Most studies have found low levels of vitamin D are linked to an increased breast cancer risk.
The body produces adequate levels of vitamin D if the skin is exposed to sufficient sunlight. Other sources include diet or supplements.
Studies have not shown that taking vitamin D supplements reduces breast cancer risk. Although taking them can restore serum vitamin D levels to a healthy state and are recommended for people unable to achieve sufficiency through sunshine and diet.
The main function of vitamin D is maintaining calcium levels.
Vitamin D may help prevent breast cancer by reducing breast cell growth and the production of oestrogen in the body.
What is vitamin D and where does vitamin D come from?
Vitamin D refers to a group of fat-soluble compounds made in skin cells in humans and some animals. It is naturally present in certain foods, added to others (see Table 1) and may be taken as a dietary supplement.
Vitamin D production in the skin is the primary natural source of vitamin D. Synthesis is triggered when the sun’s ultraviolet (UV) rays interact with the skin.
As vitamin D is fat-soluble it is better absorbed if eaten with fat.
It is not possible to produce toxic levels of vitamin D from sunlight, but it is possible from supplements.
Vitamin D production and function
After vitamin D is produced in the skin through exposure to sunlight or acquired from food or supplements, it is stored in the body’s fat cells. Here it remains inactive until it’s needed. Through a chemical process, it is converted in the liver and kidney to the active form, calcitriol.
Vitamin D has many functions. It promotes calcium absorption and maintains adequate blood calcium concentrations for healthy bones and bone growth. Without sufficient vitamin D bones can become thin. Together with calcium, vitamin D helps protect older adults from osteoporosis.
Vitamin D reduces inflammation and helps control processes such as cell growth, nerve, muscle and immune function. It has been investigated for its potential role in reducing the risk of many diseases, including cardiovascular disease and cancer, such as breast cancer.
What is a healthy level of vitamin D?
Your vitamin D level depends on many factors, such as how much time you spend outdoors, what you eat, what medications you take, but also your age and skin type.
Most people in the northern hemisphere can make enough vitamin D from daily sun exposure with their forearms, hands or lower legs uncovered (without sunscreen) from late March to the end of September. For example, fair-skinned types need 10-15 minutes; for darker skin types, 25-40 minutes is recommended.
Vitamin D is stored by the body and helps maintain adequate levels in winter. A diet which contains liver, eggs or oily fish, such as salmon or herring, can provide small amounts of vitamin D.
Vitamin D deficiency, use of supplements and vitamin D toxicity
Due to limited sunlight, especially during winter months, and the often limited time spent outdoors, vitamin D deficiency is common in the UK population.
In infants, it can lead to rickets, which causes bone deformities, although this condition is rare in the UK. In adults, deficiency can result in osteomalacia, a condition causing bone pain, and in older adults, osteoporosis.
Vitamin D supplements can be used to prevent and treat these conditions.
Those at higher risk of vitamin D deficiency include, among others, people around 65 years of age or older, people who rarely get outside, people with darker skin and babies who need to be protected from direct sunlight in the first year of life.
To detect vitamin D deficiency, a laboratory blood test is necessary. This can be arranged through your GP.
Vitamin D toxicity
While you can’t get too much vitamin D through sunshine you can through supplements, which can lead to health problems. Ingesting more than 4000 IU per day (100 micrograms/day) increases the risk of harmful health effects.
Is there a link between low vitamin D levels and an increased breast cancer risk?
Most, but not all, human population studies have found low blood vitamin D is associated with increased breast cancer risk and suggest a protective relationship between high levels of vitamin D and breast cancer.
In addition, studies of breast cancer and vitamin D have also shown that increased sunlight exposure is linked to reduced breast cancer incidence as well as breast cancer mortality.
How could vitamin D protect against breast cancer?
Vitamin D may affect breast cancer development through several mechanisms of action. Among other things, it controls normal breast cell growth, decreases cancer cell growth, has anti-inflammatory effects and decreases the conversion of androgens (male hormones) to oestrogens, reducing serum oestrogen levels. High oestrogen levels are associated with an increased risk of breast cancer.
Do vitamin D supplements reduce breast cancer risk?
Despite a link between low levels of vitamin D and increased breast cancer risk, most studies do not demonstrate that taking vitamin D supplements reduces breast cancer risk. The reasons for this are unclear, and more studies are needed to confirm this finding.
Table 1: Vitamin D content of selected foods.
Vitamin D content of selected foods
|Naturally occurring vitamin D|
|Gouda cheese, 45% fat (dry matter)|
|Whole milk, 3.5% fat|
|Fortified with vitamin D|
|Multivitamin Whole Fresh Milk|
|Wholegrain oats (Ready Brek)|
For more information on vitamin D and breast cancer, see our science briefing.
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