Tuna

Tuna

Tuna (in Hebrew “דג טונה”) is a large predatory fish of the mackerel family (Scombridae), and one of the most widely eaten fish in the world. Nutritionally, it provides protein at a very high density for a low calorie cost — but two key figures about it, its omega-3 content and its mercury content, vary dramatically between species and between forms of serving.

Tuna nutrition facts per 100 grams

100 grams of light tuna canned in water and drained provide 116 calories, 25.5 grams of protein, less than a gram of fat and zero carbohydrates — one of the best protein-to-calorie ratios of any food. The version in oil has nearly twice the calories, and fresh tuna is similar to light tuna in protein but much lower in sodium:

One can of tuna (about 140 grams drained) provides about 33 grams of protein for roughly 160 calories. The two figures that don’t fit in a single table and vary dramatically between species — omega-3 and mercury — are the ones that determine which tuna is worth buying and how often.

The figure that varies: omega-3 by species

The basic assumption that “tuna is an oily fish rich in omega-3” is only partly true:

The difference between the most available and cheapest version (canned light) and fatty tuna is fivefold or more. The common canned tuna is not as strong an omega-3 source as salmon (2,000-2,500 mg) or sardines (1,400-1,500 mg).

An important point about tuna canned in oil: the vegetable oil in the can draws in some of the fish’s omega-3, and draining the oil pours it down the sink. Tuna in water preserves omega-3 better.

Mercury — the difference that sets the frequency

Tuna is a predatory fish that lives many years, so it accumulates methylmercury up the food chain. The concentration depends on the size of the species and its lifespan:

• Light tuna (skipjack / “light”) — a small, short-lived fish. Relatively low average mercury, about 0.12 ppm.
• White tuna (albacore) — larger. Average mercury about 0.35 ppm — almost three times as much.
• Bluefin and bigeye tuna — the highest.

The guidelines of the US FDA and EPA for sensitive populations — pregnant women, breastfeeding women, women planning a pregnancy and young children:

• Canned light tuna: up to 2-3 servings a week.
• White tuna (albacore): up to one serving a week.
• Bluefin or bigeye tuna: avoid.

For a healthy adult outside these groups, eating 2-3 servings a week of light tuna is considered safe. Mercury mainly harms the developing nervous system, hence the emphasis on pregnancy and childhood.

Benefits

• Exceptionally dense protein: 25.5 grams per 100 grams for only 116 calories. A complete protein of high biological value, supplying all the essential amino acids. One can of tuna (about 140 grams drained) provides about 33 grams of protein — more than two servings of meat.

• A high amount of selenium: 100 grams provide about 90 mcg — over 100% of the daily intake. Selenium plays an interesting role here: it binds to mercury and reduces its bioavailability, which partly moderates the risk (a measure called the “Selenium Health Benefit Value”).

• Vitamin B12: 100 grams provide about 2.2 mcg, close to the full daily intake.

• Vitamin D: one of the few food sources — about 270 IU per 100 grams in canned tuna.

• Convenience and cost: canned tuna is one of the cheapest and most convenient sources of quality protein. Long shelf life, no need for refrigeration or cooking — a real practical advantage.

• Low in saturated fat: unlike other animal protein sources.

Downsides and risks

• Mercury: as described above. The main consideration, and it depends entirely on the species and the population.

• Sodium in canned tuna: regular canned tuna contains 300-400 mg of sodium per 100 grams. “No salt added” or “reduced sodium” versions exist. Briefly rinsing the tuna in water removes some of the sodium.

• Histamine (scombroid poisoning): fish of the mackerel family, including tuna, produce histamine if not kept properly chilled after being caught. The result is an allergy-like reaction — flushing, rash, headache and palpitations. Histamine is heat-stable and is not destroyed by cooking. Avoid fresh tuna whose smell or appearance is suspicious.

• Tuna in oil vs. in water: the version in oil contains nearly twice the calories (about 200 per 100 grams), and it also loses omega-3 when drained. The version in water is almost always the better choice.

• Fish allergy: one of the major allergens, and it can be severe.

• Sustainability: bluefin tuna stocks are in critical condition in some regions. Fishing methods using fish aggregating devices cause bycatch of sea turtles and dolphins. “Pole and line” labels or the MSC label indicate more sustainable methods.

Practical use

What to buy: light tuna in water, preferably reduced-sodium. This is the best balance between low mercury, preserved omega-3 and calories.

Frequency: 2-3 servings a week for a healthy adult. One serving a week of albacore for pregnant women and children.

How to make up the omega-3: since light tuna provides only about 270 mg, it is worth also including salmon, sardines or mackerel in your weekly menu — smaller fish, richer in omega-3 and lower in mercury.

Draining: drain the liquid well from the version canned in oil. With the version in water you can use the liquid.

Best pairing: tuna with olive oil, lemon and vegetables — the lemon contributes vitamin C, which improves iron absorption, and the olive oil supplies the fat missing from the version in water.

Summary

Canned tuna is an excellent, cheap and convenient protein source — 25.5 grams of protein for 116 calories — and it provides selenium, B12 and vitamin D in significant amounts. The two figures that really matter are the species and the canning method: the common light tuna contains about a quarter of the omega-3 of salmon, but also about a third of the mercury of white tuna. For most people the right choice is light tuna in water, 2-3 times a week, alongside other oily fish that will actually supply the omega-3.