One of the authors, Cedric Garland, adjunct professor in the UCSD School of Medicine Department of Family Medicine and Public Health, says their study is the first to put numbers on this relationship, as he explains:
Vitamin D, which is produced by the body through exposure to sunshine, helps the body control calcium and phosphate levels. It was Prof. Garland and his late brother Frank who first linked low vitamin D with cancer in the 1980s. They found people who lived at higher latitudes and thus had less access to sunlight had lower levels of vitamin D and were more likely to develop bowel cancer.
Since then, further studies by the Garland brothers and others have found links between low vitamin D and other cancers, including cancers of the breast, lung and bladder.
Much debate about recommended level of vitamin D
The only accurate way to measure vitamin D in the body is to measure the level of 25-hydroxyvitamin D in the blood. The kidneys convert 25-hydroxyvitamin D into the active form that helps control calcium and phosphate levels.
There has been much debate in recent years about what the recommended blood levels of vitamin D should be. In 2010, the Institute of Medicine (IOM) recommended a target of 20 ng/ml for bone health, which could be met in most healthy adults (aged 19-70), with the equivalent of 600 IU of vitamin D each day. Since then, other groups have argued that the target level should be higher, at 50 ng/ml or more.
In the new study, Prof. Garland and colleagues wanted to find out what blood level of vitamin D effectively reduces cancer risk.
They took an approach that is not normally used. They used the results from two different types of study: one a clinical trial of 1,169 women and the other a prospective study of 1,135 women. For some of their analysis, they kept the two data sets separate and compared them, and in another part, they pooled the data to create a larger sample.
Vitamin D level of 40 ng/ml or higher tied to 67% lower cancer risk
The median blood level of 25-hydroxyvitamin D in the participants in the clinical trial was 30 ng/ml, and in the participants in the prospective study, it was 48 ng/ml.
The researchers found that the rate of cancer incidence in the clinical study group (that had the lower median vitamin D level) was higher than in the prospective study group. The figures were 1,020 cases per 100,000 person-years and 722 per 100,000 person-years, respectively.
They also found that cancer rates went down as 25-hydroxyvitamin D levels rose; women whose vitamin D level was 40 ng/ml or higher had a 67% lower risk of developing cancer than women whose vitamin D level was 20 ng/ml or lower.
The researchers did not say what the optimum intake level of vitamin D should be – or how it should be generated, whether by greater exposure to sunlight, dietary changes or supplements.
Prof. Garland says their findings simply show that it is possible to see reduced cancer risk when blood levels of vitamin D reach 40 ng/ml, and that higher than this, the risk drops even further. He and his colleagues conclude:
“Primary prevention of cancer, rather than expanding early detection or improving treatment, will be essential to reversing the current upward trend of cancer incidence worldwide. This analysis suggests that improving vitamin D status is a key prevention tool.”