Folic acid. The name sounds like it might be dangerous, a caustic acid substance and not good for health. But it is actually a very important vitamin and therefore essential to your life and your health. It is also the most commonly deficient vitamin in Americans and the average American diet provides only about half the RDA, the amount recommended by the Food and Nutrition Board. Folic acid is found in green leaves, such as spinach, asparagus, beans (legumes) and especially in brewers yeast and liver. If you don't like any of those foods, try eating butterfly wings for an exotic source health food.