A new systematic review of evidence, published in The Lancet Diabetes & Endocrinology, suggests that low levels of vitamin D levels are a consequence of ill health, rather than a cause of it, casting strong doubt on the value of vitamin D supplements. Daniel Lock and his PhD supervisor Nigel Belshaw are investigating the effects of vitamin D in colorectal tissue homeostasis. Substantial epidemiological data indicate the protective effect of vitamin D sufficiency, but the molecular mechanisms which under-pin the process at the epigenetic level are yet to be elucidated.

Here they answers some questions on vitamin D related to this new review

Should we still take vitamin D supplements?

That depends upon who you are: this paper does not suggest that taking vitamin D supplements cannot be useful in some cases for some purposes. Neither does it rule out a health advantage of increasing vitamin D levels in the blood for those who are deficient. Generally supplementation should not be necessary if you are vitamin D sufficient already – unfortunately the only way to establish this is by a blood test. It is true that many people are vitamin D deficient by the current definition. Vitamin D supplements have the ability to increase the concentration of vitamin D in the blood, and for some people who are very deficient in vitamin D, this can help improve the health of their bones. Generally a dose of up to 2000 units is unlikely to pose any risk of adverse health effects to most people in the population. Anyone considering supplementation should most certainly consult their doctor before doing so, as people with high levels of vitamin D using high dose supplements may be at risk of kidney stones and other undesirable side effects.

Is it still good for our bones?

Vitamin D is essential for bone health as it allows the body to absorb calcium. Vitamin D also controls bone growth and maintenance – this paper did not assess vitamin D’s role in bone health, which has already been established.

Do we need more vitamin D and where can we get it in the diet?

Probably, yes, particularly between October through February when there is less sunlight, but this is hard to achieve. Oily fish, dairy products, mushrooms and fortified cereals are the best dietary sources, but even a healthy balanced diet containing these will not provide everybody with sufficient vitamin D in the short term through the winter months.

Why would vitamin D levels decrease as we age or when we have chronic illnesses?

As you age your skin becomes less efficient at making vitamin D in the presence of sunlight, often this is compounded by poor mobility (getting out less), decreased food intake and reduced kidney function (this is where vitamin D is activated). Any disease which affects your liver or your kidneys (like diabetes or cancer) will impair your ability to convert vitamin D to its active form.

Does this review mean Vitamin D doesn’t play any role in disease prevention?

This paper is very useful because it highlights the need for more long term intervention studies specifically looking at the effect of proper vitamin D supplementation on disease risk. When we look at vitamin D levels in the population we find that low vitamin D status is associated with an increased risk of disease, but currently we can’t tell whether low vitamin D levels themselves promote disease, or if conversely, the diseased state indirectly lowers vitamin D levels in the blood. If the former were true, we should be able to reduce disease risk directly by supplementation, but this review highlights that the evidence supporting low vitamin D as a causal factor for a wide variety of diseases at present is insufficient to draw conclusions about vitamin Ds role in actually preventing disease.

However the paper doesn’t undermine other benefits of maintaining a healthy vitamin D status, which may be achievable through supplementation. Regardless of whether low vitamin D is a cause or an effect of disease, increasing your vitamin D level may still confer additional advantages: vitamin D is still essential for bone health, orchestrating absorption of calcium in the gut and maintaining bone density into later life (which reduces the risk of osteoporosis). It’s also true that unwell patients with higher vitamin D levels over-all fare better than those who are deficient.

The authors themselves observe that including people with normal vitamin D levels could have masked any preventative effect of supplementation, equally vitamin D may act over many years to maintain health – the longest study in this review was 21 years – long, but not a lifetime.