You can apply the same science to any food high in resistant starches (plantains, legumes, rice, etc.), but Gundry’s favorite source is a purple sweet potato. This tuber, a staple in the Okinawan diet (which is categorized as a Blue Zone), is remarkably high in resistant starch—and if you let it cool, the number may even skyrocket. “Roast your purple sweet potato, throw it in the refrigerator, reheat it, and you will turn almost 50% of that sweet potato into resistant starch,” says Gundry. Simple as that.