Nutritional yeast is a microorganism known as Saccharomyces cerevisiae, which is grown on a molasses medium and then deactivated with heat (so it has no leavening or fermentation properties) and dried into flakes. Although nutritional yeast is the same species as baker's yeast and brewer’s yeast, each of these products serves different purposes. The primary culinary function of nutritional yeast is to contribute a delicious nutty, cheesy taste to savory recipes.
Moderate amounts of nutritional yeast not only taste good, but when the yeast is the fortified variety, it can also add many valuable nutrients to a person's diet, such as folic acid, B-complex vitamins (including vitamin B12), and several important minerals and amino acids. However, large amounts of nutritional yeast used on a daily basis is not wise. A health-supportive vegan diet requires the inclusion of a variety of wholesome whole foods. Reliance on one particular item, no matter how beneficial, can establish a nutritional imbalance. The same is true of nutritional supplements -- too much of a good thing isn't good. Nutritional yeast is both a food and a nutritional supplement.
Nutritional yeast is high in purines. Large quantities of purines in the diet create an abundance of uric acid, which has been associated with several ailments, including gout. Furthermore, overreliance on a single food in the diet may eventually cause a sensitivity or possibly even an allergy to that food.
The recommended daily amount of nutritional yeast is approximately 2 tablespoons of large flakes, 1-1/2 tablespoons of miniflakes, or 1 heaping tablespoon of powder. Occasionally having larger servings than this should not pose a problem, but on a regular basis, it would be smart to stay close to these guidelines.