And why do so many celebrities drink it?
EC: What Is Yerba Mate? 
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The other day, I was reading an interview with Camila Alves on Bon Appetit, and found out that she is a huge fan of yerba mate. This drink keeps coming up: it's in that one tea the Kardashians shilled on Instaram; it's what got Viggo Mortensen in trouble going through airport security; and it's what you always see Pope Francis sipping through a straw, when he's greeting his audience. But what is yerba mate exactly?

Yerba mate is a tea made from a plant called mate. It's closely related to the holly tree, and grows in the South American rainforest. Leaves and twigs from the mate are collected and dried. Then, just like every other tea, you add almost-but-not boiling water to the dried plant matter and let it steep, (Though there are myriad ways to drink it—iced, with milk or sugar, and even with other flavorings, like citrus rind.)

It's extremely popular in South America, especially in Paraguay and Uruguay, though Brazilians and Argentinians are also fond of the beverage. Traditionally, it's consumed in a gourd-shaped cup—as a nod to the traditional drinking vessel, a hollowed-out calabash gourd—through a metal straw.

The health claims and benefits for yerba mate are wildly wide-ranging. The amount of caffeine is somewhere between green tea and coffee, but many people claim that it won't give people the same jittery feeling that coffee can. It's thought to be good for focus and energy, to promote weight loss through appetite suppression, to reduce the risk of diabetes, and to improve mood. While there have been some legitimate studies done on yerba mate's health benefits, few have been conclusive.

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Still, a number of celebrities swear by yerba mate, especially for the alleged health benefits. Others, it seems, are just in the habit of drinking it. The most frequent celebrity consumers of yerba mate have connections to the countries where it's most popular: Alves is Brazilian; Mortensen spent much of his childhood in Argentina and Venezuela; and the Pope is Argentinian.

The Kardashians, though? They might just be in it for the Instagram cash.