Tuna is a food item made by cooking raw tuna. Compared to many other fish, it is not the most potent healing item, but its low cost and high availability make it a useful training food, as it heals up to 750 life points, depending on your constitution level.
Tuna is often used by players that train on the lesser demons in Karamja volcano. This training spot is fairly close to the Karamja fishing pier, where players fishing for swordfish may drop the less-valuable tuna that is caught along with them. This tends to draw in the players that train melee.
Members can use it with sweetcorn to make tuna and corn, which heals a bit more than regular tuna. The cost of tuna can be made to profit this way. It requires 30 cooking to cook, yielding 100 experience, 110 experience if cooked on a bonfire and 121 experience if used on a portable range. Potato with butter can also be added with tuna-and-sweetcorn to create a tuna potato, yielding another 10 experience and a higher healing food.
Tuna is also often used to train the Cooking skill. Factoring in the time/cost ratio, it is probably the most efficient way to achieve 99 cooking in free-to-play. Players will rarely burn tuna beginning at level 60 and will stop burning tuna at level 65 cooking (66 on fire). If you're a member you can use cooking gauntlets which makes you stop burning tuna at level 63. Cooking tuna yields around 150,000 experience per hour, where the maximum number of fish that can usually be cooked in an hour is around 1,500. If one buys raw tuna on the Grand Exchange and cooks them, it will result in a gain of 0 coins.