Vitamin A contains retinol (vitamin A alcohol) or its esters from edible fatty acids (chiefly acetic and palmitic acids). It is available from various manufacturers under many names. Dosage forms vary from oral tablets, capsules, and solutions to topical ointments and creams. It is also available as an injectable in the form of vitamin A palmitate (Aquasol A) that is intended for intramuscular (IM) injection. Vitamin A is also available as a component of multivitamins from many different sources.

The provitamin A carotenoids are also widely available, primarily ¡-carotene, from various sources under many names. Oral liquids, tablets, and capsules are available, containing ¡-carotene, either singly or in combination as multivitamin preparations.

Retinoate analogs are also available for the treatment of dermatological conditions and cancer. Table 28.6 lists the current products on the market and their approved uses, whereas Figure 28.5 shows their corresponding structure.

Vitamin D

The recognition in 1919 that rickets was the result of a nutritional deficiency led to the isolation of antirachitic compounds from food products.56 The role of sunlight in the prevention of rickets was noted at the same time.57 Early studies showed that vitamin A preparations available at the time, in addition to their growth-promoting properties, could also cure xerophthalmia and rickets. Thus, the early assumption was that all of these were because of the deficiency of vitamin A. A preliminary report by Funk and Dubin58 in 1921 was the first to suggest the presence of a vitamin in yeast extracts that was distinctly different from vitamin A and vitamin B, which the authors named vitamin D. It is interesting that the authors believed the new vitamin D was another of the B vitamins, which at that point in history was known to consist of at least two different vitamins. McCollum et al.59 clearly showed in 1922 that vitamin A and the antirachitic factor were, in fact, two separate vitamins. The synthesis of both major forms of vitamin D, ergocalciferol and cholecalciferol, was accomplished in 1977.60

The highest natural source of vitamin D is fatty fish such as herring, catfish, salmon, mackerel, and tuna or oils derived from fish such as cod liver oil. Eggs, from hens fed vitamin D, and butter also contain small amounts of vitamin D. The most common source of vitamin D in developed countries, such as the United States, is fortified foods such as milk, ready-to-eat cereals, and some fruit juices. Cholecalciferol or ergocalciferol is usually used in fortification or supplementation.

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