What It Is: The Flat Belly Diet, launched by Prevention magazine editor-in-chief Liz Vaccariello, is built around a 1,600-calorie-per-day strategy that allows dieters to eat four meals per day selected from hundreds of meal possibilities.
Central to the diet is the principle that every meal must contain a source of MUFA, short for monounsaturated fatty acids. According to the diet's proponents, these predominantly nut-based oils can target-reduce dangerous belly fat. The diet also commences with a four-day jumpstart to get rid of abdominal bloating, during which a dieter drinks what the proponents of the diet call Sassy Water, a lemon- and ginger-containing beverage named after Prevention nutrition director Cynthia Sass.
And according to a small MRI imagine study released Monday out of the Yale University Prevention Research Center, the diet reduced visceral belly fat on average by 33 percent in 28 days for nine overweight women. Other risk factors, such as high cholesterol and blood pressure, were also reduced in the study participants.
Katz: "[The diet is] generally healthful, with an emphasis on foods noted as being good for insulin resistance. My lab actually studied the effects of this short term, and they were quite good."
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."