Vitamin D


Dr Noureen Ahmad

General Practitioner, Belgium

Vitamin D – “the sunshine vitamin” – is one of the fat-soluble vitamins and plays a big role in regulating bone growth alongside several other functions. It exists in many forms (D1, D2, D3, D4, D5) but it is the 1,25 (OH)2 of vitamin D3 which is the active and the most important form.

Vitamin D is essential in the coordination of the uptake and regulation of calcium and phosphate levels in the blood. Normal calcium and phosphate levels in the blood are important for the growth of the skeleton, muscle contractions, nerve conductance and bone mineralisation.

When there is less vitamin D in the body, less calcium is taken up from the food. This results in low calcium levels in the blood. To maintain a normal calcium level, the body tries to regain it from the bones and starts to demineralise them. Therefore, when vitamin D keeps lacking in the body, the body will keep on retrieving calcium from the bones, which, in the long term, can result in weak and soft bones. Severe deficiency of vitamin D in children can lead to a condition called “rickets”, which is permanent bone malformation of the legs, skull and can create bone pains. However, this is usually seen in undeveloped countries while in the resource-rich countries this is uncommon. 

As mentioned above, vitamin D has several other functions, like cell differentiation. Many studies show that there is a connection between vitamin D deficiency and many disorders such as cancers (bowel, breast, prostate) and autoimmune diseases (diabetes mellitus type 1, inflammatory bowel diseases, rheumatoid arthritis, multiple sclerosis) and mood disorders (depression). Some studies also suggest that vitamin D deficiency could lead to food allergies, asthma and hair loss. 

Luckily, vitamin D is a provitamin; this means that it can be produced in our body through ultraviolet B rays (UVB) in sunlight or from dietary intake. When an inactive form of vitamin D is retrieved from the skin or from food, it is metabolised in the liver and subsequently in the kidneys to the active form of vitamin D. Very often, vitamin D deficiency is not properly recognised. This is because many believe that vitamin D will be enough from exposure to sunlight or by dietary intake. Therefore, the vitamin D status may not be checked regularly in standard blood tests. 

Good sources of vitamin D are in oily fish such as herring, salmon and mackerel, however they are hardly consumed by people, especially children. Meat and eggs also supply vitamin D, but approximately 6-8 times less, so not eating these oily fish can easily lead to vitamin D deficiency. With regard to this, some countries have created a standard regulation of adding vitamin D to many basic foods like dairy products, bread and margarines. 

Risk factors for vitamin D deficiency include skin pigmentation and low sun exposure, which depends on the quantity of UVB that can be absorbed by direct contact on our skin. In dark-skinned individuals, less UVB is absorbed due to the natural sunblock effect by the dark skin pigmentation. So skin pigmentation has a huge impact on the vitamin D synthesis. 

If we make a comparison based on this, then Asians would need 3 times more sun exposure and darker-skinned people will need 6-10 times more sun exposure, in comparison with a light-skinned individual to achieve the same equal dose of vitamin D. This makes it more difficult in winter where there is already low sun exposure. 

Vitamin D deficiency can also occur in diseases with impaired fat malabsorption from the intestines such as bowel diseases and pancreatic disorders. 

The vitamin D status can be obtained from a blood test where the level of the active form of vitamin D needs to be checked. The referential values are as follows, but these can vary slightly from country to country. Vitamin D deficiency is when it is below 20 nanogram/mL (50 nanomole/L), vitamin D insufficiency is when it is between 20-30 nanogram/mL (50- 75 nanomole/L) and vitamin D becomes toxic when it is above 100 nanogram/mL (250 nanomole/L). 

Occasionally vitamin D is expressed in “IU”, which stands for “international unit”. 1 microgram of the active vitamin D is equivalent to 40 IU. There is a slight variety in the consensus of vitamin D supplementation in each country, but the main guidelines are comparable. Supplementation of vitamin D is recommended in children (0-6 years), high-risk populations such as individuals with increased skin pigmentation and elderly people.

Children, from birth to the age of 6, require daily supplementation of vitamin D of 400 IU daily for light-skinned children and 600-800 IU/daily for dark-skinned children. This can be provided by liquid vitamin D drops available in most countries. Supplementation of vitamin D for children is essential for their growth and health. However, young children should not be exposed directly to the sun to obtain vitamin D and supplementation of vitamin D as “multivitamin” tablets is not advised if the diet is healthy and variable. Children after this age can be provided with 600 IU daily and with a diet containing good sources of vitamin D. 

Supplementation and dosage of vitamin D in adolescents depends of the vitamin D status from a blood test, which one should consult their doctor about. Vitamin D can be given in tablets or in liquid form; the latter is preferred, as vitamin D is known as a natural fat-soluble vitamin. It is also advised that vitamin D be taken with food. 

Elderly population, approximately around the age of 70, require 800 IU of vitamin D daily to prevent weak bones and fractures. Usually, calcium supplementation is combined with vitamin D in elderly people. 

Please always discuss with your doctor what supplements you want to take or are taking so your wellbeing can be properly catered for.

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