How does your gut microbiome influence your hormone balance?

Gut bacteria produce an enzyme that converts oestrogen from an inactive to an active form. When the diversity of your gut flora declines, so does this enzyme, and with it your active oestrogen level. Researchers link this to conditions such as metabolic syndrome, endometriosis and PCOS. The connection is biologically well explained and is supported by several studies, although the direction of cause and effect in humans has not yet been fully established.

Bacteria that ferment fibre produce short-chain fatty acids. These substances stimulate specialised cells in the gut wall to produce satiety hormones. Two hormones play a major role here: GLP-1, which also regulates blood sugar levels, and PYY, which suppresses the sensation of hunger. Disruptions in this system are associated with obesity and diabetes. Interestingly, metformin, a widely used diabetes drug, also works in part through this microbiome-hormone channel, although the exact contribution of the gut flora to that process has not yet been precisely defined.

Your thyroid gland also has a connection to your gut microbiome. Dysbiosis disrupts the absorption of nutrients the thyroid needs, such as iodine, selenium and zinc. This can impair the production and conversion of thyroid hormones. Dysbiosis is also linked to autoimmune diseases of the thyroid, such as Hashimoto's. Probiotics showed a positive effect on thyroid hormones in several studies, but these are still small-scale investigations.

Through the so-called gut-brain axis, gut bacteria also influence neurotransmitters and the hormone balance that regulates your mood. Dysbiosis is associated with an increased risk of depression. The precise cause-and-effect relationship here remains unclear, however: it has not yet been established whether a disrupted gut flora causes depression, whether depression disrupts the gut flora, or whether both occur simultaneously. Targeted treatments therefore do not yet exist.

A recent study (published in Science) revealed an interesting chain of events in osteoarthritis. Patients with osteoarthritis had lower levels of a particular gut bacterium, resulting in reduced production of a bile acid compound that normally stimulates GLP-1 release. Lower GLP-1 in the joint appears to worsen joint inflammation. In mice, restoring this compound had a protective effect. In humans, only associational data exist; it is too early to draw clinical conclusions from this.