The study reinforces links between vitamin D status and rates of breast, lung and bladder cancer first made in the 1980s.
Findings from this analysis could help to shape healthcare guidelines by placing vitamin D status as a key prevention tool.
Information gleamed from this study could also form a basis for primary prevention to slow or reverse upward trends in cancer incidence, treatment burden, mortality, and associated costs.
There were 14 million new cases of cancer worldwide in 2012 and 8.2 million cancer-related deaths. In the years ahead, the annual number of new cases is projected to increase to 22 million within the next two decades.
According to the World Health Organisation (WHO), cancer causes 20% of deaths in the European Region. With more than three million new cases and 1.7m deaths each year, cancer is the most significant cause of death and morbidity in Europe after cardiovascular diseases.
The cohort approach
The objective for researchers at the University of California, San Diego School of Medicine, was to determine what blood level of vitamin D was required to effectively reduce cancer risk.
The team were specifically looking for 25-hydroxyvitamin D, the main form of vitamin D in the blood, as a marker for vitamin D levels.
Following an unusual approach, the team combined data from two previous studies. One was a randomised clinical trial of 1,169 women (Lappe trial cohort) and the other was a prospective cohort study of 1,135 women (GrassrootsHealth cohort).
By combining the two studies, the researchers were able to work from a larger sample size and a greater range of blood serum levels of 25-hydroxyvitamin D (25(OH)D).
It is generally accepted that the most accurate measure of vitamin D levels in a person is a blood test. In the Lappe trial cohort, the median blood serum level of 25(OH)D was 30 nanograms (ng) per millilitre (ml). In the GrassrootsHealth prospective cohort, it was higher: 48 ng/ml.
In drawing conclusions, the team found that the age-adjusted cancer incidence was 1,020 cases per 100,000 person-years in the Lappe cohort and 722 per 100,000 person-years in the GrassrootsHealth cohort.
In other words, rates of cancer declined with increased 25(OH)D levels. Females with 25(OH)D concentrations of 40 ng/ml or greater had a 67% lower risk of cancer than women with levels of 20 ng/ml or less.
"These findings support an inverse association between 25(OH)D and risk of cancer," said Dr Cedric Garland, adjunct professor in the UC San Diego School of Medicine Department of Family Medicine and Public Health and member of Moores Cancer Center at UC San Diego Health.
“It highlights the importance for cancer prevention of achieving a vitamin D blood serum concentration above 20 ng/ml, the concentration recommended by the Institute of Medicine for bone health."
Strong vitamin D cases
The results suggest the importance of vitamin D for the prevention of cancer and are supported by a number of additional studies that have also found a similar reduction in risk for individual cancers.
One study demonstrated in a hospital-based case control study that women with serum concentrations of more than 60 ng/ml had an 83% reduction in breast cancer risk compared to women with concentrations less than 20 ng/ml.
A population-based case control study found a 63% lower risk of breast cancer for women with 25(OH)D concentrations of 30 ng/ml compared to women with concentrations of less than 20 ng/ml, with a 71% lower risk among post-menopausal women.
A recent nested case-control study found a 55% lower risk of colorectal cancer in women with 25(OH)D concentrations 29 ng/ml compared to women with concentrations of less than 18 ng/ml.
The wealth of studies has made recommending an optimum blood serum level of vitamin D difficult to establish.
In 2010, the IOM concluded that levels lower than 12ng/ml represented a vitamin D deficiency and recommended a target of 20 ng/ml. Other regulatory bodies have argued for higher blood serum levels of 50 ng/ml or more.
Many vitamin D supporters now advocate 800 to 1,000 International Units of vitamin D (IUs) daily (800-100 micrograms); more for persons older than 70 and pregnant or lactating women.
Garland did not identify a singular, optimum daily intake of vitamin D or the manner of intake, which may be sunlight exposure, diet and/or supplementation.
He stated that the current study clarified that reduced cancer risk became measurable at 40 ng/ml, with additional benefit at higher levels.
Source: PLOS one
Published online ahead of print, doi.org/10.1371/journal.pone.0152441
“Serum 25-Hydroxyvitamin D Concentrations 40 ng/ml Are Associated with >65% Lower Cancer Risk: Pooled Analysis of Randomized Trial and Prospective Cohort Study.”
Authors: Sharon McDonnell, Carole Baggerly, Christine French, Leo Baggerly, Cedric Garland, Edward Gorham, Joan Lappe, Robert Heaney.