If the intake is not sufficient, bones will stop growing before the soft tissues are fully mature. It is also important in the maintenance of healthy epithelial tissues. Without vitamin A, the epithelial cells become dry and flat and gradually harden to form scales that shed. Vitamin A is also necessary for the health of the membranes lining the stomach, intestinal wall, bladder and urinary passages and for the health of the sex glands and uterus. Lastly, vitamin A works better when there are sufficient body levels of zinc and an adequate intake of protein.
Vitamin D is required for calcium and phosphorus absorption and utilization.
It is necessary for growth, development and maintenance of bones and teeth in adults and children. In its active form, vitamin D works with calcium to control bone formation. This leaves the red blood cells more fully supplied with the pure oxygen that the blood carries to the heart and other organs. Vitamin E plays an essential role in cellular respiration of all muscles, especially cardiac and skeletal. Vitamin E makes it possible for these muscles and their nerves to function with less oxygen, thereby increasing their endurance and stamina.