Tuna is an excellent source of selenium, vitamin B3 (niacin), vitamin B12, vitamin B6, and protein. It is a very good source of phosphorus as well as a good source of vitamin B1 (thiamin), vitamin B2 (riboflavin), choline, vitamin D, and the minerals potassium, iodine, and magnesium. Some of the selenium is found in the form of selenoneine, which may be especially valuable in terms of its antioxidant properties. In addition, tuna provides valuable amounts of omega-3 fatty acids (including eicosapentaenoic acid or EPA, and docosahexaenoic acid or DHA).