The vitamin D status depends on the production of vitamin D3 in the skin under the influence of ultraviolet radiation and vitamin D intake through the diet or vitamin D supplements. The serum 25-hydroxyvitamin D (25(OH)D) concentration is the parameter of choice for the assessment of vitamin D status.
Low serum levels of calcium and phosphate and an elevated level of alkaline phosphatase can also point to vitamin D deficiency. Usually, between 50% and 90% of vitamin D in the body is coming from the production in the skin and the remainder is from the diet.
The production of vitamin D3 in the skin depends on sunshine exposure, latitude, skin-covering clothes, the use of sun block and skin pigmentation. In general, serum 25(OH)D is lower with higher latitudes and with darker skin types, but there are exceptions.
Vitamin D deficiency (serum 25(OH)D<25 nmol/l) is highly prevalent in India and China while vitamin D status is better in Japan and South-East Asia. Vitamin D deficiency is very common in the Middle-East and there is a relationship with skin covering clothes and staying outside of the sun. A poor to moderate vitamin D status is also common in Africa, probably caused by the dark skin types and cultural habits of staying outside of the sunshine. Vitamin D status is much better in North America where vitamin D deficiency is uncommon but vitamin D insufficiency (serum 25(OH)D between 25 and 50 nmol/l) is still common. In the United States and Canada milk is usually supplemented with vitamin D and the use of vitamin supplements is relatively common. Vitamin D status in Latin America usually is reasonable but there are exceptions and vitamin D insufficiency still occurs quite often. In Australia and New Zealand a poor vitamin D status was seen in the elderly who were often vitamin D deficient and also in immigrants from Asia.
Vitamin D deficiency also occurred in children when the mother was vitamin D deficient. Within Europe, vitamin D status usually is better in the Nordic countries than around the Mediterranean. This may be due to a lighter skin and sun seeking behaviour and a high consumption of cod liver oil in the Northern countries while in Southern Europe people stay out of the sunshine and have a somewhat darker skin. A very poor vitamin D status was observed in non-western immigrants, especially in pregnant women. In conclusion, vitamin D deficiency and insufficiency are globally still very common especially in risk groups such as young children, pregnant women, elderly and immigrants.
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