Vitamin D plays an essential role in many aspects of human health .This article outlines current understanding which is still evolving .
Everyone seems to be talking about vitamin D. Almost each week a study is published suggesting vitamin D deficiency causes disease. Despite this wealth of research, very few solid conclusions have been made about vitamin D deficiency and its role in disease development and progression.
It’s not surprising that so many links are made between low vitamin D levels and disease. Vitamin D deficiency is relatively common, particularly in the elderly population and so too are the illnesses thought to be linked to the deficiency –heart problems, cancer and cognitive decline to name a few.
The great majority of studies on vitamin D are observational. These studies commonly measure vitamin D levels in a certain group of people, measure disease levels in the same population and assume that the two are linked. However, observational studies are notoriously bad at determining causation.
Despite this murky area of research, we can be certain that vitamin D plays a vital role in the body and is worth knowing about.
So, what is it? How do we get it? And should we be getting more of it?
What is vitamin D?
Vitamin D is not actually (strictly speaking) a vitamin but rather a hormone involved in the regulation of blood levels of calcium and phosphorous.It is present in many different forms within the body – including vitamin D3, vitamin D2, cholecalciferol and calcidiol.
Vitamin D3 is produced in the skin through a reaction between UV light (from the sun) and an inactive version of vitamin D. It is then processed in the liver to form calcidiol, also known as 25-OH vitamin D. This is the level of vitamin D that blood tests measure.
Vitamin D is then further metabolised by the kidney into its biologically active form - calicitriol. People with kidney failure are at a higher risk of developing vitamin D as a result.
Vitamin D is also found in the diet, however not in sufficient quantities to allow us to rely solely on food intake to meet daily requirements. It is most commonly found in milk products, certain fish species and eggs.
It is important to maintain high levels of calcium in the diet alongside vitamin D intake, because the two nutrients are very tightly intertwined and won’t work as well without the other.
The main roles of vitamin D
Vitamin D’s main role is to regulate the levels of calcium and phosphorous in the blood.
Calcium and phosphorus are vital elements in the body; most importantly responsible for maintaining strong bones but also participate in a number of other biological reactions.
Vitamin D is also produced by cells of the immune system and is involved in stimulating the immune system to fight against bacteria and viruses.
Vitamin D deficiency
Rates of vitamin D deficiency are thought to be around 5% in adults in New Zealand, with a further 25% having lower blood vitamin D levels than the recommended amount.
People who are at a particularly high risk of vitamin D deficiency include frail, elderly people who live in institutional care and may spend less time outside in the sun, as well as people with dark skin. This is due to a reduced production of vitamin D in pigmented skin.
In its most severe form, vitamin D deficiency causes a very low bone density, commonly known as rickets (in children) or osteomalacia in adults.
More subtle forms of vitamin D deficiency have been hypothesised to result in an increased risk of cancer, heart disease and cognitive decline. However, the evidence to suggest this is still in its preliminary phases and the exact mechanism behind why this may be is unclear.
There is strong evidence to suggest that genetically determined low vitamin D levels increase the risk of multiple sclerosis.
Vitamin D supplementation
Bone health and fracture risk
There is a strong link between vitamin D levels and bone health due to the effects of vitamin D on calcium, which is a vital component of bone.
However, the role of vitamin D supplementation alone on bone strength in those with low to normal vitamin D levels, particularly the elderly, has not seen convincing results.
There is evidence to suggest that vitamin D supplementation in combination with calcium supplementation may lower fracture risk slightly, particularly in women, however this effect was seen at relatively high doses of vitamin D and calcium which may bring with it negative side effects.
A Cochrane review in 2014 (a type of study that analysis the highest quality data available on a particular topic) concluded that vitamin D supplementation in the general population was found to have a moderate benefit on reducing overall mortality and mortality from cancer, and this was particularly pronounced in the elderly population, both those living independently and in institutional care.
However an overall assessment of the quality of the studies included in the review suggests that this evidence is still not strong enough to convincingly recommend or refute routine vitamin D supplementation in those without a significant deficiency.
Researchers have been hopeful that vitamin D may play a beneficial role in preventing the progression of chronic diseases such as cancer, heart disease, autoimmune disease and cognitive decline. However the current research to support this is lacking.
Side effects of Vitamin D supplementation
Vitamin D supplementation can induce hypercalcaemia (increased calcium levels in the blood) which may in turn lead to an increase in the frequency of kidney stones. It is also associated with diarrhoea and gastrointestinal upset.
So what should I do?
The research is vast but relatively inconclusive on the role of vitamin D supplementation and disease prevention in the general population.
Studies do suggest that those at a high risk of deficiency, particularly elderly, frail women living in institutionalised care may benefit from supplementation due to a slightly decreased fracture risk and overall decrease in mortality. A risk of deficiency should be suspected in those who are unable to spend regular time outside in the sun, particularly people with darker skin.
Testing for vitamin D levels in the general population is not recommended because of the lack of evidence supporting supplementation benefit and the debate that still exists surrounding what normal vitamin D levels are.
It is likely that one of the main reasons vitamin D deficiency is linked to so many chronic conditions is that it is a marker of frailty. These older, frailer people who have low vitamin D levels are far more likely to have disease in the first place, and it is often for reasons other than their low vitamin D levels that they develop disease.
Further research is required before we can fully understand and make use of the complex role of vitamin D in disease.