 In 1936, Albert Sanjurghi, who won the Nobel Prize for discovering vitamin C, described a vitamin P, which we now know encompasses a class of thousands of phytonutrients called flavonoids. Some, like quercidin, are widespread in plant-based foods. You can tell something is widespread in the plant kingdom when you can even find it in iceberg lettuce. Fruits, however, are only found in specific plant families. For example, hasperidin is found primarily in citrus fruits. This may be one of the reasons out of all the different types of fruits that have been looked at, citrus may help cut our risk of stroke the most. The citrus phytonutrient hasperidin increases blood flow. Using a machine called a Doppler fluximeter, you can measure blood flow through the skin using a laser beam. A laser Doppler fluximeter sounds like something from back to the future. And if you give people the amount of hasperidin found in two cups of orange juice, blood flow goes up, though if you instead just give them the orange juice itself, that works even better. So there's other beneficial stuff besides just the hasperidin in citrus. For example, if you measure the changes in genetic expression, orange juice consumption induces changes in the expression in 3,000 of our genes, whereas hasperidin alone only modulated the expression of about 2,000. Still, nearly 2,000 stretches of our DNA expressed differently because we consumed just one of the thousands of phytonutrients in plants, pretty mind-blowing. When these changes in blood flow are not just kind of in theory, researchers have taken volunteers with cold sensitivity— cold hands, cold feet—put them in an air-conditioned room and measured the temperature of their fingertips after drinking a placebo drink, like orange Kool-Aid, versus drinks with two doses of actual citrus phytonutrients. In the Kool-Aid group, their fingers got colder and colder, dropping nearly 9 degrees Fahrenheit. But the fingers of those consuming low or high doses of citrus didn't as much. That's because their blood flow remained steady. Here's that laser test again. When we're exposed to cold temperature, our body starts to clamp off peripheral blood flow to keep our core warm. But if you eat a bunch of oranges before you go skiing, your risk of frostbite may go down since you're keeping up your blood flow to your fingers and toes. They even took these poor women and plunged their hands into some chilly water. And as you can see, their finger temperature rebounded faster towards normal in the citrus group. Of course, having warm hands is nice, but maintaining blood flow to your fingers is not as important as maintaining blood flow to your brain.