One class of compounds thought to possibly be antioxidants in the body are xanthophylls, a subclass of carotenoids. The most famous of carotenoids is beta carotene, the compound which colors carrots yellow-orange. Synthetic beta-carotene is used as food coloring, notably to color margarine yellow. Xanthophylls are also yellow. They are made by plants, and occur in chloroplasts, the complex structures in green leaves which perform photosynthesis.
Antioxidant xanthophylls contain an -OH group.
The reactivity of the -OH group of these xanthophylls allows them to function as antioxidants, which means they can give up electrons to oxidants. The most common xanthophylls are lutein and zeaxanthin. They enter the diet primarily as constituents of leafy green vegetables. They are present in blood. The concentrations in blood decline in the elderly.