Tastiness. What odds do you give a diet if you don't like what you're eating? That was one of the judges' problems with Medifast, for example: The limited menu, small portions, and powdered, just-add-water food will likely grow old fast. Packaged meals are convenient, but excitement and variety, which help dieters stay on board, are missing. Experts also looked for overemphasis on unusual ingredients. They were wary about the raw food diet, which calls for plant-based foods never heated above 115 degrees. That translates to dehydrated bananas, buckwheat/sunflower-seed pizza crust, mashed avocado, and unpasteurized milk. To gauge whether you might find a particular plan tasty enough to adhere to, Giancoli advises, "check out the recipes first. How many ingredients do most meals require, and are you familiar with them? Or do they call for things you've never heard of?"
Special requirements. Flexibility and convenience are vital. When they're minimal or absent, diets can be difficult to follow. The Abs diet, for example, requires lots of prep work, and each meal has to include at least two of 12 Abs "powerfoods" like almonds, beans, and instant oatmeal. That's a potential challenge for busy dieters. As for convenience, consider the Zone Diet's strict regimen: breakfast within an hour of waking up, and snacks and meals every five hours, with precise percentages of carbs, protein, and healthy fat. Point-counting, carb-counting, and other calculations detract from a plan's ease. Other considerations: Are restaurant meals allowed, or even doable? Can you eat what the rest of your family eats? Do meals require tedious stovetop toil? Do you have to buy expensive kitchen gadgets like the dehydrator and blender recommended for the raw food diet?
What's hard or easy for one dieter, of course, may be just right or all wrong for another. Some people need lots of rules and restrictions; others can't handle such rigidity. "It boils down to what you feel is sustainable," says registered dietitian Elisabetta Politi, who serves on the U.S. News Best Diets expert panel. "It depends on what fits your personality, what you enjoy, and what is or isn't too bothersome."
Updated 01/07/2013: This is an updated version of a previously published story.