Consumption of microplastics through water may enhance the risk of the polycystic ovarian syndrome (PCOS), an endocrine disorder that affects up to 10 per cent women in their reproductive years, scientists at Presidency University, Calcutta, have reported.

While earlier studies had identified microplastics as pervasive sources of exposure to endocrine-disrupting chemicals, the Calcutta scientists have through experiments on zebrafish provided insights into the biological mechanisms through which microplastics can induce PCOS.

Medical researchers had long suspected that a combination of genetic and environmental factors contribute to PCOS — a condition marked by irregular periods, persistent acne, excessive hair growth, infertility and other symptoms — but did not know what exactly causes PCOS.

Zoologist Kousik Pramanick and his colleagues at Presidency’s life sciences department set out to explore whether and how the consumption of polystyrene microplastics at doses equivalent to the human exposure might impact the zebrafish, a tropical freshwater species widely used in biological studies.

“The zebrafish shares roughly 70 per cent of its genome with humans,” Pramanick told The Telegraph.

Several earlier studies in Germany, Italy and the US had detected microplastics or nanoplastics — plastic fragments ranging from 5 millimetres to one micrometre or even smaller — in samples of water, including mineral water in plastic bottles.

A 2018 study by scientists at the State University of New York had detected an average of 325 microplastic fragments per litre of bottled water. But researchers at Columbia University, New York, earlier this year used a refined chemical imaging technology to estimate 240,000 fragments per litre of bottled water, 90 per cent of them nanoplastics, less than one micrometre.

However, the potential health risks of microplastics remain unclear. Researchers used to believe microplastics are too large to be absorbed in the body through the intestine and are excreted unabsorbed. But nanoplastics are small enough to be absorbed into the human bloodstream and reach different organs.

In their experiments, the Presidency scientists exposed three groups of zebrafish in lab tanks to either plain freshwater, a drug called letrozole known to induce PCOS, or polystyrene microplastics at 40 micrograms per litre concentration, alongside normal diet.

After 21 days, they found that the zebrafish in the letrozole group and the microplastics group had significantly higher levels of ovarian and brain testosterone, a key endocrine change observed in PCOS, than the zebrafish in freshwater.

The zebrafish exposed to letrozole and microplastics also displayed ovarian changes and hormone disruptions that are found in PCOS.

The study also suggested that exposure to microplastics or letrozole enhanced the activity of three genes that trigger inflammatory responses and contribute to ovarian fibrosis, or excessive proliferation of connective tissue in the ovaries, one of the outcomes of PCOS.