Vitamin A is essential for maintaining good vision and promoting normal growth. It is necessary for the health of epithelial cells and is required for the digestion of protein. Vitamin A is essential for lactation, reproduction and the formation of steroid hormones. It is used to form the cells lining the digestive, respiratory, reproductive and urinary tracts and in all tissue linings of the body. Vitamin A is best absorbed when taken with oil or fat. It occurs in animal tissues as retinol but in plants as carotene, a precursor of vitamin A. 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. It is vital to the health of the nervous system and kidneys.