotic fiber, alone” (Verdino, 2017: 4).

The immune and neuroendocrine systems share a common set of hormones and receptors. Glucocorticoids, such as corticosterone and cortisol, regulate inflammation levels and have effects both on the innate and adaptive immune responses. Additionally, vitamin D affects immune cell responses by enhancing antigen presentation. Moreover, sex hormones affect the immune response in numerous ways. The effects of hormones on microbiota are summarized in Figure 2.11.

Figure 2.11 Host effects on gut microbiota

A variety of host factors (such as diet, exercise, mood, general health state, stress and gender) lead to alterations in hormonal levels, which in turn lead to a variety of effects on the microbiota (including growth, virulence and resistance)

Source: Reproduced with permission from Neuman et al. (2015)