DEAR DR. BLONZ: I use canned tuna to make tuna salad for sandwiches. How much of a difference does it make whether the tuna is packed in oil or water? Is there more of the omega-3 fats present in the oil-packed kind? I also wanted to know about the risks of mercury in tuna. -- S.S., Phoenix
DEAR S.S.: The oil used for canned tuna is usually soybean or canola oil, not fish oil. Oil-packed tuna will contain more fat, but no extra amounts of the healthful long-chain omega-3 fats (abbreviated as EPA and DHA).
One cup of light tuna, canned in oil (drained), will contain 12 grams of fat, of which 0.3 grams will be omega-3 fatty acids and 0.4 grams will be omega-6 fatty acids (the omega-6 coming primarily from the oil). By contrast, 1 cup of light tuna, canned in water, contains 1 gram of fat, of which 0.4 grams will be omega-3 fatty acids and 0.14 grams will be omega-6 fatty acids. (What could explain there being less omega-3 in the oil-packed? Consider that a bit of the fish's omega-3 fat mixes with the oil while they are hanging out in the can, and then exits when the oil gets drained.)
Albacore (white) tuna contains more omega-3 than the yellowfin and skipjack varieties used to make light tuna. One cup of albacore tuna, canned in oil (drained), will contain 14 grams of fat, of which 0.8 grams will be omega-3 fatty acids and 0.4 grams will be omega-6 fatty acids. One cup of albacore, canned in water, will contain 5 grams of fat, of which 1.5 grams will be omega-3 fatty acids and 0.1 grams will be omega-6 fatty acids. You can see the difference.
Your concern about mercury in fish is valid. Because tuna is relatively high on the food chain, it tends to contain higher contaminants such as mercury, a dangerous toxin. A study in the April 2013 issue of the journal Environmental Science and Technology reported that over 70% of the mercury in the human body comes from that present as a contaminant in fish. Since mercury distributes itself throughout the fish, cleaning and method of preparation can't solve the problem.
The most commonly consumed fish found to have higher levels of mercury contamination include swordfish, shark, orange roughy, tilefish and certain, but not all, types of tuna. (Note: Salmon is not of serious concern in this regard.) Size matters with tuna, because the longer the animal lives, the more mercury it can accumulate. When selecting canned tuna, consider opting for tuna caught via troll, or pole-and-line.
Two resources for follow-up: Check out the EPA's advice regarding fish; the site's information is targeted to children and pregnant women, which are the most vulnerable populations in this regard. This page has a downloadable table: b.link/h96jx6. Then, check Seafood Watch (seafoodwatch.org) for more on harvest methods for all types of fish.
Send questions to: "On Nutrition," Ed Blonz, c/o Andrews McMeel Syndication, 1130 Walnut St., Kansas City, MO, 64106. Send email inquiries to firstname.lastname@example.org. Due to the volume of mail, personal replies cannot be provided.