Traditional weight-loss diets are the most effective and safest, says a report from the U.S. Department of Agriculture. It says any diet that limits calories sufficiently will take the weight off, but only moderate-fat and high-carbohydrate diets will keep it off. The report is critical of high-protein, low carbohydrate diets. It says the traditional moderate fat diet is the most nutritionally adequate and showed the best improvements in cholesterol levels, blood fats and blood sugar. Fat is limited to 30 percent of the calories in a moderate fat diet. The recent USDA report is part of a continuing project studying the effectiveness of weight loss diets.