Fish is a great source of protein, omega-3 fatty acids, and other nutrients that we need. It’s also one of the staples in the famously healthy Mediterranean diet. But fish can have mercury in it, and some types of tuna are known for high levels of mercury. So how much is too much?
The FDA sets guidelines for children and for pregnant adults
The FDA has a chart of nutritional recommendations for children (under age 12) and for people who are pregnant or breastfeeding. If you aren’t in those categories, it sets no specific limits or guidelines for avoiding mercury in fish. (The one exception: They suggest also following the guidelines before you get pregnant, if you are planning to do so, as mercury stays in your body for a while.)
For adults, the U.S. Dietary Guidelines recommend eating at least eight ounces a week to realize the benefits of eating fish. If you’re pregnant, the recommendation is a little more specific: eight to 12 ounces of low-mercury fish; any more than that and you may be getting too much mercury. A five-ounce can of tuna contains four ounces of fish (the other ounce is water or oil).
The FDA also has a list of fish it considers to be high, medium, and low in mercury, based on tests.
Canned “light” tuna is low in mercury (so you can eat up to three cans a week if you are pregnant, if that is the only fish you eat)
Canned skipjack tuna is also low in mercury
Albacore or “chunk white” tuna is in the medium mercury list
Yellowfin is also in the medium mercury list
So if you are an adult who is not pregnant (and not planning to become pregnant), you’re officially free to eat as much tuna as you want—but you may prefer to pick the lower mercury options. If you are pregnant, planning to become pregnant, or breastfeeding children, you’re advised to stick with light tuna or other low-mercury fish.
Some cans of tuna have more mercury than others
Are the guidelines good enough to keep us safe? There isn’t a clear answer to that, since the guidelines make a judgment call. If you are concerned about mercury, you might decide to eat a lot less fish (maybe even skip out on the eight-ounce minimum). There are also people out there eating albacore every day for lunch—probably not a great idea, but if you’re an adult, it would take a lot of mercury to potentially harm you. The emphasis is on pregnancy and children because mercury can have detrimental effects on a growing child’s brain, heart, and blood vessels.
Consumer Reports recently tested mercury levels in cans of tuna and came up with their own set of recommendations. These are not backed by governmental guidelines or large amounts of evidence, but similarly represent a judgment call by the folks at Consumer Reports. They recommend that people who are not pregnant eat no more than one to two cans of albacore per week, depending on brand, but that up to three cans per week of most brands of chunk light tuna are fine. Chicken of the Sea, Safe Catch, and Starkist brands showed lower levels of mercury in their tests.
Consumer Reports recommend avoiding tuna entirely if you are pregnant, because some cans had unpredictably higher levels of mercury than others. But that doesn’t mean that other types of fish are safer; they didn’t test tilapia, salmon, or other types of fish that are on the FDA’s low mercury list to compare. Avoiding all low-mercury fish in pregnancy would mean avoiding fish entirely, and that doesn’t jibe with the FDA recommendations, either.
Consumer Reports’ take seems a bit overblown to me. I’d rather go with recommendations from a government agency that has tested a large amount of fish and made recommendations that balance risks and benefits over a single publication’s spot tests and quotes from a few experts. But as I said earlier, it’s a judgment call either way. If you eat tuna, sticking with “light” over albacore is likely to get you lower mercury levels.