They may be one of the world’s favourite supplements but, according to a recent study, more than one in 10 fish oil capsules are rancid. Most of the oil comes from Peruvian anchovetas, a type of anchovy, which is also used to feed pigs, poultry and farmed fish. And despite catching more than 4m tonnes a year of Peruvian anchovetas to cater to the global demand, large industry players want to scale this up even further. 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 https://www.theguardian.com/sciencepod">theguardian.com/sciencepod