Vitamin D and Reducing Cancer Risk

             Given that the potential role of vitamin D in cancer prevention has been widely touted, many people were surprised that cancer-related considerations didn't figure prominently in the new Dietary Reference Intakes for vitamin D established by the Institute of Medicine (IOM). An IOM committee on which we served, charged with determining the population needs for vitamin D in North America, reviewed the evidence linking vitamin D with both skeletal and nonskeletal health outcomes. The committee concluded that vitamin D plays an important role in bone health and that the evidence provides a sound basis for determining the population's needs. For outcomes beyond bone health, however, including cancer, cardiovascular disease, diabetes, and autoimmune disorders, the evidence was found to be inconsistent and inconclusive as to causality.

             Based on vitamin D's importance to bone health, the recommended dietary allowances (RDAs) are 600 IU per day for persons 1 to 70 years of age and 800 IU per day for persons over 70 — intakes corresponding to a serum 25-hydroxyvitamin D level of at least 20 ng per milliliter (50 nmol per liter). Because of wide variation in skin synthesis of vitamin D and the known risks of skin cancer, we derived the RDAs under the assumption that sun exposure would be minimal. The committee also concluded that the prevalence of vitamin D inadequacy in North America has been overestimated. Most North Americans have serum 25-hydroxyvitamin D concentrations above 20 ng per milliliter, which is adequate for bone health in at least 97.5% of the population.

Key Points

• Vitamin D is essential for the formation, growth, and repair of bones and for normal calcium absorption and immune function. It is obtained primarily through exposure of the skin to ultraviolet radiation in sunlight, but it can also be obtained from some foods and dietary supplements (see Questions 1 and 4).
• Some studies suggest that higher intakes of vitamin D from food and/or supplements and higher levels of vitamin D in the blood are associated with reduced risks of colorectal cancer; however, the research results overall have been inconsistent (see Question 7).
• Whether vitamin D is associated with reduced risks of other cancers, including breast, prostate, and pancreatic cancers, remains unclear (see Questions 8, 9, 10, and 11).
• The National Cancer Institute (NCI) does not recommend for or against the use of vitamin D supplements to reduce the risk of colorectal or any other type of cancer (see Question 14).
  
  Note: The information in this fact sheet is not to be used as the basis for making health claims about products containing vitamin D.

• What is vitamin D?
• How much vitamin D is needed for health?
• What are the health effects of vitamin D deficiency?
• How much vitamin D is in fortified foods and supplements?
• Is it safe to take vitamin D supplements?
• Is there a role for vitamin D in reducing cancer risk?
• Is there evidence that vitamin D can help reduce the risk of colorectal cancer?
• Is there evidence that vitamin D can help reduce breast cancer risk?
• Is there evidence that vitamin D can help reduce prostate cancer risk?    
• Is there evidence that vitamin D can help reduce pancreatic cancer risk?
• Is there evidence that vitamin D can help reduce the risk of other rare cancers?
• What are the possible mechanisms by which vitamin D may modify cancer risk?
• How can people get enough sunlight for vitamin D synthesis while minimizing the risk of skin cancer?
• Does NCI recommend the use of vitamin D supplements to prevent colorectal or other cancers?         

1. What is vitamin D?
   Vitamin D is technically not a vitamin. It is the name given to a group of fat-soluble prohormones (substances that are precursors to hormones that usually have little hormonal activity by themselves). Two major forms of vitamin D that are important to humans are vitamin D₂, or ergocalciferol, and vitamin D₃, or cholecalciferol. Vitamin D₂ is made naturally by plants, and vitamin D₃ is made naturally by the body when the skin is exposed to ultraviolet radiation (in particular, UVB radiation) in sunlight. Vitamin D₂ and vitamin D₃ can also be commercially manufactured. The active form of vitamin D in the body is 1,25-dihydroxyvitamin D, or calcitriol, which can be made from either vitamin D₂ or vitamin D₃. To make the active form, vitamin D₂ and vitamin D₃ are modified in the liver to produce 25-hydroxyvitamin D, which travels through the blood to the kidneys, where it is modified further to make 1,25-dihydroxyvitamin D.
   Vitamin D is involved in a number of processes that are essential for good health, including the following:
   
   • It helps improve muscle strength and immune function.
   • It helps reduce inflammation.
   • It promotes the absorption of calcium from the small intestine.
   • It helps maintain adequate blood levels of the calcium and phosphate needed for bone formation, mineralization (incorporating minerals to increase strength and density), growth, and repair (1–3).
   Most people get the vitamin D they need through sunlight exposure. It can also be obtained through the diet, but very few foods naturally contain vitamin D. These foods include fatty fish, fish liver oil, and eggs. Smaller amounts are found in meat and cheese. Most dietary vitamin D comes from fortified foods, such as milk, juices, yogurt, bread, and breakfast cereals. Vitamin D can also be obtained through dietary supplements. Fortified foods and dietary supplements usually contain either vitamin D₂ or vitamin D₃. A person’s vitamin D status is usually checked by measuring the level of 25-hydroxyvitamin D in their blood serum.
2. How much vitamin D is needed for health?