Canned Tuna: Doing the Mercury Math Instead of Guessing

Canned tuna occupies a strange spot in American eating: simultaneously the cheapest high-quality protein in the store and the food people feel vaguely guilty about. A 5 oz can of light tuna is roughly 100 calories and 22 g of protein for about $1.50 — nothing else in the aisle competes. The guilt is about mercury, and it is not baseless. It is just usually applied without doing the arithmetic, which is a shame, because the arithmetic is easy and it almost always comes out permissive.

At roughly 22 g of protein per 100 calories, canned light tuna has the best protein-per-calorie ratio of any common animal food — better than chicken breast, better than nonfat Greek yogurt. That is the reason to care about getting the mercury question right rather than avoiding the aisle.

Why tuna and not other fish

Mercury enters the ocean, gets converted by bacteria into methylmercury, and then does something that matters enormously: it bioaccumulates. Small organisms absorb a little, small fish eat many of them and retain it, bigger fish eat many small fish, and the concentration multiplies at each step up the chain. A long-lived predator at the top of that chain concentrates decades of accumulated mercury from everything below it.

That single mechanism explains the entire advisory landscape. Salmon, sardines and shrimp are low because they are short-lived, small, or eat low on the chain. Swordfish, king mackerel, shark and bigeye tuna are high because they are large, long-lived apex predators. Tuna is interesting precisely because it straddles the line — different species, sold in the same aisle, with genuinely different numbers:

Doing your own math

The EPA's reference dose for methylmercury is 0.1 micrograms per kilogram of bodyweight per day — a level set with a substantial safety margin built in, meaning it is not a cliff but a conservative planning number. Turning that into cans is three steps:

Reading the can

• Light vs white — the mercury decision, covered above. 'Chunk light' is skipjack; 'solid white' is albacore.
• Water vs oil — water-packed is ~100 kcal per can; oil-packed is ~150–200 depending on drain. Oil-packed tastes noticeably better and the fat is usually soybean or olive oil. Neither is wrong; know which one you logged. One subtlety: draining oil-packed tuna removes some of the fish's own omega-3 along with the added oil, while draining water-packed loses very little.
• Sodium — regular cans run ~300 mg; 'no salt added' versions run ~60 mg for the same fish at the same price. If you eat tuna several times a week this is a free 700+ mg weekly reduction.
• Pouches vs cans — pouches are drier (less liquid to drain), often slightly pricier per ounce, and identical nutritionally. Convenience, not nutrition.
• 'Skipjack' or 'pole and line' on the label — usually indicates both the lower-mercury species and a fishing method with less bycatch. When it is the same price, it is a free upgrade.

The selenium footnote, sized honestly

You will encounter the argument that selenium in fish 'neutralizes' mercury — selenium and mercury bind tightly, and fish high in selenium relative to mercury are proposed to be protective. There is real chemistry underneath it, and canned tuna is genuinely selenium-rich (~90% DV per can). But the concept has not been adopted by the FDA or EPA into consumer guidance, the human outcome data is thin, and it is frequently deployed to argue that mercury limits do not apply at all. Treat it as an open research question, not a permission slip. The math above is the operational answer.

Where the can wins

Cost per gram of protein is where canned tuna is genuinely unbeatable: roughly $0.07 per gram of protein at $1.50 a can, against chicken breast around $0.09–0.12 and Greek yogurt closer to $0.15. It is shelf-stable for years, requires no cooking, needs no refrigeration until opened, and delivers a complete amino acid profile. For anyone building protein intake on a tight budget or without reliable kitchen access, it is the single most practical item in the store.

The classic tuna salad problem is worth naming, since it is where those good numbers usually die: a can at 100 calories plus a quarter cup of mayonnaise at ~360 turns a lean protein into a mostly-fat dish. Swapping the mayo for plain Greek yogurt — the swap covered in our Greek yogurt guide — takes the same tuna salad from ~460 calories to ~140 while adding protein. It is probably the single highest-yield swap in this entire series.

What is actually in the can besides fish

Canned tuna is cooked twice — once before canning, once inside the sealed can during retort sterilization — and that double cook is why it is shelf-stable for years without preservatives. It also explains two things people notice. The texture is firm and flaked rather than tender, because the protein has been thoroughly denatured. And the omega-3 content is lower than fresh tuna: some of the fat renders out during processing, and in oil-packed cans it disperses into the packing oil that most people pour down the sink.

Which is the underrated argument for water-packed if omega-3 matters to you: draining water-packed tuna loses very little of the fish's own fat, while draining oil-packed removes the added oil and a share of the omega-3 that migrated into it. If you buy oil-packed, using the oil in the dish rather than discarding it recovers both.

On BPA: it was historically used in can linings and tuna cans were part of the conversation. Most major US brands moved to non-BPA linings over the last decade, and many now state it on the label. If it concerns you, pouches sidestep the question entirely — though pouch linings are their own multi-layer laminate, so this is a preference rather than a resolution.

Common mistakes

• Buying albacore by default. Three times the mercury for the same price and the same protein.
• Avoiding tuna entirely out of vague worry. The math is permissive for most adults — usually 2–3 cans of light weekly — and the protein is hard to replace at that price.
• Ignoring the sodium line. No-salt-added exists, costs the same, and saves ~240 mg per can.
• Drowning it in mayo. The dressing is usually 3–4× the calories of the fish.
• Assuming pouch = better. Same fish, less liquid, higher price per ounce.
• Applying adult math to children. Mercury limits scale with bodyweight; a child's allowance is a fraction of an adult's.

Who should adjust

Pregnancy and breastfeeding is the case the advisories were written for — methylmercury crosses the placenta and the developing nervous system is the sensitive target. FDA/EPA advice is 2–3 servings a week from the Best Choices list (canned light tuna qualifies, albacore is limited to one serving), and it is worth following literally rather than approximately. Young children get the same treatment for the same reason, scaled down further by bodyweight. And anyone eating fish at genuinely high frequency — several servings daily, which does happen — should raise it with a physician; blood mercury testing exists and is straightforward.