Before you buy another book or throw out half your groceries, get the skinny on three of the hottest diet contenders.

Popular diet plans have always played musical food groups: no carbs, more protein, low fat, high carbs.

Today, having been there and done 'em all, Americans are heavier than ever, which is why the newest approaches are taking a different tack, claiming to manipulate the body in some way—say, by "flushing" fat or "revving" metabolism. The question is: Do they work? "The golden rule with weight loss is always 'calories in versus calories out.' No matter what the gimmick is, if fad diets lead to weight loss, they operate by making you take in fewer calories," says Donald Hensrud, MD, associate professor of preventive medicine and nutrition at the Mayo Clinic in Rochester, Minnesota.

We asked some of the country's top nutrition experts to help evaluate the high-concept diet books making the rounds.