Ayoob: "You may have a flatter belly in 32 days but that's because you're losing weight, pure and simple. Make no mistake, your body will determine where you lose weight. It tends to come from the belly first anyway -- that's usually the body's first preference -- but it's the weight loss that's flattening your belly, not some diet miracle."
Ikeda: "This diet is based on the premise that a higher intake of monounsaturated fatty acids will result in a flat abdomen. There is nothing in the scientific literature that substantiates this claim. The best way to get a flat belly is to increase the strength of abdominal muscles by exercising them."
What It Is: The so-called full-fat diet builds off of research in January 2007 in which Swedish researchers found that women who had at least one serving of milk (whole milk, to be exact) or cheese each day experienced less weight gain over the following nine years than their counterparts who did not. Some concluded from this research that other full-fat daily foods may also provide these weight loss benefits -- though the researchers behind the Swedish study were hesitant to delve so deeply into the results to make a similar claim.
While the full-fat diet has many different versions, one of the most prominent proponents of eating foods in their full-fat form is New York-based nutritionist Esther Bloom, who delivers such advice in her book, "Eat, Drink and Be Gorgeous."
Ayoob: "I like the non-diet mentality of this. It's the idea that you can 'have your cake and eat it too.' But it's clear about this: you can't have it all the time and in all amounts. It's the idea that nothing is forbidden, and that's good."
Ikeda: "Although Esther Blum is an RD [registered dietitian], which gives her a plus for credibility in my book, her enthusiasm for supplements negates the plus with a minus. None of the endorsements on her Web site come from credible nutrition experts."
Katz: "I didn't find enough information about this to make a judgment, but the usual promises were made."
What It Is: The Atkins Diet was first popularized in 1972, with the release of the book "Dr. Atkins' New Diet Revolution." Controversial from the start, the diet is built around the idea that consuming too many carbohydrates is the main factor behind overweight and obesity. Therefore, by drastically reducing the intake of carbohydrates and shifting over to a diet high in protein in fat, a person can force his or her body to burn stored fat more efficiently.
The Atkins diet gained momentum at the beginning of the decade, and diet authors have published a host of new books aimed at further delving into the benefits of a low-carb approach to weight loss.
Ikeda: "Is this old thing still around? If it worked, obesity would no longer be a problem in this country, since a good percentage of the population has tried it."
Katz: "I think this is a silly diet at odds with health. It restricts choice very severely, which in turn restricts calories severely -- so it of course produces short term weight loss. But cutting out 'carbs' long term makes no sense; everything from lollipops to lentils is a 'carb,' so this diet throws out the baby with the bathwater."