In the strange cycle of feeding the animals that we end up eating, it's not unusual for farmers to use human manure to fertilize the grass that they graze on. But given the extreme nature of the human diet, it's not surprising that our excreta is not the healthiest product with which to fertilize farm land.
Turns out that what is referred to as "human sewage sludge-derived fertiliser" could be having a direct impact on mammalian reproduction—not just in farm animals, but in humans. That's the gist of a new study from British and French scientists who looked at the effects of manure exposure on pregnant ewes.
"The study highlights potential risks associated with the common practice of grazing livestock on pastures on which human sewage sludge-derived fertiliser has been used," Dr. Richard Lea—of the University of Nottingham and lead author of the study—said in a statement. "More worryingly, since low-level chemical exposure poses a threat to human reproductive development, the consumption of products from animals grazing such pastures may be of considerable environmental concern."
More specifically, their results suggested that chemical contaminants in human-based manure can mimic sex hormones and disrupt ovary development, with the potential for long-term damage to adult female fertility.
"The biggest effects on the foetal ovary were seen when the sheep were switched to sewage sludge fertilised fields in the last two to three months of pregnancy," said Professor Paul Fowler of the University of Aberdeen, who coordinated the 2.9-million-euro study funded by the European Commission.
Like humans, or any other mammal, for that matter, the totality of a ewe's reproductive eggs are developed while she is still a foetus in her mother's womb. Researchers found that sheep exposed to human waste for as little as 80 days were found to have a reduced number of eggs in their foetus' ovaries.
By extension, researchers found that eating meat from animals who grazed on land treated with commonly used agricultural fertilisers could have "serious implications for pregnant women" and "the future reproductive health of their unborn children."
In other words, using human waste as fertilizer might be making humans infertile. Nice work, humanity.