Every couple of years a food or beverage is crowned with what nutrition experts call a "health halo." Some of the foods — wild salmon, blueberries, flax seeds — deserve it. But others gain status for no apparent reason. Acai berry, anyone? It's not that the trendy food is unhealthful. It's just that if you're already eating a well-balanced diet it's unnecessary. And possibly expensive. The latest entrant in this category: coconut water. Coconut water — the mildly sweet liquid from the center of young, green coconuts — has been popular in tropical areas since, well, as long as people have lived among coconut palm trees. In recent years it has found its way onto the shelves of mainstream grocery stores, often with fruit juice or sugar added. A 14-ounce serving can cost as much as $3.50. "Coconut water is a good source of potassium , but bananas and potatoes contain just as much or even more, and they're much less expensive," says Joa
