Selenomethionine

Selenomethionine, abbreviated SeMet, is an organoselenium amino acid and the selenium analogue of methionine. In SeMet, the sulfur atom of methionine is replaced by selenium, giving a compound that is chemically similar to methionine but with selenium in place of sulfur. It is a major form of organic selenium found in foods and supplements.

Selennomethionine occurs naturally in foods that accumulate organic selenium, such as cereals grown in selenium-rich soils and especially in selenium-enriched yeast used for supplements. It is also produced industrially by chemical synthesis or by fermentation with yeast under selenium-supplemented conditions. In many dietary sources, SeMet accounts for a large portion of the organic selenium content and contributes to overall selenium bioavailability.

Biochemically, SeMet can be non-specifically incorporated into body proteins in place of methionine, serving as a storage form of selenium. Upon protein turnover, SeMet is released and can be used to support the synthesis of selenoproteins or converted to other selenium forms for metabolism or excretion. It generally shows high oral bioavailability compared with inorganic selenium forms such as selenite.

Uses and regulation: SeMet is used as a dietary supplement and as a selenium source in animal feeds. In humans, it is commonly sold as a supplement to improve selenium status, with typical labeled doses ranging from about 100 to 200 micrograms per day. It is one of the preferred organic selenium forms for supplementation due to its stability and bioavailability.

Safety and research: Selenium is essential for enzymes such as glutathione peroxidases and thyroid deiodinases, but excessive intake can cause selenosis, with hair and nail changes, gastrointestinal symptoms, and fatigue. The tolerable upper intake level for adults is about 400 micrograms per day from all sources. Evidence on cancer prevention and other outcomes is mixed and dose-dependent; SeMet is regarded as a biologically active form of selenium.