3 January, 2019

We’ve all heard the message that we don’t get enough vitamin D, especially in the winter, when days are short and we are well wrapped up against the cold and any fleeting sunshine. Vitamin D is important for bone health, and for teeth and muscles, and that is why the NHS recommends a vitamin D supplement, especially in the autumn and winter. Now scientists are researching the possibility that vitamin D supplements could help prevent some cancers including breast cancer.

There is quite a lot of evidence showing that women with breast cancer often have low levels of vitamin D, although this doesn’t necessarily indicate a causal link. More interesting, perhaps, is the limited evidence which suggests that taking high strength vitamin D supplements could offer an element of protection against breast cancer and other cancers.

However, a word of caution: taking vitamin D in very high amounts is toxic, with serious health consequences. To protect the public from taking toxic amounts of vitamin D, the European Food Standards Agency has set the upper limit for vitamin D supplements at 100 micrograms per day (4000 IU), which is actually at least double the strength used in the studies which suggested a protective effect from the supplements.

In 2016 the UK Scientific Advisory Committee on Nutrition considered the evidence on vitamin D and many aspects of health, and published guidelines which were adopted by the NHS. The committee looked at vitamin D and cancer prevention (including breast cancer), and decided not to include cancer prevention and vitamin D in the guidelines until there was evidence from large scale randomised control clinical trials.

Randomised control clinical trials involve an intervention over a period of time (such as giving the participants vitamin D supplements), and comparing levels of breast cancer in these women to those in a group of women given a placebo, or given nothing. The evidence is usually strongest when these trials use a large number of participants over a long period of time.

Interestingly one such trial is now nearing completion. Over the past five years, a randomised control clinical trial has involved over 25 thousand male and female participants in their fifties and older. One of the questions the trial is examining is whether high-strength daily vitamin D supplements (50 micrograms or 2000 IU per day) can reduce the risk of cancer, including breast cancer. The trial phase will be completed at the end of December 2017 and scientist will next be analysing the data and reporting their findings. Results from this study could help clarify if supplements provide a level of protection against breast cancer in the over 50s.

As we wait for these research results, it is interesting to look at the strength of the evidence to date for a link between breast cancer and low levels of vitamin D, and the protective effect of taking vitamin D supplements.

Over the past ten years there have been many studies looking at vitamin D and cancer. The simplest form of study is called a retrospective cohort study. This kind of study looks at levels of vitamin D in the blood of women diagnosed with breast cancer. Most of these studies have found that women diagnosed with breast cancer also have low levels of vitamin D. However, these studies simply show that both conditions exist at the same time in these women, and so are rather weak evidence, especially as it is thought that up to 39% of the UK population have ’low vitamin D status’ in the winter months. The studies don’t provide evidence of cause and effect, or even strong evidence of an association, and they don’t compare the results with levels of vitamin D in women without breast cancer.

Studies which compare levels of vitamin D in women diagnosed with breast cancer with women who don’t have breast cancer are called case-control studies. The ‘controls’ are the women who don’t have breast cancer. These women are selected to be as similar as possible to the ‘cases’ in characteristics which are unrelated to breast cancer. The evidence from this kind of study is stronger than that from retrospective cohort studies, although the study design can vary, making it hard to evaluate them. Many, but not all, recent case control studies have found a significant relationship between low levels of vitamin D in the blood and breast cancer incidence, and some studies have found a particularly strong relationship between low levels of vitamin D and triple negative and invasive breast cancer types.

The third kind of study is called a randomised control clinical trial, such as the one described above. Results from previous studies have been contradictory. For example, a strong protective effect of vitamin D against cancer in post-menopausal women was shown in one study using a relatively high concentration supplement of vitamin D, while another, which used a lower concentration supplement on post-menopausal women found no effect. It is possible that the difference in vitamin D amount is behind the differing results of these trials.

In addition to the study types described above, there are studies aimed at understanding the way vitamin D works inside cells or in animals, to reduce cancer risk. In our bodies vitamin D is converted to the hormone calcitriol, and this hormone has been found to have potential anti-cancer effects, including anti-proliferative, anti-invasive and anti-metastasis effects (reducing the spread of cancer), inducing the cell death of some cancer cells, stimulating some cancer cells to change into more normal cell types, reducing inflammation, and inhibiting the development of tumour blood supply. Clearly calcitriol has some strong anti-cancer properties. However, these tests were carried out on cells or on animals in the lab, so results only provide an indication of what might happen in our bodies, and some tests used concentrations of vitamin D which would be toxic to humans.

Finally, review studies summarise recent research in a particular area, and reach conclusions. Many reviews come out in support of the idea that having low levels of vitamin D in the blood might increase the risk of developing cancer, and that taking supplements might provide some protection, although the research is not always clear. There is some suggestion that the effect of low levels of vitamin D is stronger in the obese, and that the risk of developing aggressive breast cancer sub-types in particular might be increased by low levels of vitamin D in the blood.

Based on evidence from research, it seems that taking vitamin D supplements at no more than recommended levels is unlikely to do us any harm, and might offer some protection against breast cancer. So, how much is it recommended we take?

A dose of up to 25-50 micrograms vitamin D per day (1000-2000 IU) was used in the recent clinical trials, but consult your pharmacist before buying as different brands of supplement may vary and a pharmacist can advise you on any possible side effects or things you need to be aware of when taking supplements. Also, you must check first with your GP before taking vitamin D supplements if you have a medical condition or are on medication, as there are some conditions which can be made worse by vitamin D. Finally, remember not to take too much, as daily doses of 250 micrograms or 10 000 IU vitamin D can lead to toxic side effects which are dangerous.

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