Vitamin A was the first vitamin to be discovered. It is essential in maintaining good vision and promoting normal growth. It occurs in animal tissues as retinol but in plants as carotene, a precursor of vitamin A. This vitamin is necessary for the health of epithelial cells, skin, the immune and digestive systems, respiratory, reproductive and urinary tracts . It is essential for lactation, reproduction and the formation of steroid hormones. It is best absorbed when taken with oil or fat.
Vitamin D works synergistically with vitamin A. It plays a role in absorbing and regulating calcium and phosphorus. This vitamin helps the body synthesize protein and helps build strong bones, teeth and skin. It is vital to the health of the nervous system and kidneys. It can be derived from the sun's action on oils secreted and reabsorbed by the skin. But if skin is tanned or older, it is less capable of producing vitamin D.