A four-ounce serving of tuna contains a full 26.77 grams of protein and is very low in saturated fat. This nutritionally rich food also contains several important vitamins and minerals, including vitamins D, B12, and B6, selenium, potassium, iron, and iodine. Selenium has antioxidant properties that help regulate body functions and protect your cells from the free radicals that can cause chronic health conditions like heart disease and some cancers.