The women's health initiative--curse or blessing?

Vitamin D resistance and colon cancer prevention

Observational studies have been largely consistent in showing an inverse association between vitamin D and an individual's risk of developing colorectal cancer. Vitamin D protection is further supported by a range of preclinical colon cancer models, including carcinogen, genetic and dietary models. A large number of mechanistic studies in both humans and rodents point to vitamin D preventing cancer by regulating cell proliferation. Counterbalancing this mostly positive data are the results of human intervention studies in which supplemental vitamin D was found to be ineffective for reducing colon cancer risk. One explanation for these discrepancies is the timing of vitamin D intervention. It is possible that colon lesions may progress to a stage where they become unresponsive to vitamin D. Such a somatic loss in vitamin D responsiveness bears the hallmarks of an epigenetic change. Here, we review data supporting the chemopreventive effectiveness of vitamin D and discuss how gene silencing and other molecular changes somatically acquired during colon cancer development may limit the protection that may otherwise be afforded by vitamin D via dietary intervention. Finally, we discuss how understanding the mechanisms by which vitamin D protection is lost might be used to devise strategies to enhance its chemopreventive actions.

Mendelian randomization: how it can--and cannot--help confirm causal relations between nutrition and cancer

Observational epidemiologic studies of nutrition and cancer have faced formidable methodologic obstacles, including dietary measurement error and confounding. We consider whether Mendelian randomization can help surmount these obstacles. The Mendelian randomization strategy, building on both the accuracy of genotyping and the random assortment of alleles at meiosis, involves searching for an association between a nutritional exposure-mimicking gene variant (a type of "instrumental variable") and cancer outcome. Necessary assumptions are that the gene is independent of cancer, given the exposure, and also independent of potential confounders. An allelic variant can serve as a proxy for diet and other nutritional factors through its effects on either metabolic processes or consumption behavior. Such a genetic proxy is measured with little error and usually is not confounded by nongenetic characteristics. Examples of potentially informative genes include LCT (lactase), ALDH2 (aldehyde dehydrogenase), and HFE (hemochromatosis), proxies, respectively, for dairy product intake, alcoholic beverage drinking, and serum iron levels. We show that use of these and other genes in Mendelian randomization studies of nutrition and cancer may be more complicated than previously recognized and discuss factors that can invalidate the instrumental variable assumptions or cloud the interpretation of these studies. Sample size requirements for Mendelian randomization studies of nutrition and cancer are shown to be potentially daunting; strong genetic proxies for exposure are necessary to make such studies feasible. We conclude that Mendelian randomization is not universally applicable, but, under the right conditions, can complement evidence for causal associations from conventional epidemiologic studies.