Latest research shows that a good vitamin D intake may protect against breast cancer in postmenopausal women. Researchers came to this conclusion by assessing the results from three studies, involving 5,038 women aged 55 or older, who lived in 57 countries world-wide (although over 90% were from the United States or Canada).

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Vitamin D and breast cancer

In two of the studies, women were taking vitamin D3 and calcium supplements, or placebos, and had their blood vitamin D levels measured every year. In the third study, women sent in a home vitamin D blood spot test and recorded any supplements they were taking. All had at least two follow up-health assessments to record any cancer diagnoses.

Out of the 5,03 women, 77 were diagnosed with breast cancer over the relatively short follow-up period of around 4 years. Women with a vitamin D concentration of 60 ng/ml or greater were 82% less likely to develop breast cancer than those with a vitamin D concentration of less than 20 ng/ml.

After adjusting for other factors such as age, weight, smoking and calcium intakes, this reduced slightly so that women with vitamin D concentrations of at least 60 ng/ml had an 80% lower risk of breast cancer than women with concentrations of less than 20 ng/ml.

Overall, 99.3% of women with these higher blood levels of vitamin D remained breast cancer free compared with 96.8% of those with blood vitamin D levels of less than 20 ng/ml.

Between these ranges, there was a consistent reduction in breast cancer risk as vitamin D blood levels increased, suggesting a direct protective effect that increased with increase vitamin D intakes. There was no evidence of increased risk at higher concentrations.

Although the below slide provided by the publisher, PLOS ONE, looks complicated, the important bit is the nice descending black line showing how the risk of breast cancer consistently falls as blood vitamin D levels increase.

This confirms the results from 14 pervious studies, involving over 25,000 women, in which every 10 ng/mL increase in blood vitamin D levels was associated with a 3.2% reduction in breast cancer risk.

How might vitamin D protect against breast cancer?

Vitamin D is made in the skin on exposure to ultraviolet light, and was first suggested to protect against breast cancer back in 1990. Researchers found a link between the average annual sunlight and breast cancer mortality rates in 87 regions of the United States. In regions with the greatest amount of sunlight, women were 80% less likely to develop breast cancer than in those receiving the least sunlight.

Since then, vitamin D was found to prevent the development and growth of breast cancer cells by binding to a vitamin D receptor that is present in breast tissues. This triggers a variety of protective signals which have been shown to:

• regulate growth and division of cells
• tell cells to remain as normal breast cells (a process called differentiation)
• help cells literally stick together better (adhesion)
• protect cells from DNA damage
• regulate the production of immune factors (cytokines)
• activate immune cells
• Damp down inflammation.
• tell abnormal cells to self-destruct (apoptosis)

All these protective processes combine to reduce cancer formation and may even suppress the growth of established tumours.

Sources of vitamin D  

Dietary vitamin D3 (cholecalciferol) is obtained from eating:

• oily fish (sardine, herring, mackerel, salmon, tuna)
• liver
• eggs
• fortified milk, margarine and cereals
• butter.

Vitamin D2 (ergocalciferol) is a plant form that is mainly found in mushrooms that have been exposed to ultraviolet light (eg maitake, chanterelle, morel, oyster, shiitake, enoki, portobello and chestnut mushrooms).

Sun bathing without sunscreen, in order to get more vitamin D, is not advisable due to the risk of skin cancer, but short sun exposures without burning will help you make useful amounts of vitamin D.

Vitamin D deficiency is common

The latest UK National Diet and Nutrition Survey shows that among women aged 19 to 65 years, average dietary intakes of vitamin D from food was just 2.1mcg. Overall, 17% of adults have vitamin D levels that are low enough to cause deficiency diseases such as osteopaenia (soft bones) and osteoporosis (brittle bones). It may even be that vitamin D deficiency is associated with an increased risk of certain cancers (breast, prostate, bowel) although this is not yet certain.

As vitamin D has so many beneficial effects, Public Health England recommend that everyone takes a 10mcg vitamin D supplement during Autumn and Winter when UV levels are too low to trigger  vitamin D production in the skin. This dose is too small to achieve the most protective blood levels seen in the above study, however.

Optimal blood levels of vitamin D

Vitamin D (25-hydroxyvitamin D) blood level ranges vary from lab to lab, but in general blood levels are defined as follows:

• Deficiency: below 30 nmol/L (12 ng/ml)
• Insufficiency: 30 – 50 nmol/L (12 – 20 ng/ml)
• Adequate: 50 – 80 nmol/L (20 – 32 ng/ml)
• Optimum: > 80 nmol/L (32 ng/ml)
• Risk of toxicity: > 125 nmol/L (50 ng/ml)
• Toxicity: > 220nmol/L (88 ng/ml)

The dose of vitamin D you need to achieve a particular blood level is highly individual. One study in the United States, for examples, found that an intake of 100 mcg (4000 IU) was needed to maintain vitamin D levels in all older women.

Taking more than 100mcg vitamin D per day, which is the upper safe level from supplements, is not advised.

For general health, I usually recommend a vitamin D3 dose of 25mcg for adults under 50 years, and a dose of 50mcg for adults aged 50 and over. As vitamin D is fat soluble, take it with a meal, or in oil based capsules such as those that include olive oil.

If you have a health condition, especially cancer, or are taking any prescribed medicines, always check with your doctor before taking any supplements, including vitamin D. As vitamin D levels are low in between 20% and 60% of people when first diagnosed with cancer, vitamin D supplements may be advised to support cancer treatment but only take them with your doctor’s knowledge and permission.

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NB No claims are made for any particular supplement and any protection against cancer.

Image credits: pixabay; PLOS ONE;