Gut bacteria that recycle oestrogens back into the bloodstream are far more abundant in people from industrialised societies than in hunter-gatherers and rural farmers, according to a new study. Researchers found up to seven times greater recycling capacity in urban populations. The findings raise questions about potential health impacts from elevated hormone levels.
Rebecca Brittain at Jagiellonian University Medical College in Poland and her team analysed gut microbiome data from hundreds of people across 24 populations worldwide. These included hunter-gatherers in Botswana and Nepal, rural farmers in Venezuela and Nepal, and city dwellers in Philadelphia and Colorado. The study measured genetic sequences for beta-glucuronidase enzymes, which remove tags from excreted sex hormones like oestrogens, allowing reabsorption into the blood. Industrialised groups showed up to seven times more of these sequences and twice the diversity compared to non-industrial groups. Age, sex and BMI had no effect on these patterns, but formula-fed babies had up to three times the capacity and 11 times the diversity of breastfed infants. “We don’t know how the body would respond to this increased input,” Brittain said. “But the implications could be quite large.” The team noted that higher recycling might lead to elevated blood oestrogen levels, potentially affecting fertility and cancer risk, though it could benefit those with low hormones. “The assumption is usually that higher oestrogen recycling is harmful,” Brittain added. “I don’t think that’s a fair assumption. For some people with really low oestrogen levels this could be a good thing.” Katherine Cook at Wake Forest University School of Medicine called it an interesting study highlighting the microbiome's role in health, but noted limitations, such as all industrialised samples being from the US. Brittain's group plans further research into lifestyle factors driving these differences. The findings appear in PNAS.
