For years now, coconut water brands have been thought to deliver the ultimate hydration benefits (yes, including for a hangover). That's not entirely so, a registered dietitian has just warned. If you're not sure of exactly what you're buying, coconut water can lead to one rather undesirable effect. Registered dietitian Lizzie Streit, M.S., R.D., L.D.N., has explained to Healthline that while coconut water is a natural source of minerals that provide hydration, it can have one uncomfortable effect. RELATED: The One Vitamin…