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By Dr. Natasha Klemm, ND, PCN Co-Chair
There has been increasing interest in the use and efficacy of supplements for cancer prevention, particularly Vitamin D. Early epidemiological studies show lower incidence and mortality of certain cancers in southern latitudes where sunshine is more plentiful. Vitamin D is synthesized in the skin upon sunlight exposure, and therefore seen as the possible mechanism for lowering cancer risk.
In addition to sunlight exposure, the body takes in vitamin D3 in fortified dairy products, eggs, fish and supplements. It is then converted to the active form, 25(OH)D or “calcitriol,” in the liver. Calcitriol is a hormone that stimulates calcium absorption from the intestines, kidneys and bone, and is well known for promoting bone health. Initially calcitriol was thought only to affect these tissues; however, we now know that the vitamin D receptor (VDR) is also found in breast, prostate, colon, skin and pancreas tissues, indicating that vitamin D affects additional cell functions.
Laboratory research has demonstrated numerous effects on cancer development related to the presence of VDR in many types of tissue. In rats given the carcinogen N-methyl N-nitrosurea (NMU), vitamin D prevented or reduced breast tumor incidence and progression. Another study reported that vitamin D supplementation reduced the incidence and severity of prostate and colorectal cancers in rodents administered NMU. It was also shown that mice lacking the VDR are susceptible to developing skin cancer, indicating a potential protective effect via VDR stimulation.
Results of the animal studies led researchers to study the mechanism by which vitamin D affects cancer growth. Both the production of VDR and conversion of Vitamin D to its active form are involved in preventing or slowing cancer progression. Vitamin D shows promise in preventing cancer because it induces cellular differentiation, inhibits cancer cell growth, stimulates cell death and impairs angiogenesis (growth of new blood vessels).
Human studies are difficult to conduct because of the variety of ways that humans obtain Vitamin D. Most of the research to date has been performed on colorectal, breast, prostate and pancreatic cancers – tissues that express the VDR. Several studies have indicated a reduced colorectal cancer risk, and possibly breast cancer risk, with adequate Vitamin D levels. Other studies have showed no significant benefit with supplementation; however, these negative results can be attributed to poor study design, including a low supplemental dose (400 IU).
General consensus is that a previous recommendation for 400 IU Vitamin D daily is too low. Current Canadian guidelines recommend a dietary allowance of 600 IU for people from 1 to 70 years of age, although supplemental doses of up to 4000 IU are considered safe and may be necessary to counteract minimal dietary intake and sunlight exposure.
Vitamin D exists in several forms, but 25(OH)D is the one best measured in the bood. Adequate levels have been reported to be above 50 nanomolar (nM), based on a limited sample of Caucasians who are more susceptible to the effects of sunlight, in northern latitudes where there is less sunlight to produce vitamin D. Individuals with darker complexions have lower levels. Other recent studies have recommended that sufficient levels be increased to 75 nM based on its multiple health benefits, including cancer prevention.
Vitamin D is a fat-soluble vitamin, so it is best absorbed when consumed with a fatty meal. Consumption of other fat-soluble vitamins, including vitamins A and K must be considered when supplementing with vitamin D. The literature shows that we need to balance intake of vitamins and minerals, and that we can do this with a healthy diet and possibly careful supplementation.
The research on Vitamin D is promising for cancer prevention and slowing progression. During dark Canadian winters, about 40 percent of Canadians havae less than 50nM vitamin D in their blood. In addition to eating a balanced diet with vitamin D sources and having sufficient exposure to sunlight, you may want to talk to your health care practitioner about ways to get additional vitamin D.
US National Institutes of Health, Office of Dietary Supplements. Dietary Supplement Fact Sheet: Vitamin D.
Grassroots Health. D*Action, an international public health project to solve the vitamin D deficiency epidemic.
Natasha Klemm ND is a Toronto-based doctor and is co-Chair of Prevent Cancer Now