In 1936, Albert Szent-Györgyi, who won the Nobel Prize the following year for discovering vitamin C, suggested that a class of phytonutrients called flavonols should also be considered a vitamin. (He suggested “vitamin P.”)505 The most common flavonol in the diet is quercetin,506 which is found concentrated in onions, kale, and apples.507 It’s what gives apple peels their bitter taste.508 Researchers had been testing dozens of different compounds on cells scraped from umbilical cords and then irradiated to force senescence. In 2015, they announced their results: Quercetin was a natural
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