This excellent fish provides many nutrients and is an excellent source of protein. The concern about tuna is that certain type of it contain high levels of mercury, an element that can be dangerous, particularly to children, pregnant women, and those nursing or looking to become pregnant (source: Health Canada). Almost all fish contain some level of mercury, and other fish with higher levels of mercury are shark, swordfish, marlin and roughy. The concern over tuna is that inexpensive as it is, and readily available cooked and packaged in tins, it tends to be the most widely and frequently consumed.
This is not to swear off tuna altogether, but to consider it as part of a balanced diet – enjoy a reasonable portion size on occasion. More important, is to understand that all tuna is not the same. The cold-water blue-fin tuna (favourite for sushi restaurants) tends to have much higher levels. Because mercury tends to deposit in muscle rather than fat, blue-fin tuna being large fish have greater muscle mass and therefore higher concentration. The warm-water yellow-fin tuna is a more fatty fish, therefore having lower levels.
Tinned tuna tends to be yellow-fin and most supermarkets sell yellow-fin in their frozen seafood section. Look for light tuna rather than white or Albacore. (Edit: the levels in light tinned tuna is relatively low, making it safe to enjoy 2 or 3 times per week, according to Health Canada.) Health Canada recommends including some form of fish or seafood at least twice each week, and even better, 3 to 4 times per week. A portion is about 3 to 4 ounces (85 to 110 gr), meaning it’s easy enough to have a couple of servings of tuna each week and avoid high levels of mercury.
Update: Look for sustainable seafood, as tuna, and seafood in general, is being overharvested, and fished by boats manned frequently by slave labour.