Selenium - a scoping review for Nordic Nutrition Recommendations 2023

Abstract

Selenium is an essential trace element in humans, critical to the normal physiology in all animal species. The main form of selenium in food is selenomethionine, selenocysteine and a variety of organic compounds, while inorganic salts mainly occur in food supplements. In animals and humans, selenium occurs as selenocysteine in selenoproteins encoded by 25 genes (specific selenium pool). Several selenoproteins are part of the antioxidant enzyme system and serve as oxido-reductases and in thyroid hormone regulation. SelenoproteinP (SELENOP) transports selenium to peripheral tissues, is the main plasma selenoprotein, and has been used as biomarker of selenium status and intake. SELENOP in plasma represents a saturable pool of selenium and is maximised at a selenium concentration in plasma of about 110 µg/L or an intake of selenomethionine at about 1.2 µg/kg body weight in adults. In Finland, with an estimated selenium intake of 88 µg/day in men and 68 µg/day in women, the average selenium concentration in plasma is about 110 µg/L. Imported wheat from selenium rich areas is an important dietary source in Norway. Dietary intakes in the Nordic and Baltic area vary from 39 to 88 µg/day in men and 22 to 68 µg/day in women, the highest levels were from Finland. Most intervention trials on the effect of selenium supplementation on health outcomes have been carried out in 'selenium-replete'-populations and show no beneficial effect, which from a nutritional point of view would rather not be expected. Some intervention studies conducted in populations low in selenium have showed a beneficial effect. Observational studies suggest an inverse relationship between selenium status and risk of cardiovascular diseases (CVDs), cancer and all-cause mortality, and some other outcomes at low levels of intake (<55 µg/day) or in plasma or serum (<100 µg/L). However, a lack of quantitative data and inconsistencies between studies precludes these studies to be used to derive dietary reference values. At high intakes above 330 to 450 µg/day selenium may cause toxic effects affecting liver, peripheral nerves, skin, nails, and hair. An upper tolerable level (UL) of 255 µg selenium/day in adults was established by EFSA.

Keywords: antioxidants; nutrition recommendations; selenium; selenoprotein; trace elements.

Fig. 1

Overview of selenium absorption, distribution, and excretion. The distribution of selenium in plasma is representative for a selenium replete individual (9). Reproduced with permission (1).

Fig. 2

Overview of selenium biotransformation. Glutathione (GSH, glutathione reductase (GR) glutaredoxin (Grx), thioredoxin (Trx), thioredoxin reductase (TXNRD), selenocysteine (SeCys), methylselenocysteine (MeSeCys), selenomethionine (SeMet), selenocystathione (SeCystat), seryl-tRNA (Ser-tRNA), methylselenide (MeSe^-), dimethylselenide (Me₂Se), trimethylselenonium ion (Me₃Se⁺). Reproduced with permission (1).

Fig. 3

Illustration of regulated and saturable pool of selenium and relation to unregulated and non-saturable pool of selenium.