Whether one prefers grapefruits, oranges or tangerines, citrus fruits are high in vitamin C, which is needed to fight off colds and nasty flu viruses. These are often called winter citrus fruits for this reason, since consumption levels peak during that season! Oranges, tangerines and grapefruits have been proven to prevent many kinds of cancer, too, such as prostate, lung and breast cancers. So start off your day right with a glass of freshly squeezed orange juice.