Nearly every year for the last two decades, the news has been the same: Americans are fatter now than they were last year. The country’s adult obesity rate, currently at 34 percent, has doubled since 1995. And as our waistlines have expanded, so have the number of trendy diets and nutritional fads that promise fast results.

Thanks to the Duchess of Cambridge, or rather her svelte mother, Carole Middleton , 2011 was the year of the Dukan diet, a regimen similar to Atkins that favors a high-protein, low-carb pattern. Last year, it was a juice diet. In 2010, Michael Pollan authored the top-selling diet book, Food Rules. But whether practical or extreme, which diet actually will work?

To find out, The Daily Beast revisited its annual diets ranking. This year, we added two more diet regimens that have garnered greater mainstream attention in the last few years. A Paleolithic diet, which focuses on eating the same energy sources as our ancestors did, and a vegan diet, which forbids any animal-based food, have both been the subject of numerous stories. But, neither diet has undergone the type of clinical study that provides enough identical data points to compare it with other diets in our ranking. Nevertheless, given the positive results of several published articles on each, we felt it was only logical to include them to encourage readers to do their own research.

As with last year, our ranking was derived by combing journals of nutrition and medicine to find clinical studies on specific diets. To be ranked, the dietary studies had to include six-month and 12-month data on weight loss and participant retention, as well as 12-month change in body mass index, or BMI. Each data point was weighted one-fifth of the final score for each diet. If there were multiple studies for diets, results were averaged.

Most Effective Diets for 2012, from Weight Watchers to Vegan (Photos)

The Daily Beast analyzes the most recent scientific studies to figure out which diet will whittle the excess most effectively.

12.30.11 4:45 AM ET