Vitamin D plays a number of important roles in the body, starting from its participation in maintaining calcium-phosphorus homeostasis and proper bone mineralization, as well as its influence on the proper functioning of the endocrine, autoimmune, nervous, and muscular systems.
Many people in our climatic zone suffer from vitamin D deficiencies, not only in the autumn-winter period. The groups of people who should particularly take care to provide adequate amounts of this vitamin include infants and children during periods of intensive growth, due to the risk of developing rickets, and elderly individuals, in whom the production of active forms of this vitamin in skin cells decreases, as well as its conversion in the kidneys and liver.
Food products can provide us with an average of about 20% of the daily recommended dose of vitamin D. It occurs in food of animal origin in the form of cholecalciferol - vitamin D3, and in plant form as ergocalciferol - vitamin D2. Even if we take into account various sources of this vitamin in our daily diet, we may have difficulty achieving the proper level in a blood serum test.
Vitamin D occurs in food in small amounts, mainly in the animal world. Liver oils from fish (cod liver oil), fish (eel, herring, mackerel), eggs, liver (veal, pork, beef), mushrooms (chanterelles, porcini, mushrooms), lard, and butter are particularly rich sources.
Plant products generally do not contain vitamin D, with only trace amounts occurring in cereal products and vegetable oils. In many countries, margarines and cereal products, as well as milk and its products, are enriched with vitamin D. In USA, margarines and powdered milk for infants are enriched.
Witamina D is quite stable and does not break down under the influence of high temperature or during prolonged storage. However, it is destroyed by ultraviolet radiation, and under oxygen conditions in an aqueous environment, it undergoes auto-oxidation (self-oxidation).
Source: M. Łyszczek - Supplements from A to Z. N. Kmieciak - Vitamin D. Functions, sources, norms and supplementation. J. Gawęcki - Human nutrition.