Why is a popular--and healthy--meat substitute under such relentless attack? Tony Soprano has nothing on the Center For Science In The Public Interest. Fresh from deep-sixing Olestra, Procter & Gamble's noncaloric fat substitute, it's now trying to whack a popular and healthy meat substitute called Quorn , from U.K.-based Marlow Foods Ltd. , a unit of AstraZeneca . Made from a fungus called "mycoprotein," it's used in 90 imitation beef and chicken products and is Europe's top-selling meat alternative. No matter that Quorn received approval from the Food & Drug Administration early this year, after a rigorous five-year process. "This...