Pop quiz! What's the single biggest source of calories for Americans? White bread? Big Macs? Actually, try soda. The average American drinks about two cans of the stuff every day. "But I drink diet soda," you say. "With no calories or sugar, it's the perfect alternative for weight watchers...Right?"Not so fast. Before you pop the top off the caramel-colored bubbly, know this: guzzling diet soda co
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