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food source · 3
←Tuna
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.
Sea fish are a good source of selenium, and salmon is no exception. Selenium is built into the active site of a group of antioxidant enzymes, a part those enzymes need in order to work.
Mechanism · The screw in the enzyme
Selenium is not an antioxidant itself but a part fitted into the active site of a group of enzymes: the enzymes do the work, and selenium is the screw that lets them. The selenium in sea fish is steadier than in grain, because it depends less on the soil where the food was grown.
Safety · Selenium is no excuse for mercury
Selenium binds mercury, and by the chemistry it can offset a little of the effect of the trace mercury in sea fish; but the selenium is used up in the process and the offset is only partial, so do not treat it as a reason to stop worrying about high-mercury fish.
One oyster brings zinc, selenium and B12 together. Selenium is built into the active site of antioxidant enzymes, a part those enzymes need in order to work.
Mechanism · One atom at the cutting edge
The body makes selenium into a special amino acid and sets it into the active site of antioxidant enzymes as the cutting edge. Without that edge, the enzyme can fold and still not work. For a nutrient that serves as a part, once enough enzyme has been made, extra has nowhere to go, so the idea that more selenium means more antioxidant power was wrong from the start.
In practice · Selenium in food is not a pill
The selenium in an oyster comes with the whole oyster, in the natural range for food, which is a different thing from swallowing a high-dose supplement. The iron column is often written backwards: Pacific oysters are not low in total iron, but only about 0.85 mg per 100 g is heme iron, so it is absorbed less well than the iron in red meat.
synergy · 1
Glutathione peroxidase, a selenium-containing enzyme, breaks down hydrogen peroxide in the watery part of the cell, and vitamin E stops the chain oxidation of fats in the cell membrane. They intercept reactive oxygen in different places, and when one is short, the other has to carry more of the load.
cofactor · 4
The Toulis 2010 meta-analysis pooled 4 randomized trials (most patients were already on levothyroxine): after 3 months of selenium, anti-thyroid peroxidase (TPO) antibody titers were lower; effects on thyroid function, dose needs and thyroid structure were either not seen or not reported. Selenium has a U-shaped dose response and too much is harmful, so do not megadose.
The deiodinases that convert thyroxine (T4) into active T3 are all selenium-containing proteins, so with too little selenium, tissue T3 can run short even when T4 is adequate. Selenium enzymes also clear the hydrogen peroxide produced as the thyroid makes hormone, protecting thyroid cells. Iodine is the raw material, selenium handles processing and protection, and both need to be adequate.
Selenocysteine sits at the active center of glutathione peroxidase. When immune cells kill microbes with an oxidative burst, they oxidize themselves too, and selenium enzymes are how they protect themselves.
Glutathione neutralizes peroxides only with the help of glutathione peroxidase (GPx), an enzyme whose active site is a selenium-containing amino acid, selenocysteine. So this antioxidant defense in the liver depends on selenium.