Placido Osuna points to a crudely-stitched scar on his belly and thanks God for the little green herb that his sons encouraged him to grow instead of tobacco and cotton.

"Stevia saved my life," says the 68-year-old farmer, dressed in flip-flops and an open shirt, with a machete tucked into his belt. Without the income from this new cash crop, which is native to Paraguay and whose leaves contain compounds up to 300 times sweeter than sugar but naturally calorie-free, Mr Osuna says he could never have afforded gall-bladder surgery.