Vitamin D is often referred to as the “sunshine vitamin” because the first step in making it occurs in the skin and is stimulated by the ultraviolet light in sunshine. This produces a preliminary form called vitamin D3, which is one of the common forms of vitamin D found in food and in supplements. The next step occurs in the liver, with a final step to make fully active vitamin D occurring mainly in the kidneys. The form made in the liver goes by the scientific name 25-hydroxy-vitamin D. This appears to be the storage form of the vitamin, and is also the form of vitamin D measured in blood tests for the vitamin.
Blood levels of vitamin D in people living in northerly countries such as Canada and the Scandinavian countries are measurably much lower in the winter than in the summer. This led medical researchers in Norway to investigate whether this difference could be related to various measures of health – and it was. Among other associations, they saw that people diagnosed with cancer of the breast, colon or prostate gland had a 15% – 25% lower risk of dying if they were diagnosed in summer or fall, compared to those diagnosed in winter or spring. This led to many more studies around the globe looking at vitamin D and cancer, and again higher levels correlated with better outcomes for many patients.
The latest study of this type comes again from Norway and it looked at vitamin D levels in patients diagnosed with cancer of the breast, lung and colon, as well as the immune system cancer called lymphoma. A strong point of the study was that over 650 people with these cancers were followed for a long time, from 1984 to 2004. Their vitamin D levels were tested within 90 days of their diagnosis. The researchers adjusted for the seasonally varying levels of the vitamin in their statistical analysis of the outcomes. For example, a value of ‘70’ in the winter might be equivalent to 100’ in the winter.