Oranges rich in vitamin C offer another important yet lesser-known nutritional bonus: citrus limonoids. In laboratory tests with animals and with human cells, citrus limonoids have been shown to help fight cancers of the mouth, skin, lung, breast, stomach and colon.

Agricultural Research Service scientists in northern California, led by chemist Gary D. Manners, are uncovering new details about these compounds. Manners and coinvestigators have reported their findings in studies published during the past several years. They've demonstrated, for example, that each time we bite into a citrus slice or drink a glass of orange juice, our bodies can readily access a limonoid called limonin.