3 January, 2019

We’ve all heard we don’t get enough vitamin D, especially in the winter. We are well wrapped up against the cold and fleeting sunshine when days are short.

Vitamin D is important for bone health and teeth and muscles. The NHS recommends a vitamin D supplement, especially in the autumn and winter. Now scientists are researching how vitamin D supplements could help prevent some cancers, including breast cancer.

Evidence shows that breast cancer women often have low vitamin D levels. However, 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). This is at least double the strength used in the studies, which suggested a protective effect from the supplements.

The research

In 2016 the UK Scientific Advisory Committee on Nutrition considered the evidence on vitamin D and many aspects of health and published guidelines that the NHS adopted. The committee looked at vitamin D and cancer prevention (including breast cancer). They 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 some time, such as giving the participants vitamin D supplements. Then comparing levels of breast cancer in these women to those in a group of women given a placebo or nothing. The evidence is usually strongest when these trials use many participants over a long 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. The trial examines 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. Scientists will next be analysing the data and reporting their findings. Results from this study could help clarify if supplements protect 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.

What do we already know about vitamin D and cancer?

Over the past ten years, many studies have looked at vitamin D and cancer. The simplest form of study is called a retrospective cohort study. This study looks at vitamin D levels 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 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 cause and effect or strong evidence of an association. They don’t compare the results with vitamin D levels 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 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. Some studies have found a particularly strong relationship between low levels of vitamin D and triple-negative and invasive breast cancer types.

Could vitamin D help reduce my risk?

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. In contrast, 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 how vitamin D works inside cells or in animals to reduce cancer risk. In our bodies, vitamin D is converted to the hormone calcitriol. 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). Including 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. Calcitriol has some strong anti-cancer properties. However, these tests were carried out on cells or animals in the lab. The results only indicate what might happen in our bodies. Some tests used concentrations of vitamin D which would be toxic to humans.

The result

Finally, review studies summarise recent research in a particular area and reach conclusions. Many reviews support 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 vitamin D levels is stronger in the obese. The risk of developing aggressive breast cancer sub-types, in particular, might be increased by low vitamin D levels in the blood.

Based on evidence from research, taking vitamin D supplements at no more than recommended levels is unlikely to harm us. And might offer some protection against breast cancer.

How much is it recommended we take?

A dose of up to 25-50 micrograms of vitamin D per day (1000-2000 IU) was used in recent clinical trials. But consult your pharmacist before buying as different brands of supplement may vary. A pharmacist can advise you on any possible side effects or things you must 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, some conditions can be made worse by vitamin D. Finally, remember not to take too much. Daily doses of 250 micrograms or 10 000 IU of vitamin D can lead to dangerous toxic side effects.

For more visit our Vitamin D factsheet

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