Sources of vitamin D
Sunshine
The low incidence of rickets in tropical climates suggested that sunshine might playa role in its prevention. Even after it had been demonstrated conclusively that the ultraviolet light from sunshine aided in the healing of rickets, it was difficult to understand the connection between this effect oflight and the similar effect of vitamin D from such sources as cod-liver oil. Eventually, the puzzle was solved when it was discovered that vitamin D activity could be produced by irradiation. The amount of ultraviolet light in sunlight varies with the season and the locality, as does the total amount of sunlight. These rays are also filtered out by fog, smoke and ordinary window glass. It is obvious that an adequate natural source of ultraviolet light is impossible in northern climates during the winter months. For this reason, some other source of vitamin D is needed.
Similarly, the pigments in the skin that protect against overproduction of vitamin D in dark-skinned peoples living in the tropics also reduce the effectiveness ofthe smaller amount ofirradiation in temperate climates. As a result, the incidence of rickets is higher in dark-skinned babies living in temperate zones than in either lightskinned babies in this zone or dark-skinned infants in the tropics.
Foods and supplements
The natural distribution of vitamin D in common foods is limited to small, often insignificant amounts in fatty tissue, cream, butter, eggs, and liver. Consequently, we have come to depend on fortified foods, fish-liver oil, or concentrates for preventive and therapeutic use.
Because it was necessary to decide on one food, commonly used by children, that would be fortified with a standard amount of vitamin D, the Council on Foods and Nutrition of the American Medical Association made the following decision:
Of all the common foods available, milk is the most sui table as a carrier of added vitamin D. Vitamin D is concerned with the utilization of calcium and phosphorus, of which milk is an excellent source.
Vitamin D milk now on the market is produced by adding a vitamin D concentrate to homogenized milk; the present standard of10 mcg (400 IV) per quart means that all brands of evaporated milk aho have vitamin D added, and strong recommendations to fortifY nonfat milk solids with vitamins A and D have also been made by the American Medical Association.39 Indiscriminate fortification of a variety of other foods wi.th vitamin D does not seem to be either necessary or desirable.
Among the numerous fish-liver oils investigated there is a wide range of potency. This seems to vary with the season ofthe ca tch and the oil content ofthe livers. The highest-potency oil is often yielded from fish that give the lowest amount of oil. Concentrates are made from the natural fish-liver oils or by irradiating pure ergosterol and cholesterol. Such preparations are labeled with the exact units per dose or per capsule and are prescribed accordingly. A protective dose to meet the daily requirement is considerably lower than what may be prescribed as a curative dose.
In a study of the consumption of vitamin D by children (birth-18 years) the average daily intake for all age groups was above 400 IU. Fortified milk supplied the largest amount of vitamin D, the percentage increasing with age. Vitamin preparations were more important in the infant and preschool groups than with older children. Fortified foods contributed to intakes of vitamin Dover the recommended dietary allowance, particularly in the school age group.
It is becoming evident that changing social, cultural, and environmental conditions are contributing to the reemergence of rickets as a nutritional problem in the Western countries. Many Children cases involve severely restricted vegetarian diets, and many of the infants were breast-fed, with no vitamin supplements. None of the mothers had received supplements during pregnancy or lactation. Other cases of rickets involving children of white vegetarians also have been reported.
Stability
Vitamin D in foods and in food concentrates is remarkably stable to heating, aging, and storage. Vitamin D milk that is warmed for the baby is still a reliable source of this factor.
