Survivorship. Barrie Cassileth

exposed to sunlight, and is also found in plants, fish, and dairy products. Its effects in the body are widespread: thousands of genes are regulated directly or indirectly by vitamin D. Scientists know that it is important for bone formation and that it assists in the absorption of calcium; adequate vitamin D levels are necessary to prevent osteoporosis and bone fractures. Although far more equivocal, some studies suggest that vitamin D may play a role in cancer, cardiovascular disease, diabetes, physical functioning, autoimmune diseases, infections/immune function, and mental health, among other diseases. Many argue that vitamin D supplementation can prevent or improve these conditions. But study results have been mixed, and evidence is lacking for virtually all uses except bone health.

      Our need for this vitamin is not surprising; through most of human evolution, vitamin D was readily available. Our hunter-gatherer ancestors spent most of their time outside in the sun and presumably produced high levels of vitamin D. But our modern indoors lifestyle has made vitamin D deficiency a widespread problem. Deficiency is even more prevalent among darker-skinned people, who make less vitamin D to begin with than their lighter-skinned counterparts.

      Of particular relevance for this book, there is limited but promising evidence that vitamin D may have a role in decreasing the risk or progression of some cancers. While vitamin D supplementation has not been shown to improve outcomes in cancer patients, some studies, especially those involving colon cancer, are suggestive of anticancer effects.⁸ However, the numerous efforts to delineate a causal relationship between vitamin D and a wide array of human cancers have produced conflicting results, and large-scale randomized trials will be required to provide definitive answers.⁹