This all sounds well and good, but what happens when you leave them in the fridge overnight? Well, says Gundry, “The process of cooling and then reheating actually makes much more of the starch resistant.” The theory is that while those starches lose some of their structure during the cooking process, they form a brand new structure once they cool. Research backs it up, showing that foods high in resistant starch have increased amounts after reheating them from cooled.
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.
The only caveat we should mention: Make sure to gently reheat your sweet potatoes, as cooking them again may blast the starch structure once again. (Of course, sweet potatoes are still healthy—cooled or cooked!—but if you’re trying to increase the amount of resistant starches, you might want to lightly reheat or eat them chilled on top of a salad.)