Vitamin D3: Function in the body, deficiency and health outcomes
The recommended form of vitamin D for the body is vitamin D3 (cholecalciferol). It is a fat soluble vitamin meaning that it can pass through cell membranes quite easily, and to optimise bioavailability should be consumed with some type of fat or oil. However most of our bodily vitamin D3 is synthesised in the skin when the skin is exposed to UVB light (sunshine!) In the UK vitamin D3 is not synthesised in the skin between October and March, mainly because light at the correct wavelength does not reach earth during the winter months. It is therefore recommended by SACN (The Scientific Advisory Committee on Nutrition) that a reference nutrient intake of 10 micrograms of vitamin D3 are consumed per day throughout the year by everyone in the general population aged 4 years and older.
SACN advises that safe intake of vitamin D3 can be obtained from all dietary sources such as natural food sources, fortified foods and supplements. However, care must be taken not to overdose on vitamin D3 supplementation. According to EFSA (European Food Safety Authority) a tolerable upper intake level is 100 micrograms per day (all age groups). Excessive sun exposure does not cause toxicity by overproduction of vitamin D3.
Why is vitamin D3 important?
Vitamin D3 largely facilitates the metabolism of calcium. Vitamin D3 either synthesised in the skin or taken orally via food or supplementation is processed in the liver and released into the bloodstream as calcidiol. Circulating calcidiol in the blood is a measurement of vitamin D status. Calcidiol is activated in the kidney to form calcitriol. Calcitriol then has various target tissues all associated with the management of calcium in the body. In the intestine calcitriol primary function is to increase the absorption of calcium and phosphorus. In the kidney calcitriol enhances reabsorption of calcium from urine via various pathways. In bone, calcitriol (along with parathyroid hormone) mobilises calcium and phosphorus to normalise blood calcium concentration via release or uptake of calcium. Bone is the major storage site for calcium and the level of calcium in the blood largely depends upon the movement of calcium into or out of bone.
Causes of vitamin D3 deficiency
Vitamin D3 deficiency is becoming more common in developed countries, and in the UK in particular the prevalence of vitamin D3 deficiency in adults is around 40% in the winter months and can be as high as 81% in otherwise healthy Southern Asian females. It could be argued that deficiency in adults is due to various factors, which could also be relevant for children;
- Extensive covering with clothing
- Not spending enough time outdoors
- Use of high factor sunscreens. Effective skin cancer prevention advertising has seen an increase in use of sunscreen and a reduction in the availability of low factor sunscreen. Whilst it’s excellent that the general public are receptive to skin cancer prevention, the consequence in some cases is a reduction in vitamin D3 synthesis from sunlight, potentially leading to deficiency
- Inadequate diet. Best sources of dietary vitamin D3 include; meat and meat products (30%), fat spreads (19%), fish and fish products (17%), cereals (13%), egg and egg dishes (13%), milk and milk products (5%). However, unless fortified, foodstuffs are a poor source of vitamin D3, and It could be argued that as most dietary sources are from meat, fish and dairy, vegans need to pay particular attention to their diet to ensure adequate intake of vitamin D3. Supplementation is probably the best method to ensure deficiency does not occur.
- Obesity. Fat cells tend to ‘hold on’ to vitamin D3, so obese people may have enough intake but due to excess weight the vitamin D3 is not bioavailable. There is also an argument that obese people may not undertake outdoor activities, so not exposing their skin to sunlight leading to lack of vitamin D3 synthesis and hence a deficiency
- In the over 65’s all the above and age related reduced gut absorption can lead to deficiency.
The most common diseases caused by a vitamin D3 deficiency are:
1. Rickets. This is where bones are under mineralised as a result of poor calcium absorption in the absence of vitamin D3. This disease is evident in the legs of children who are vitamin D3 deficient. ‘Bow-legs’ or ‘knock knees’ can be observed. Deformities of the chest, pelvis and skull, fractures in severe cases and delayed dentition and poor growth are also risks.
2. Osteomalacia. The adult equivalent of rickets that results from demineralisation of bone rather than failure to mineralise it in the first place. Bones lose minerals and become porous, soft and flexible, which causes progressive weakness, pain in the pelvis, lower back and legs. Women who have little exposure to sunlight or not taking supplements are especially as risk after several pregnancies due to strain on their marginal reserve of calcium.
3. Osteoporosis. Where bones become brittle and porous.
Vitamin D3 receptors are also widely dispersed in the body (e.g. gastrointestinal, endocrine, renal, cardiovascular, immune, hepatic, etc) indicating that vitamin D3 is required for many bodily functions. Recent evidence points to a link between vitamin D3 deficiency and increased risk of cardiovascular disease.
It is becoming more evident that vitamin D3 plays a major role in health outcomes, at a time when many people are becoming more deficient. As such supplementation of 10 micrograms per day is recommended for everybody by SACN to reduce the risk of deficiency.