Testosterone

How Gut Bacteria Influence Male Hormones

The animal evidence here is unusually direct, while the human work is still catching up. Testosterone is the main male sex hormone. It is made in the testes, on instructions sent from the brain. Gut bacteria appear to sit inside that system rather than alongside it. They can affect how much testosterone circulates and how much of it is bound up and unavailable to the body. Explanations of low testosterone usually stop at age, weight and sleep. Those all matter. The gut adds another route into the same system.

An experiment that moved the hormone rather than measuring it

Most research on hormones and bacteria can only show that two things occur together. One mouse study went further. Researchers used a strain of mice bred to develop type 1 diabetes, in which females get the disease far more often than males. That sex difference disappeared when the mice were raised germ-free, meaning with no gut bacteria at all.

The team then transferred gut contents from adult males into young females. The recipients' gut bacteria shifted. Their testosterone rose. Inflammation in the pancreas fell, and far fewer of them went on to develop diabetes.

The final step is what makes this persuasive. When researchers blocked the receptor that testosterone acts through, the protection vanished. The bacteria were working through the hormone rather than around it.

Source: Sex differences in the gut microbiome drive hormone-dependent regulation of autoimmunity

What the human studies have found so far

Mice are not men, and the human research sits at an earlier stage. A systematic review gathered the studies measuring gut bacteria alongside testosterone in men. It found a consistent positive relationship between the two across those studies. Ruminococcus, a group of bacteria that ferment fibre, showed the strongest link of any single group.

A separate study looked at older men specifically. Overall bacterial variety did not differ between those with high and low testosterone. Individual groups did differ. Several members of the Firmicutes, a large family containing many fibre fermenters, were more common in men with higher total testosterone. Their abundance rose in step with the hormone.

These are snapshots rather than experiments. No one has yet deliberately changed men's gut bacteria and watched what testosterone does in response.

Source: Potential relationship of the gut microbiome with testosterone level in men: a systematic review

Source: Firmicutes in Gut Microbiota Correlate with Blood Testosterone Levels in Elderly Men

Why fibre fermenters keep turning up

The same kinds of bacteria appearing in both sets of findings is worth pausing on. There is a proposed route that would explain it. The gut wall normally keeps bacteria and their fragments out of the bloodstream. When that barrier loosens, pieces of bacterial cell wall can slip through. The immune system reads them as a threat and mounts a low, constant inflammatory response.

The cells in the testes that manufacture testosterone are sensitive to that signal. Researchers have put this forward as one reason testosterone tends to run lower in men carrying excess weight, since more body fat is associated with both a looser gut barrier and more bacterial fragments in the blood.

This is a proposed mechanism rather than a settled one. It does fit the pattern, because fibre fermenters produce the compounds that help keep the gut barrier tight in the first place.

Source: Potential relationship of the gut microbiome with testosterone level in men: a systematic review

What this means for you

The case adds up like this. In mice, changing the gut bacteria changed testosterone directly, and the effect ran through the hormone itself. In men, the bacteria that track with higher testosterone are repeatedly the ones that ferment fibre. The proposed mechanism connecting them runs through the gut barrier those same bacteria help maintain. Symptoms such as persistent fatigue, low libido or erectile difficulties deserve a blood test and a proper conversation with a doctor rather than a dietary experiment. Alongside that, feeding fibre fermenters is a reasonable thing to do, since that is the group the evidence keeps pointing at. Pulses, whole grains, vegetables, nuts and seeds each support different members of that community. That is the idea GutLab is built on, where everyday gut health starts with regularly feeding the microbiome, the community of microbes living in the gut, a broad range of plant-rich ingredients.