How Vitamin D Protects Against Cancer

Find out the truth about vitamin D and how much you need, depending on where you live, to cut your cancer risk.

Halve your risk of colon cancer The vitamin D and cancer connection, first proposed by Dr Cedrick Garland in 1980, led to research that showed a strong association between risk of colon cancer and dietary intake of vitamin D and calcium. [2] An eight year study of 25,802 people from the state of Maryland in the USA found that those with blood levels of vitamin D equivalent to 10mcg (400iu) or more had half the risk of colon cancer compared to those with lower levels. [3] This is twice the US RDA of 5mcg (200iu).

In the UK, there is no recommended daily dietary intake for vitamin D if you are aged between four and 50 and live a ‘normal life-style’. However, ‘normal’ means spending time every day outdoors in the sunshine, which is not always possible given the UK weather and people’s lifestyles. For those confined indoors, experts recommend 10mcg per day. [4] Since 1980, many researchers have confirmed Dr Garland’s hypothesis. A scientific review undertaken by the National Cancer Institute in 2007 found that vitamin D was beneficial in preventing colorectal cancer. Although the study found no link between vitamin D status and overall cancer mortality, the study did show that blood levels of 80 nanomoles per litre (nmol/L) or higher were associated with a 72% reduction in colorectal cancer mortality. [5] As you’ll see later, to achieve blood levels of 80nmol/L, you need to have just over 25mcg (1,000iu) a day.

Similar blood levels and vitamin D intake were found to protect against colon cancer in a study that followed 1,500 people for 25 years. At 80nmol/L the rate was cut by 50%, whereas levels of over 100nmol/L reduced colon cancer incidence by 66%, according to research published in the American Journal of Preventative Medicine. When five studies were ‘pooled’ together, researchers found that a 50% lower risk of colorectal cancer was associated with a blood level of greater than, or equal to, 83nmol/L, compared to less than or equal to 30nmol/L. They concluded that 1,000-2,000iu per day of vitamin D could reduce the incidence of colorectal cancer with minimal risk. [6]

Reduce your risk of breast and ovarian cancer by a third Several teams of researchers have found that adequate levels of vitamin D also lower the chances of developing breast cancer. Low blood levels of vitamin D have been correlated with breast cancer disease progression and the spread of cancer to the bones. [7] Women with advanced breast cancer that had spread to their bones were less likely to die of the disease when they had high amounts of active vitamin D in their blood. [8] A team of cancer prevention specialists at the University of California, San Diego, found that women with the highest level of vitamin D in their blood had a 50% lower risk of breast cancer than those with the lowest level. [9] “The results were very clear,” said co-author Dr Cedric Garland. “The higher your level, the lower the risk.” To have a blood level that would cut your risk by 50%, the researchers said that you would have to take 50mcg (2,000iu) daily and also spend 10-15 minutes in the sun.

Researchers from the same university also reviewed 63 studies, published worldwide between 1966 and 2004, and found that taking 25mcg (1,000iu) of the vitamin daily could lower an individual’s cancer risk by 50% in colon cancer, and by 30% in breast and ovarian cancer. [10a] [10b] In fact, Dr Garland has estimated that 600,000 cases a year of breast and colorectal cancer could be prevented by adequate intakes of vitamin D. [11] Protection from prostate cancer Vitamin D also helps protect men from prostate cancer. A team led by a researcher from Harvard Medical school followed 15,000 men and found that those who had below average levels of the vitamin in their blood had a “significantly increased risk of aggressive prostate cancer”.

In this study, published in PLOS Medicine, 50nmol/L counted as deficient and even 80nmol/L was described as ‘sub-optimal’. Taking the US Recommended Daily Allowance (RDA) of vitamin D (10mcg, or 400iu/day) was found to reduce the risk of pancreatic cancer by 43% in a sample of more than 120,000 people from two long-term health surveys. [12] The Harvard researchers said further work was necessary to determine if consuming vitamin D in the diet, or through sun exposure, might have even more of an effect than taking supplements. How does vitamin D work? The anti-cancer activity of vitamin D is thought to result from its role in a wide range of mechanisms central to the development of cancer, such as regulating cell growth, and apoptosis (programmed cell death). [13]

Programmed cell death is a normal process that should happen in our bodies if a cell is damaged in any way. This is one of the problems with cancer – the damaged cell continues to replicate. One clinical study of 92 colon cancer patients showed that supplementing the diet with calcium and vitamin D appeared to increase the levels of a protein (called BAX) that controls this process in the colon and might therefore be pushing the pre-cancerous cells into programmed cell death. [14]

Another possible explanation for the cancer-protective effect of vitamin D is that it’s needed to enable calcium to be used properly in the body, and that calcium is important for proper immune function. Vitamin D is converted into the hormone, which works with another hormone, parathormone – produced in the parathyroid gland – to control calcium balance in the body. Calcitriol also plays an important role in the immune system because it is able to suppress pro-inflammatory chemicals that are often implicated in the cancer process. It has been found to kill cancer cells in laboratory and animal studies, and high levels of calcitriol have been shown to be beneficial in patients with advanced prostate cancer. [15]

How much is enough? Just how much vitamin D is required for optimum health varies depending on where you live in the world (your latitude), the colour of your skin, the time of year, your use of sun block and your level of sun exposure. The amount you need from diet depends on the amount you produce in your skin. Although you make the most vitamin D in the summer, the relative conversion of sunlight, acting on cholesterol to convert it into vitamin D, is apparently at its highest in the autumn; perhaps this is as an evolutionary adaptation so that the body can store it for the winter months ahead.

Dr Reinhold Vieth, professor at the Department of Nutritional Sciences at the University of Toronto and one of the world’s top vitamin D experts, estimates that ‘humans in a state of nature’ probably had about 125 to 150nmol/L of vitamin D in their blood all year long. In our modern world, these levels are only likely to be achieved for a few months a year by the minority of adults: those who spend a lot of time in the sun, such as lifeguards or farmers. In the remainder of the population, vitamin D levels tend to be lower and crash in winter. In testing office workers in Toronto in winter, Professor Vieth found that the average was only about 40nmol/L, or about one-quarter to one-third of the amount humans would have ‘in the wild’.

Varying recommendations Text books will tell you that you should be getting between 5 and 10mcg (200–400iu) a day. Some of this we can get from food, notably fatty fish and cod liver oil, and the rest from sunlight, by exposing the face and hands for 20 minutes a few times a week (which should be enough to prevent bone softening that manifests itself as rickets). If exposure to sunlight is limited, Dr Michael Holick, a top expert on vitamin D, maintains that a minimum of 25mcg (1,000iu) per day from food and/or supplements is required to maintain a healthy concentration in the blood. [16] Most experts now agree that this is the level needed daily to maintain a healthy blood level of 25-hydroxyvitamin D – this is the form of vitamin D measured in the blood, also known as 25(OH)D – of between 75 and 125 nmol/L. [17]

Why recommended levels need to rise
In March 2007, Robert Heaney, professor of medicine at Creighton University in Nebraska, called for improved levels of vitamin D in the general population on the grounds that this would reduce the risk of bone fractures caused by osteoporosis, as well as protecting against ‘various cancers and autoimmune disorders’. [18] The average UK diet will provide about 3.75mcg (150iu) of vitamin D per day. The low vitamin D intake in several European countries is due to the fact that only a few foods are naturally good sources of vitamin D. [19] Fatty fish such as herring, mackerel, pilchards, sardines and fresh tuna, as well as eggs, are rich sources. There’s a little in milk, meat and fortified foods. If you eat these foods regularly and expose yourself to natural daylight for half an hour a day, you will achieve the equivalent of about 15mcg (600iu). The ideal intake is at least 30mcg so, to achieve this, you would need to supplement an additional 15mcg of vitamin D each day. A good multivitamin may provide this, but most multis don’t. However, if you have cancer and don’t get enough sun exposure, for example in winter, I would recommend supplementing 30mcg a day – twice this amount.

Who is at risk of deficiency?
Vitamin D deficiency among the elderly is far from uncommon. According to a survey published in the New England Journal of Medicine, 57% of 290 senior citizens in hospital had low blood levels of this vitamin. [20] But it’s not just the elderly who are at risk. At least 60% of the adult population in the UK do not get enough vitamin D according to the National Diet and Nutrition Survey for 1995–2004. The survey also showed that 12% were actually deficient. Vitamin D inadequacy is common among post-menopausal women, [21] vegetarians, those eating low-fat diets, and those not getting enough sunlight.

Others at risk include people living in urban northerly regions, and immigrants who rarely expose their skin to sunlight. So, for example, an Indian who has dark skin, and remains covered up much of the time, who is also vegan (and therefore does not eat meat, fish, eggs or dairy products) is at risk of vitamin D deficiency. This causes rickets (in children) and osteomalacia (in adults) – diseases in which the bones become malleable. People with dark skin – no doubt due to their high melanin content – are most susceptible to rickets especially when living in countries with little sunlight. As discussed earlier, experts believe that current recommended levels of vitamin D are too low and that there is an urgent need to recommend a more effective intake. [22]

According to Dr Reinhold Vieth, one of the world’s leading authorities on vitamin D: “Current dietary guidelines for vitamin D in the UK are incorrect in stating that adults below age 50 require no vitamin D and specify too little for older people. Sun avoidance advice makes the vitamin D problem even worse in the UK. The result is an unacceptably high occurrence of what should be regarded as toxic vitamin D deficiency.” The importance of latitude A study using data on over 4 million cancer patients from 13 countries showed a marked difference in risk of a number of cancers between countries classified as ‘sunny’ and those classified as ‘less sunny’. [23]

Research also suggests that cancer patients who have surgery or treatment in the summer have a better chance of surviving their cancer than those who undergo treatment in the winter, when they are exposed to less sunlight. [24] To make vitamin D, you need sunlight that contains UVB rays, but these can be blocked by cloud. Fewer and fewer UVB rays reach the ground the further north you go. (It’s the UVA rays that age you the most.) Liverpool in the UK is approximately on the same latitude as Edmonton in Canada (53 degrees north) where it has been found that people’s bodies make little vitamin D for up to six months of the year. Furthermore, according to a study by Dr Hypponnen, Scottish people were twice as likely to be vitamin D deficient as those south of the border. Even in Boston in the US – 42 degrees north – you will be unable to make vitamin D from sunlight for four months of the year, from November to February.

In the UK, even in summer, you only really get sufficient UVB rays between 10.00am and 2.00pm; exposing your hands, face and arms during those hours gives you about 5-10mcg (200-400iu), which isn’t nearly enough for the new recommended levels. To get those, the American experts are suggesting you need to expose 50-80% of your skin to the sun’s rays for about 20 minutes. Modern life keeps us indoors away from the sun, which supplies 90% of the vitamin D we need.

A billion or more people in Europe obtain insufficient sunlight and vitamin D, putting them at increased risk of many of the common cancers as well as other conditions, including diabetes, arthritis and multiple sclerosis. In fact, some experts believe that the epidemic of chronic disease caused by a lack of vitamin D is probably as large as the epidemics caused by smoking and obesity, but the importance of vitamin D deficiency is still not properly recognised by governments. The problem with sunscreens

In 2005, a conference was held at the House of Commons to address the issue of ‘Sunlight, vitamin D and health’ and the issue of sun safety. It was pointed out that those regularly using sunscreen at factor 8 or above, or avoiding sunshine, were putting themselves at risk of vitamin D deficiency. This includes women who regularly use foundation cosmetics containing sunscreen. Leading vitamin D campaigner Dr Oliver Gillie, editor of the House of Commons conference report, is highly critical of the SunSmart advice put out by Cancer Research UK, which encourages people to stay out of the sun and use sunscreens. “That program has probably caused many more deaths from cancer than it has prevented,” claims Gillie. “It may also be partly responsible for apparent increases in chronic diseases such as multiple sclerosis and diabetes.” [25] Dr Gillie’s advice is to sunbathe safely (without burning) at every opportunity, without using sunscreens. ‘Safely’ means starting slowly (2-3 minutes each side) and gradually increasing from day to day. Only brief full-body exposure to bright summer sunshine – of 10 or 15 minutes a day – is needed to make high amounts of the vitamin.

How do I know if I’ve got enough?
The vitamin D that you make from sunlight, or get from either a supplement or your diet, is stored in your blood in a form known as 25(OH)D. A level of 27.5nmol/L of 25(OH)D is what you would make with around 10mcg (400iu) vitamin D a day. “That’s enough to prevent rickets but not enough for your health,” says Dr Heaney. In fact in the latest studies, it’s a level that counts as deficient; 50nmol/L is considered to be just about acceptable. In a 2007 study, published in the American Journal of Clinical Nutrition, the optimum level was 80nmol/L and over. [26] This is the level that most other studies find of benefit, but to get that you need about 40mcg (1600iu), although this also depends on your age. An average 70-year-old, according to Professor Meir Stampfer, an epidemiologist at Harvard Medical School, can make only about a quarter of the vitamin D from the sun compared to a 20-year-old.

A study of nearly 7,500 middle-aged men and women published in 2007 came to the startling conclusion that most people in Britain have insufficient vitamin D in their blood for at least six months of the year. [27] “There is a now a strong case for fortifying foods with vitamin D,” says the author of the study, Dr Elia Hypponnen, of the Institute for Child Health in London. “You can only make vitamin D from sunlight for about half the year in the UK, so by around Easter 90% of the population are seriously depleted in the amount they have.” She also believes that vitamin D supplements of between 40 and 50mcg (1600 and 2000iu) – four to five times the current recommended daily allowance (RDA) – should be more easily available over the counter. Heaney calculates that to increase most of the population to a reasonably healthy level of vitamin D, would require a daily supplement for everyone that was ten times the current RDA of 5mcg (200iu) – that’s 500mcg (2000iu) – on top of whatever they were already receiving from diet and sunlight.

This brings me to a very important question: could you be harmed if you take too much? Side effects of too much vitamin D More than 50mcg (2000iu) a day, according to some UK sources, can lead to the body absorbing too much calcium (a condition known as hypercalcemia), possibly damaging the liver and kidneys. Other side effects of overdosing are said to include increased thirst, nausea, vomiting and the deposit of calcium in blood-vessel walls. But even among academics there is little agreement on this. The Food and Nutrition Board of the American Institute of Medicine has set the guaranteed safe level per day at 50mcg (2,000iu), whereas many of the North American experts believe that much more is safe. Dr Reinhold Veith, at the Department of Laboratory Medicine and Pathobiology at the University of Toronto, believes that 1,000mcg (40,000iu) could be a toxic dose, but only if taken over a long time. Campaigners regularly point out that a fair-skinned person with about 80% of their skin exposed to the sun for 20 minutes at mid-day can produce about 250mcg (10,000iu) with no harm at all.

Further support for higher levels being both necessary and safe comes from a major review of Vitamin D published in July 2007 in the prestigious New England Journal of Medicine [28], which concludes that: “As long as [someone’s] combined total is 30ng per millilitre, the patient has sufficient Vitamin D.” Now, that’s an amount that translates to about 65mcg (2,600iu) a day. And as for the dangers, the review comments that: “Doses of 250mcg (10,000iu) per day for up to five months do not cause toxicity.” In the UK, however, these figures have not filtered through to the Food Standards Agency (FSA). “Most people can get all the vitamin D they require from a healthy balanced diet and exposure to sunlight,” said a spokesperson. As for supplements, the Agency recommends no more than 25mcg (1,000iu) because “intakes above this amount could be harmful”.

Getting the correct levels for you
To summarise, the evidence clearly shows that we should all be making sure that our vitamin D levels are adequate. In fact, the benefits of vitamin D are now so widely recognised that the pharmaceutical industry is trying to develop a patentable drug version of vitamin D that can be given in high doses without the side effects, namely excess calcium accumulation, which occurs with very high-dose vitamin D. At least that’s their story. Personally, I would recommend sticking with the natural vitamin D molecule, and not taking more than 50mcg a day.

In the pursuit of optimum nutrition, I recommend eating oily fish three times a week (a serving of salmon or mackerel provides around 8.75mcg or 350iu); eating six free-range eggs a week (an egg provides 0.5mcg, or 20iu); exposing your skin to the sun every day (you’ll make around 10mcg or 400iu) with 20 minutes exposure between 10.00am and 2.00 pm); and supplementing at least 15mcg (600iu) daily. That will give you around 30mcg (1200iu) a day in total. If you are mainly vegetarian and don’t eat vitamin D-enriched foods, eggs or dairy produce, and do not get substantial exposure to sunlight, I would recommend supplementing your diet with 15-30mcg (600–1200iu) of vitamin D. This level is found in some multivitamins and also in some calcium and magnesium supplements. During the winter months (November to April in the northern hemisphere), it’s probably worth supplementing an additional 25mcg (1,000iu) and possibly more, especially if you are older and live in the far north. If you live in a hot country, however, and are exposed to substantial amounts of sunlight, be careful that you are not getting too much dietary vitamin D, because an excess has a negative effect on calcium balance, as I have explained.


1. J.M. Lappe, et al., ‘Vitamin D and calcium supplementation reduces cancer risk: results of a randomized trial’, American Journal of Clinical Nutrition, 2007 June;85(6):1586–91

2. C. Garland, et al., ‘Dietary vitamin D and calcium and risk of colorectal cancer: A 19–year prospective study in men’, Lancet, 1985 Feb. 9;1(8424):307–9

3. M. Elias, ‘Vitamin D may help beat cancer’, USA Today (26/1/89)

4. Department of Health, ‘Dietary Reference Values for Food Energy and Nutrients for the United Kingdom’, London: TSO, (2003)

5. D.M. Freedman, et al., ‘Prospective study of serum vitamin D and cancer mortality in the United States’, Journal of the National Cancer Institute, 2007 Nov. 7;99(21):1594–602

6. E.D. Gorham, et al., ‘Optimal vitamin D status for colorectal cancer prevention: A quantitative meta analysis’, American Journal of Preventive Medicine, 2007 Mar.;32(3):210–6

7. N. Buyru, et al., ‘Vitamin D receptor gene polymorphisms in breast cancer’,Experimental and Molecular Medicine, 2003 Dec. 31;35(6):550–5

8. M.P. Saunders, et al., ‘A novel cyclic adenosine monophosphate analog induces hypercalcemia via production of 1,25-dihydroxyvitamin D in patients with solid tumors’, Journal of Clinical Endocrinology and Metabolism, 1997 Dec.;82(12):4044–8

9. C.F. Garland, et al., ‘Vitamin D and prevention of breast cancer: pooled analysis’, Journal of Steroid Biochemistry and Molecular Biology, 2007 Mar.;103(3–5):708–11

10a. C.F. Garland, et al., ‘The role of vitamin D in cancer prevention’, American Journal of Public Health, 2006 Feb.;96(2):252–61.

10b. E.D. Gorham, et al., ‘Optimal vitamin D status for colorectal cancer prevention: a quantitative meta analysis’, American Journal of Preventive Medicine, 2007 Mar.;32(3):210–6

11. C.F. Garland, et al., ‘What is the dose-response relationship between vitamin D and cancer risk?’, Nutrition Reviews, 2007 Aug.;65(8 Pt 2):S91–S95

12. H.G. Skinner, et al., ‘Vitamin D intake and the risk for pancreatic cancer in two cohort studies’, Cancer Epidemiology, Biomarkers and Prevention, 2006 Sept.;15(9):1688–95

13. B.A.Ingraham,etal.,‘Molecular basis of the potential of vitamin D to prevent cancer’, Current Medical Research and Opinion, 2008 Jan.;24(1):139–49

14. V. Fedirko, et al., ‘Effects of vitamin D and calcium supplementation on markers of apoptosis in normal colon mucosa: A randomized, double-blind, placebo-controlled clinical trial’, Cancer Prevention Research (Phila Pa), 2009 Mar.;2(3):213–23

15. T.M. Beer and A. Myrthue, ‘Calcitriol in the treatment of prostate cancer’, Anticancer Research, 2006 July;26(4A):2647–51

16. M.F. Holick, ‘Vitamin D: Importance in the prevention of cancers, type 1 diabetes, heart disease, and osteoporosis’, American Journal of Clinical Nutrition, 2004 Mar.;79(3):362–71

17. G.E. Mullin and A. Dobs, ‘Vitamin D and its role in cancer and immunity: A prescription for sunlight’, Nutrition in Clinical Practice, 2007 June; 22(3):305–22

18. R.P. Heaney, ‘The case for improving vitamin D status’, Journal of Steroid Biochemistry and Molecular Biology, 2007 Mar.;103(3–5):635–41

19. A. Zittermann, ‘Vitamin D in preventive medicine: Are we ignoring the evidence?’, British Journal of Nutrition, 2003 May;89(5):552–72

20. M.K. Thomas, et al., ‘Hypovitaminosis D in medical inpatients’, New England Journal of Medicine, 1998 Mar. 19;338(12):777–83

21. S. Gaugris, et al., ‘Vitamin D inadequacy among post-menopausal women: A systematic review’, QJM., 2005 Sept.;98(9):667–76

22. R. Vieth, et al., ‘The urgent need to recommend an intake of vitamin D that is effective’, American Journal of Clinical Nutrition, 2007 Mar.;85(3):649–50

23. P. Tuohimaa, et al., ‘Does solar exposure, as indicated by the non-melanoma skin cancers, protect from solid cancers: vitamin D as a possible explanation’, European Journal of Cancer, 2007 July;43(11):1701–12

24. W. Zhou, et al., ‘Vitamin D is associated with improved survival in early-stage non-small cell lung cancer patients’, Cancer Epidemiology, Biomarkers and Prevention, 2005 Oct.;14(10):2303–9

25. O. Gillie (Editor), ‘Sunlight, Vitamin D & Health: A report of a conference held at the House of Commons in November 2005’, Health Research Forum Occasional Reports: No. 2

26. E. Hypponen and C. Power, ‘Hypovitaminosis D in British adults at age 45 y: Nationwide cohort study of dietary and lifestyle predictors’, American Journal of Clinical Nutrition, 2007 Mar.;85(3):860–8

27. E. Hypponen and C. Power, ‘Hypovitaminosis D in British adults at age 45 y: Nationwide cohort study of dietary and lifestyle predictors’, American Journal of Clinical Nutrition, 2007 Mar.;85(3):860–8

28. M.F. Holick, ‘Vitamin D deficiency’, New England Journal of Medicine, 2007 July 19;357(3):266–81