Is a Low-Fat Diet Risky?

! It used to be so simple: butter is bad. Margarine is better. Vegetable oils are best. And too much fat of any sort is not good for you. But these easy guidelines have given way to a bewildering -- and at times contradictory -- smorgasbord of dietary dos and don'ts. Salads drizzled with olive oil are O.K. Confections rich in coconut oil aren't. Fish oil gets two thumbs up. Margarine receives rancid reviews. "I swear to God, it's confusing," declares Peter Summers, a retired health-care specialist who lives in San Francisco. "First I went to margarine. But then I found...