Complex sentence: Selenium salts are toxic in large amounts, but trace amounts are necessary for cellular function in many organisms, including all animals, and is an ingredient in many multi-vitamins and other dietary supplements, including infant formula.
Split into simpler sentences: Selenium salts are toxic in large amounts, but trace amounts are necessary for cellular function in many organisms, including all animals. Selenium is an ingredient in many multi-vitamins and other dietary supplements, including infant formula.