Madam

When The Institute of Medicine (IOM) of the National Academies released its new Dietary Reference Intakes for Calcium and Vitamin D report on 30 November 2010^(1, Reference Ross, Manson and Abrams²⁾, the vitamin D research community was shocked and dismayed at the findings. The committee found a benefit only for bones, leading to the finding that a 25-hydroxyvitamin D (25(OH)D) level of 20 ng/ml was adequate and a recommended intake of 15 μg/d for most people. These are well below the recommendations of vitamin D experts: intakes of up to 50 μg/d and achieving serum 25(OH)D levels of 40–60 ng/ml⁽³⁾. Casual solar UV-B irradiance in summer in England raises serum 25(OH)D levels by nearly 40 nmol/l, equivalent to the production of about 37.5 μg/d for those aged 45 years⁽Reference Hyppönen and Power⁴⁾, far more than suggested by the IOM⁽¹⁾.

The UV-B–vitamin D–cancer hypothesis was based on an ecological study of the geographical variation of colon cancer mortality rates and sunlight doses in the USA⁽Reference Garland and Garland⁵⁾ and has been extended by subsequent ecological studies in Australia, Asia, Europe and the USA to about twenty types of cancer⁽Reference Grant and Garland^6–Reference Grant and Mohr⁹⁾. While the IOM considered some ecological studies as background information, it noted they have the primary weakness that ‘Outcome measures are not predictable at the individual level’ and, thus, are of low quality for dietary reference intakes⁽¹⁾. This summary dismissal is not warranted: in part because no mechanism other than production of vitamin D has been proposed to explain the ecological study findings, in part since the findings of ecological studies of cancer have been supported by other studies⁽Reference Grant¹⁰⁾, and in part since ecological studies integrate the effect of UV-B and vitamin D over much of the lifetime and include many cases.

A second type of study based on solar UV-B is that of cancer risk with respect to diagnosis or death from non-melanoma skin cancer (NMSC). The primary risk factor for NMSC is UV irradiance, with UV-B the most important risk factor for NMSC death⁽Reference Grant¹¹⁾. An ecological study for Spain found fifteen types of cancer inversely correlated with NMSC mortality rate after adjusting for smoking⁽Reference Grant¹²⁾. A record linkage study found significant inverse correlations between diagnosis of NMSC and incidence of gastric, liver, pancreatic and prostate cancer and non-significant inverse correlations for five other types of cancer⁽Reference Tuohimaa, Pukkala and Scelo¹³⁾. A reduced risk of prostate cancer incidence was noted with more early-life UV-B irradiance⁽Reference John, Koo and Schwartz¹⁴⁾.

A third type of study is based on solar UV-B exposure related to occupation. A death certificate-based case–control study of cancer mortality rates in the USA found significant inverse correlations for breast and colon cancer with respect to occupations with high occupational exposure to sunlight⁽Reference Freedman, Dosemeci and McGlynn¹⁵⁾. A study of cancer risk in Rhineland-Palatinate, Germany found significantly reduced risk of nearly a dozen types of internal cancer compared with incidence of NMSC plus melanoma in regions with more land devoted to winegrowing⁽Reference Seidler, Hammer and Husmann^16, Reference Grant¹⁷⁾.

A fourth type of study is the case–control study using self-reported personal sun exposure. A pooled study of this nature found a protective effect of recreational sun exposure at 18–40 years of age and in the 10 years before diagnosis for non-Hodgkin's lymphoma⁽Reference Kricker, Armstrong and Hughes¹⁸⁾.

Together with other studies such as case–control studies of vitamin D and breast cancer⁽Reference Yin, Grandi and Raum¹⁹⁾ and improved survival rate after diagnosis of non-Hodgkin's lymphoma and other types of cancer with higher serum 25(OH)D at time of diagnosis⁽Reference Drake, Maurer and Link²⁰⁾, there is strong support for a causal relationship between vitamin D and reduced risk of cancer⁽Reference Grant¹⁰⁾ which could have permitted the IOM to find a beneficial effect of vitamin D in reducing the risk of cancer.