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Vitamin D has anti-cancer properties but evidence is lacking on whether it helps prevent skin cancers.
We assessed baseline (1996) serum 25(OH)-vitamin D status and estimated intake of vitamin D from foods and supplements in Australian adults who participated in an 11-year prospective study of skin cancer. During follow-up (1997-2007), all incident skin cancers were monitored through an extensive surveillance system and histologically confirmed. Serum concentrations were dichotomised and odds ratios obtained from logistic regression analysis.
Participants with serum 25(OH)-vitamin D concentrations above 75 nmol/L versus those below 75 nmol/L more often developed basal cell carcinoma (OR=1.51 (95%CI 1.10 to 2.07, P=0.01) and melanoma (OR=2.71 (95%CI 0.98 to 7.48, P=0.05) during follow-up after allowing for all variables that caused confounding in a multivariable model as well as usual time spent outdoors. Squamous cell carcinoma incidence was lower in persons with serum 25(OH)-vitamin D concentrations above 75 nmol/L versus those below 75 nmol/L (OR=0.67 (95%CI 0.44 to 1.03, P=0.07).
Our findings do not indicate that the carcinogenicity of high sun exposure can be counteracted by high vitamin D status. High sun exposure is to be avoided as a means to achieve high vitamin D status. Our ongoing analyses will confirm whether intake of vitamin D from foods and supplements has associations with skin cancer risk independent of vitamin D status, and whether vitamin D –related polymorphisms alter any of the associations observed between vitamin D status and skin cancer risk.
We hypothesised that persons who have high levels of vitamin D in their blood, or those who eat foods and take dietary supplements that are high in vitamin D, have a lower risk of developing skin cancer than persons with low levels of this vitamin. We also hypothesised that genetic factors may influence these associations between vitamin D and skin cancer risk.
Vitamin D is formed in the skin at exposure to the sun. The body also obtains vitamin D through foods and dietary supplements. Some studies suggest that vitamin D may help prevent certain types of cancer, but it is not clear whether it may help prevent skin cancer. Genetic factors are known to influence the way that cells can process vitamin D, and thus genetic factors should be considered when we study the effect of vitamin D on skin cancer formation.
We have used data from a study in which adults in an Australian community were followed during an 11-year period to ascertain in detail who developed skin cancer. At the start of the study all participants gave a blood sample in which we measured vitamin D levels. We also measured their usual consumption of foods and supplements that contain vitamin D. We extracted DNA from the blood sample to ascertain their status of 29 genes that are relevant vitamin D. All other main risk factors for skin cancer were measured as well (e.g. time spent outdoors, skin colour) to allow for these factors in our analyses.
We found that people who have high levels of vitamin D in their blood, have an increased risk of two skin cancer types, namely basal cell carcinoma (the most common type of skin cancer) and melanoma (the most dangerous type of skin cancer). The risk of squamous cell carcinoma, a type of skin cancer that is most directly linked to sun exposure, was lower in persons with high blood levels of vitamin D.
These results show that even if vitamin D had an anti-skin cancer effect, this is not sufficiently strong to prevent skin cancer from occurring. It is therefore important to avoid sun exposure as a way to increase vitamin D levels. Ongoing analyses will show whether vitamin D intake or genetic factors influence these findings.