Tuna loin is highly nutritious and versatile in its uses, although it is most commonly used for steaks. This cut comes from the loin at the back of the fish. Tuna, in general, is an excellent source of lean protein, as well as omega-3 fatty acids, vitamin B12, niacin, and selenium. Tuna loin can be grilled, baked, broiled, or pan-fried. It can also be used in salads, sandwiches, sushi, and other dishes. Its flavor is enhanced when it is marinated in a mixture of herbs, spices, and oil before cooking.