Diets come and go, teasing and tempting us with dreams of that elusive hot body. Eat what you want! Pounds melt away overnight! The reality, as frustrated dieters know well, is that dieting is hard, and frankly, most diets don't work. Some can even threaten your health. And digging out the truth about dieting, let alone deciphering whether particular plans live up to the hype, is laborious enough to burn off a pound or two by itself.

Best Diets 2013 cuts through the clutter of claims and delivers the facts about 29 diets. Now in its third year, Best Diets is introducing four new diet profiles and a new rankings category: Best Plant-Based Diets.

Many of the 29, such as Weight Watchers, are household names, while others, such as the DASH (Dietary Approaches to Stop Hypertension) diet, should be. To create the third annual rankings, U.S. News editors and reporters spent months winnowing potential additions to our diet roster and then mining medical journals, government reports, and other resources to create in-depth profiles for those that made the cut. This year we turned the spotlight on the Flexitarian Diet, Engine 2 Diet, Traditional Asian Diet, and Anti-Inflammatory Diet.

Each profile explains how the diet works, determines whether its claims add up or fall short, scrutinizes it for possible health risks—and reveals what it's like to live on the diet, not just read about it.

A panel of nationally recognized experts in diet, nutrition, obesity, food psychology, diabetes, and heart disease reviewed our profiles, added their own fact-finding, and rated each diet in seven categories: how easy it is to follow, its ability to produce short-term and long-term weight loss, its nutritional completeness, its safety, and its potential for preventing and managing diabetes and heart disease. We also asked the panelists to let us know about aspects of each diet they particularly liked or disliked and to weigh in with tidbits of advice that someone considering a particular diet should know.

Every diet received robust scrutiny, and we converted the experts' ratings to scores and stars from 5 (highest) to 1 (lowest) to construct eight sets of Best Diets rankings. One—Best Plant-Based Diets—is brand new for 2013; the other seven have been refreshed and include the four new diets:

• Best Diets Overall combines panelists' ratings in all seven categories. All categories were not equally weighted. Short-term and long-term weight loss were combined, with long-term ratings getting twice the weight. Why? Quick results are important after the holidays or when summer looms, but a diet's true test is whether it can be sustained for years. That's especially the case for those who are overweight or obese; losing as little as 5 percent of body weight can dramatically reduce the risk of chronic illnesses such as diabetes and heart disease. And this time around, safety was double-counted, because no diet should be dangerous.

• Best Commercial Diet Plans uses the same approach to rank 12 structured diet programs marketed to the public.

• Best Weight-Loss Diets was generated by combining short-term and long-term weight-loss ratings, weighting both equally. Some dieters want to drop pounds fast; others, looking years ahead, are aiming for slow and steady. Equal weighting accepts both goals as worthy.

• Best Diabetes Diets is based on averaged diabetes ratings.