L-Se-methylselenocysteine (L-SeMC) is a natural L-type selenium-containing amino acid and a methylated derivative of selenocysteine, the 21st essential amino acid for the human body. It was first isolated from Astragalus in 1960. It is widely found in selenium-rich plants such as broccoli, garlic, onion, wild leek, etc., and is the best source of selenium for human nutrition. It is an easily absorbed form of organic selenium with multiple effects such as tumor inhibition, anti-oxidation, anti-aging, auxiliary treatment of cardiovascular diseases, detoxification, etc.