Vitamin C and vitamin E – which are ubiquitous among raw plant foods – are confirmed as dietary antioxidants, whereas vitamin A becomes an antioxidant following metabolism of provitamin A beta-carotene and cryptoxanthin. Most food compounds listed as antioxidants – such as polyphenols common in colorful, edible plants – have antioxidant activity only in vitro, as their fate in vivo is to be rapidly metabolized and excreted, and the in vivo properties of their metabolites remain poorly understood. For antioxidants added to food to preserve them, see butylated hydroxyanisole and butylated hydroxytoluene.