Nutrition Information about Canned Tuna
Canned tuna is a good way to get lean protein, omega-3 fats, selenium, vitamin D, B vitamins and other nutrients. But there are lingering concerns—notably about mercury. Here's what you need to know about canned tuna.
What's a "serving" of tuna and how many calories does it have? According to the USDA, a serving of canned tuna is 3 ounces, the same as for meat. That's a fairly small portion, but labels typically list an even smaller "serving size"—just 2 ounces. Depending on the type and how it's packed (water versus oil), tuna has anywhere from 90 to 180 calories in 3 ounces. "Canned" tuna in jars and pouches may have different serving sizes.
What kinds of tuna are canned? White tuna is albacore (it has the lightest-colored flesh of all tuna species, but it's actually still more pink or beige than white when cooked). Skipjack, yellowfin, tongol and sometimes bigeye are sold as "light" tuna; cans of light tuna may be a mix of different species.
What's the difference between "solid" and "chunk" tuna? Solid means larger pieces; chunk consists of smaller pieces, but it sometimes looks more like shredded tuna, and it can be mushy. Chunk is usually cheaper.
What's healthier—tuna in water or in oil? Water-packed is usually preferable because it has fewer calories and retains more omega-3s. Oil-packed chunk tuna absorbs more of the oil than solid white, even if you drain it. On the other hand, the oil that tuna is packed in—often soybean oil—is unsaturated and heart-healthy. Unless you are closely watching your calories, choose whichever you prefer. Most important in terms of calories is to watch the amount of mayonnaise or other dressing you add.
Is canned tuna a good source of heart-healthy omega-3 fats? In general, canned tuna is a moderately good source of EPA and DHA, the two main omega-3s in fish. It has less of these than salmon, but more than most other fish.
White tuna has more than light, according to USDA data. But, as recently shown in a study in Public Health Nutrition, the amount varies greatly between cans, and tuna packed in water has about three times as much EPA and DHA as tuna packed in oil. Why is that? When you drain the oil, some of the omega-3s in the fish go with it. In contrast, because water and oil don’t mix, draining the liquid in water-packed tuna does not reduce the omega-3s.
What about mercury in canned tuna? Nearly all fish have traces of methylmercury, a form of mercury that has neurotoxic effects, especially in developing brains; larger fish have more. White tuna generally has more than light tuna, which comes from smaller fish—though the levels vary widely. And some testing has shown that light tuna also has amounts high enough to be of concern.
The effects of low levels of mercury in adults is not clear, but a recent Harvard study concluded that relatively high intakes do not increase cardiovascular disease risk, as some had feared. Still, it's reasonable for everyone to limit their exposure. The best advice is to vary your intake of fish.
In particular, the FDA says that young children and pregnant or nursing women (and those who might become pregnant) should eat no more than 12 ounces of low-mercury fish a week, which includes canned light tuna—but no more than 6 ounces of white tuna a week. Some experts believe these limits are too high. To get a sense of how much tuna (and other fish) you can eat before exceeding limits set by the government, several advocacy groups provide "mercury calculators." One is at GotMercury.org.
Is canned tuna high in sodium? As with most processed foods, manufacturers add salt to canned tuna. A 3-ounce serving can have 300 to 500 milligrams—a fair amount, since the daily sodium limit for most adults is 1,500 milligrams. Some companies have reduced the sodium content. You can cut some sodium by draining and rinsing the fish. Or look for "low sodium" canned tuna. There's also "very low sodium" tuna, which has no added salt.
Why do some, but not all, cans of tuna carry the "heart-check mark" from the American Heart Association? Foods that meet certain criteria for heart health may qualify to carry the seal from the American Heart Association. Most canned tuna would qualify, but the certification program is voluntary and costly, and not all companies choose to participate.
What are the glass-like crystals or shards sometimes seen in canned tuna? They are a harmless compound called struvite, which will dissolve in your stomach. The minerals in struvite are naturally in fish, but may bind together during canning—akin to sugar forming harmless crystals in syrup. Some manufacturers use food additives (like pyrophosphate) to retard struvite formation, but it's not always possible to prevent it altogether.
Is it my imagination or did tuna cans get smaller? Some have indeed shrunk—from 6 to 5 ounces. And more of the contents may be water. This is just one example of food companies providing less, for the same price. Single-serve (3 ounce) cans and tuna in pouches (some only 2.6 ounces) may be convenient—but you pay more per ounce (and the packaging is more wasteful). For the most cost-effective tuna, buy larger (12-ounce) cans, if available.
Information about "Dolphin-Safe" Canned Tuna
"Dolphin-safe" sounds great, but that label doesn't tell the whole story. The vast majority of canned tuna sold in the U.S. (including imported) meets certain "dolphin-friendly" criteria. For example, fishing boats are not allowed to chase or set nets around dolphins, which swim with yellowfin tuna in the eastern Pacific.
However, according to Monterey Bay Aquarium, the fishing methods that are allowed can harm other marine life, including sharks, sea turtles, juvenile tuna and endangered albatrosses. The only canned tuna that gets an "ocean friendly" green light from this organization is "troll-caught," "pole-caught" and "pole-and-line-caught" tuna (check the labels for these terms)—methods that have little or no "by-catch." Another advantage of poll/troll-caught tuna is that these fish tend to be younger and smaller and thus have less mercury. The Marine Stewardship Council (look for the seal) also certifies albacore tuna that is caught by eco-friendly methods.
Source: Originally published in The University of California, Berkeley Wellness Letter (October 2011)