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. The team was the first to show limonin's "bioavailability." The body derives limonin from a parent compound--limonin glucoside--that's present in citrus and citrus juices in about the same amount as vitamin C, according to Manners. He's with the agency's Western Regional Research Center in Albany, Calif. In other early work, Manners and colleagues found that limonin may lower cholesterol. The researchers showed that, when exposed to limonin, human liver cells in petri dishes produced less apo B, a compound associated with higher cholesterol levels. High cholesterol levels are linked to increased risk of heart disease and other health problems. Now, Manners and coinvestigators are taking a closer look at limonin's cholesterol-lowering effects. They're doing that in a first-of-its-kind study with healthy volunteers. Manners is collaborating with researchers at Albany and at the ARS Western Human Nutrition Research Center in Davis, Calif. ARS is the U.S. Department of Agriculture's chief scientific research agency.