Selenoamino acids are a group of amino acids that contain selenium, an essential trace element. They play a crucial role in various biological processes, including antioxidant defense and thyroid hormone metabolism. The most well-known selenoamino acid is selenocysteine, often referred to as the 21st amino acid, which is incorporated into proteins during translation. Another important selenoamino acid is selenomethionine, which can be found in certain foods like Brazil nuts and fish. These compounds contribute to the body's selenium levels, supporting immune function and overall health. Adequate intake of selenoamino acids is vital for maintaining optimal physiological functions.