Vitamin A is a fat-soluble vitamin, which means that it requires fats as well as minerals to be properly absorbed by your digestive tract. It occurs in two forms: Preformed vitamin A, also called retinol, which is found in foods of animal origin. The other form is Provitamin A, also called carotene and is most commonly found in fruits and vegetables. Carotene is the primary pigment of deeply colored fruits & vegetables. In nature, there are over 500 carotenoids, but the body can only convert about 50 of them to Vitamin A.

Vitamin A is essential for numerous intrinsic processes. The most well-known and understood process is that of vision. The retinal form of vitamin A is essential for the neural transmission of light into vision. Epithelial cells are highly dependent on retinoic acid and are commonly used to treat a variety of skin diseases. A developing fetus is also highly dependent on retinoic acid, as it is essential to the growth of the eyes, lungs, ears and heart. The retinoids are not only the most active form of vitamin A, but also a current area of interest to many scientists.

Vitamin D is called the sunlight vitamin because the body produces it when the sun's ultraviolet B (UVB) rays strike the skin. It is the only vitamin the body manufactures naturally and is technically considered a hormone. It is a fat-soluble vitamin that has properties of both a vitamin and a hormone. It is required for the absorption and utilization of calcium and phosphorous. It is needed for growth, and it is especially important for the normal growth and development of bones and teeth in children. It also protects against muscle weakness and is involved in regulating the heartbeat.

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