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Food · Animal Foods · Fish
food source · 3
Raw yellowfin tuna has about 24 g of protein and less than 1 g of fat per 100 g, which makes it almost pure lean protein, with all nine essential amino acids.
Mechanism · All nine essential amino acids
The body builds protein chains in order, and if one essential amino acid is missing, the whole chain stops. Tuna has all nine essential amino acids in useful proportions, so nothing has to be made up from other foods. Once eaten, stomach acid unfolds the protein, digestive enzymes cut it into short pieces, and the small intestine passes the amino acids into the blood; muscle, enzymes and antibodies are all built from this same supply.
Numbers · Raw weight and cooked weight
Raw yellowfin tuna has about 24 g of protein, under 1 g of fat and 0 g of carbohydrate per 100 g, at about 109 kcal. Cooked figures are higher almost entirely because water is lost: the denominator shrinks, and you are not getting more protein. Water-packed canned tuna stays close to the fresh figures; oil-packed tuna still has more fat and calories than water-packed even after draining.
B12 comes almost only from animal foods, and tuna is very high in it: one serving usually exceeds a day's need. B12 maintains the insulating sheath around nerve fibers and lets the bone marrow make red blood cells of normal size.
Mechanism · Three steps to absorption
Absorbing B12 takes three steps: stomach acid frees it from food protein, intrinsic factor made by the stomach escorts it, and receptors at the end of the small intestine take it in. If any step fails, you can eat it and still not absorb it.
Clinical · Eating enough can still fall short
One serving of tuna usually supplies far more B12 than a day's need. But atrophic gastritis, stomach surgery, long-term strong acid suppression and disease at the end of the small intestine can all leave someone eating enough and still not absorbing it. Whether to test for B12 is a decision for a doctor; do not match your own symptoms against a list.
Tuna is one of the richest sources of selenium, and a serving often covers a day. Selenium is built into the active site of antioxidant enzymes as a part that lets them work; it is not itself an antioxidant that goes around neutralizing free radicals.
Mechanism · A part built into an enzyme
Selenium is made into a special amino acid, selenocysteine, which sits at the active site of enzymes such as glutathione peroxidase, and the enzyme breaks hydrogen peroxide down into water. Once the parts are in place, the enzyme works; adding more does not make it faster, and selenium has a toxic upper limit.
Safety · Selenium does not cancel extra mercury
In tuna, the link between selenium and mercury is worth remembering. By the mechanism, part of mercury's harm comes from stealing this selenium part, and the selenium in fish can tie up some of the mercury; this comes mainly from animal and mechanism studies and has not been measured directly in people. It is only a partial offset: taking selenium supplements does not make it safe to eat more high-mercury fish, and more selenium from supplements is not better.