Vitamin A. Essential in vision; enhances immunity by protecting the body from bacteria and viruses; boosts the production of white blood cells; helps build and maintain bone. Richest sources of retinal are liver, fish liver oils, whole and fortified milk, and eggs. Carotenoids (such as beta-carotene) that the body uses to make vitamin A are abundant in orange and yellow fruits and vegetables, and green leafy vegetables. Researchers are investigating the links on vitamin A's ability to boost the immune system and fight cancer, and the results look promising. The dark side of this is people are taking megadoses of vitamin A mistakenly believing that it can ward off or cure disease, often doing serious liver damage. People can achieve the level of beta-carotene in the studies simply by eating the 5 to 9 servings recommended by health authorities.