L-isomer of selenomethionine (Se-met) is a major natural food-form of selenium, synthetic L-Se-met or enriched food sources thereof such as selenium yeast are appropriate supplemental forms of Se for humans; for animals, DL-Se-met is acceptable. Ingested Se-met is either metabolized directly to reactive forms of selenium or stored in place of methionine in body proteins. Se-met metabolism is closely linked to protein turnover. Selenium, which is nutritionally essential for humans, is a constituent of more than two dozen selenoproteins that play critical roles in reproduction, thyroid hormone metabolism, DNA synthesis, and protection from oxidative damage and infection.