Mushrooms, which replace meat in the filling, are a great source of many of the same nutrients as beef, but with a fraction of the calories and no cholesterol. They are a great source of source of B vitamins, including riboflavin, niacin, and pantothenic acid. The B vitamins provide energy by breaking down proteins, fats and carbohydrates. The Niacin (a B vitamin) in mushrooms aids our digestion and helps our skin acquire a healthy glow and Riboflavin, another B vitamin, helps us maintain healthy red blood cells.