What is it and what does it do
Vitamin D is a fat soluble vitamin essential for normal calcium metabolism and normal bone production and maintenance. It promotes calcium absorption from the gut, normal blood calcium levels and normal mineralisation of bone. Bone is not static but is constantly being remodelled and for this adequate supplies of calcium are needed. Vitamin deficiency is associated with reduced blood levels of calcium (and through various mechanisms, phosphate) leading to tiredness, muscle and bone pains and aches, muscle twitching and weakness. The low calcium levels eventually lead to malformed, soft and brittle bones and bone cysts, that is, rickets in children and osteoporosis and osteomalacia in adults. Vitamin D is also involved in the maintenance of the normal immune system and chronic deficiency is thought to increase predisposition to insulin dependent diabetes, multiple sclerosis and other disorders where the body immunity becomes abnormal. There are also suggestions that vitamin D deficiency may predispose to cancer, particularly cancer of the bowel and breast and that it may increase the risk of cardiovascular disease and high blood pressure. It is thus a very important vitamin.
What are the sources
Vitamin D synthesis
The synthesis of Vitamin D is complicated. In animals and humans, a precursor, 7-dehydrocholesterol, is produced in the skin from cholesterol. Through the action of ultraviolet light this in turn becomes previtamin D3, which is converted to vitamin D3 (cholecalciferol). Plants synthesize ergosterol, which is converted to vitamin D2 (ergocalciferol), also by ultraviolet light. Thus, if there is insufficient sunlight, our Vitamin D must come from the diet. However, plant vitamin (Vitamin D2) may be less active than the animal one (D3). D2 and D3 are biologically inactive and must be metabolized further, first in the liver to 25 hydroxy vitamin D (25, OH Vit D) and, finally, to the most active form in the kidney, 1, 25 dihydroxy vitamin D (1,25 Vit D).
The vitamin D measured in laboratories is usually 25 Vit D.
Daily requirements and food sources
The daily requirements of vitamin D are variously suggested at 200 iu/day in babies and children and up to 700 IU/day or more in old people. However, an average diet in the Western world often contains no more than 100 IU/day, with potential Vitamin D deficiency further exacerbated by lack of sunshine or insufficient exposure to sunlight. It has been suggested that in certain situations even up to 10,000 IU/day may be needed.
The best dietary sources of Vit D are oily fish, followed by eggs (egg yolk) and mushrooms (partly because of exposure to U/V light to make them whiter!). However, many food products are also fortified with Vitamin D and can be easily included within a healthy diet such as breakfast foods, baby foods, some fruit juices and margarines.
Causes of Vitamin D deficiency
There are many causes of Vitamin D deficiency, starting with nutritional deficiencies (particularly vegetarians), inadequate exposure to sunlight, liver or kidney disease, gastrointestinal diseases causing fat malabsorption (eg pancreatic disease, coeliac, Crohn’s) and very low fat diets. Some medicines also cause Vitamin D malabsorption such as Orlistat. Others, such as antiepileptics, are associated with increased breakdown of the vitamin in the liver as is alcohol abuse. A potentially major cause of Vitamin D deficiency in the Western world is obesity because Vitamin D is stored in fat and in obese individuals this leads to slower release and thus availability in blood.
Vitamin D replacement
As in the majority of individuals Vitamin D deficiency is the result of life style, changes in diets (particularly eating more oily fish), more exposure to sunlight and weight loss can improve blood levels. Supplements , such as cod liver oil (given in the past routinely to babies and children particularly) can also be beneficial. It should not be though forgotten that the main action of Vitamin D is to promote calcium absorption and normal bone formation and growth. Thus, for Vitamin D to be effective, there must be also adequate dietary calcium. The best sources of dietary calcium are milk (in the water part), followed by dairy products, small fish (such as sardines if eaten with bones) followed by eggs, pulses such as beans and white flour products.
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