Progesterone

The Two-Way Traffic Between Progesterone and the Gut

This is a young field, but the mechanisms uncovered so far are unusually specific. Progesterone is the hormone that rises after ovulation and stays high through pregnancy. The relationship between it and gut bacteria runs in both directions. The hormone changes which bacteria thrive, and certain bacteria can manufacture compounds in the progesterone family themselves. Discussion of progesterone normally stays with the ovaries and the placenta. A second, smaller production line turns out to be sitting in the large intestine.

The hormone reshapes the community

Start with the better-established direction. Gut bacteria change substantially during pregnancy, and researchers wanted to know what was driving it.

They tracked gut bacteria in women and in mice through late pregnancy. Both showed a marked shift, including a rise in Bifidobacterium, a group of bacteria that ferment fibre and are the dominant bacteria in a breastfed baby's gut.

To test whether progesterone itself was responsible, the team implanted progesterone pellets into mice that were not pregnant. Bifidobacterium rose in those mice too. They then grew the bacteria in the laboratory with progesterone added and saw the same effect without any animal involved at all.

That is three lines of evidence pointing the same way. The hormone acts on the bacteria directly rather than through some other change that pregnancy brings.

The wider shift is worth noting too. As pregnancy progresses, bacterial variety falls and producers of short-chain fatty acids decline. Short-chain fatty acids are the compounds bacteria make from fermenting fibre. The overall pattern comes to resemble one usually associated with metabolic problems, yet in pregnancy it appears to be a normal part of the process.

Source: Progesterone Increases Bifidobacterium Relative Abundance during Late Pregnancy

The bacteria return the favour

The traffic runs the other way as well, and this is where it gets genuinely surprising.

The liver sends steroid hormones into the gut in bile, the fluid it makes to help digest fat. Among them are corticoids, the family that includes the stress hormone cortisol. Researchers found that two gut bacteria can strip a chemical group off those corticoids. The bacteria are Gordonibacter pamelaeae and Eggerthella lenta. Removing that group converts the corticoids into progestins, meaning compounds in the progesterone family.

One of the products is allopregnanolone. That compound is also a licensed medicine for postpartum depression, sold under the name brexanolone. Levels of it were substantially higher in stool from pregnant women.

The trigger for the reaction is the part that ties this back to food. It only ran properly when hydrogen gas was present, and hydrogen is produced by other gut bacteria as they ferment fibre. Bacteria that generated less hydrogen produced far less of the conversion.

Source: Gut bacteria convert glucocorticoids into progestins in the presence of hydrogen gas

What is still unknown

Two honest limits are worth stating before drawing any conclusions.

The conversion work was done in laboratory cultures and in stool samples. Nobody has shown that these bacterial progestins reach the bloodstream in amounts that change how a person feels or functions.

The pregnancy work shows progesterone shaping bacteria in women, but the reverse direction in humans has not been tested. Whether feeding the hydrogen producers changes anyone's hormone levels is an open question rather than an answered one.

What is established is narrower and still useful. Fibre fermentation produces the hydrogen that this reaction depends on, so the raw material for it comes from food.

What this means for you

The honest summary has three parts. Progesterone and gut bacteria influence each other. The hormone reshaping the bacterial community is well demonstrated. The bacteria producing progesterone-like compounds is real chemistry whose importance inside the body is not yet known. Irregular cycles, difficulty conceiving or unexplained heavy bleeding should be investigated by a doctor, since hormone testing is the only way to find out what is actually happening. Alongside that, the practical point follows directly from the chemistry above, because the reaction depends on hydrogen and hydrogen comes from bacteria fermenting fibre. Pulses, whole grains, vegetables and fruit are what feed that fermentation. That is the same idea behind GutLab, 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.