They may be one of the world’s favourite supplements but, according to a study from earlier this year, more than one in 10 fish oil capsules are rancid. Most of the oil comes from Peruvian anchovetas, a type of anchovy that is also used to feed pigs, poultry and farmed fish. And despite catching more than 4m tonnes a year of anchovetas to cater to the global demand, large industry players want to scale this up even further. In this episode from January 2022, Madeleine Finlay speaks to environment journalist Richa Syal about why so many fish oil pills are rancid, and hears from journalist Dan Collyns in Chimbote, Peru, about how the industry is affecting the local environment and its residents. Help support our independent journalism at theguardian.com/sciencepod