Rio has long been famous for its fruit juice bars. Any time of day or night you'll find cariocas bellying up to the bar - so to speak - to get their daily dose of natural vitamins. A couple of tropical fruits native to Brazil, cashew and acerola, pack more vitamin C than any other fruit. Then there's pineapple, passion fruit, mango, cupuaçu, papaya (left photo below), graviola (soursop in English, second from left), guavas (beautiful red ones below), etc. I never miss a chance, and was once the butt of a friendly joke. I asked for a passion fruit juice and the guy helping me shouted to the back "the lady here wants a tranquilizer" (in Portuguese "um calmante," because of the known calming properties of that fruit). It was so unexpected that everyone started to laugh. But that's typical of Rio, I think, where everybody has a stand-up comic inside, just waiting for the right moment to pop out...
The "in" thing is to eat a bowl of açaí, a little black berry from a palm that grows wild in the Amazon. It is a great source of vitamins and energy. It is now available in the U.S.
Also, if you're in luck, you may find fresh pitanga juice. This is the tiny fruit from a tree native to the Brazilian Atlantic Forest, called "pitangueira" in Portuguese. It turns a deep dark red when ripe and it's quite tart. Look at this orange-red color, that's what "pitanga" means in Tupi, the language that was spoken in Brazil before (and for years after) the Portuguese arrived.The pitangueira grows all over Brazil, from the North to the extreme South, and it's one of the things I miss the most. (It's in season in September, October, November, just like the jaboticaba.)