Modern life can cause big changes in our estrogen levels

Discarded sex hormones can be returned to the bloodstream by bacteria in the gut – and now a study has found that there are far more of these sex hormone-recycling bacteria in the guts of people in industrialized societies than in the guts of hunter-gatherers and non-industrial farmers. This could mean that as a result of modern life, some people have higher levels of certain sex hormones in their blood, which would have profound health effects.

“We don’t know how the body would respond to this increased input,” says Rebecca Brittain of the Jagiellonian University Medical College in Poland. “But the consequences can be quite large.

Sex hormones such as estrogens circulate in the blood. When levels are too high, cells in the liver add a chemical marker that leads to the hormone being excreted, often through the intestines. But that tag happens to be a sugar molecule that certain bacteria feed on. So some bacteria in the gut cut off the tags using enzymes called beta-glucuronidases.

Once the label is removed, the hormone can be reabsorbed by the body and end up back in the bloodstream. Studies indicate that a substantial part of the secreted sex hormones is recycled in this way by intestinal bacteria.

in 2011 the concept of “strobolome” was used for the first time. describe all gut bacteria that can alter estrogens, potentially affecting blood levels in both sexes. It was proposed earlier this year “testobolome” is used to describe gut bacteria that can affect testosterone levels.

The latest study by Brittain’s team compared the strobolomes of hundreds of people from 24 populations around the world, using data from previous studies in which their gut microbiomes were sequenced. These populations included, for example, hunter-gatherers in Botswana and Nepal, rural farmers in Venezuela and Nepal, and urban dwellers in Philadelphia and Colorado.

Specifically, Brittain’s team looked for genetic sequences encoding beta-glucuronidase enzymes and measured the overall proportion of these sequences and their diversity. The results suggest that the capacity of gut microbes to recycle estrogen in industrialized populations is up to seven times greater than in hunter-gatherer and rural farming populations, with double the diversity.

The team also found that formula-fed babies have up to three times more recycling capacity than breast-fed babies, and up to 11 times more diversity. However, people’s age, gender and BMI made no difference to their strobolomes.

Brittain’s team and others are now trying to determine whether the higher recycling capacity suggested by the gene sequences actually corresponds to higher levels of estrogen recycling and, most importantly, whether this leads to higher levels of the hormone in the blood. For example, it could be that human bodies can adjust hormone levels to partially or fully compensate for higher recycling.

But if some individuals have higher levels of estrogen in their blood during their lifetime because of their microbiomes, it could have a big impact on their fertility and health, such as increasing their risk of certain cancers. But in some cases these effects can be beneficial. “Typically, higher estrogen recycling is thought to be harmful,” says Brittain. “I don’t think that’s a fair assumption. For some people with really low estrogen, that might be a good thing.”

“This is an interesting study that adds to the growing evidence of the importance of gut microbiome function to human health and development,” he says. Katherine Cook at Wake Forest University School of Medicine in North Carolina, who is investigating possible links between the microbiome and breast cancer risk.

But it has its limitations, he says, including the fact that all the industrialized populations were in the US. “Additional cohorts, perhaps from Europe, may have strengthened industry associations,” says Cook.

Brittain says she and her colleagues will try to identify lifestyle factors responsible for the differences they found. “We’d like to know a lot more about these individuals, but the data didn’t exist, so we’re going to do our own study,” he says.

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