Two new vitamin D studies using a sophisticated form of analysis
called meta-analysis, in which data from multiple reports
is combined, have revealed new prescriptions for possibly
preventing up to half of the cases of breast cancer and
two-thirds of the cases of colorectal cancer in the United
States. The work was conducted by a core team of cancer
prevention specialists at the Moores Cancer Center at
University of California, San Diego (UCSD), and colleagues
from both coasts.
The breast cancer study, published online in the current
issue of the Journal of Steroid Biochemistry and Molecular
Biology, pooled dose-response data from two earlier studies
- the Harvard Nurses Health Study and the St. George's Hospital
Study - and found that individuals with the highest blood
levels of 25-hydroxyvitamin D, or 25(OH)D, had the lowest
risk of breast cancer.
The researchers divided the 1,760 records of individuals
in the two studies into five equal groups, from the lowest
blood levels of 25(OH)D (less than 13 nanograms per milliliter,
or 13 ng/ml) to the highest (approximately 52 ng/ml).
The data also included whether or not the individual had
"The data were very clear, showing that individuals
in the group with the lowest blood levels had the highest
rates of breast cancer, and the breast cancer rates dropped
as the blood levels of 25-hydroxyvitamin D increased,"
said study co-author Cedric Garland, Dr.P.H. "The
serum level associated with a 50 percent reduction in
risk could be maintained by taking 2,000 international
units of vitamin D3 daily plus, when the weather permits,
spending 10 to 15 minutes a day in the sun."
The colorectal cancer study, published online February
6 in the American Journal of Preventive Medicine, is a
meta-analysis of five studies that explored the association
of blood levels of 25(OH)D with risk of colon cancer.
All of the studies involved blood collected and tested
for 25 (OH)D levels from healthy volunteer donors who
were then followed for up to 25 years for development
of colorectal cancer.
As with the breast cancer study, the dose-response data
on a total of 1,448 individuals were put into order by
serum 25(OH)D level and then divided into five equal groups,
from the lowest blood levels to the highest.
"Through this meta-analysis we found that raising
the serum level of 25-hydroxyvitamin D to 34 ng/ml would
reduce the incidence rates of colorectal cancer by half,"
said co-author Edward D. Gorham, Ph.D. "We project
a two-thirds reduction in incidence with serum levels
of 46ng/ml, which corresponds to a daily intake of 2,000
IU of vitamin D3. This would be best achieved with a combination
of diet, supplements and 10 to 15 minutes per day in the
Vitamin D3 is available through diet, supplements and
exposure of the skin to sunlight, or ultraviolet B (UVB).
In the paper, the researchers underscored the importance
of limiting sun exposure such that the skin does not change
color (tan) or burn. For a typical fair-skinned Caucasian
individual, adequate vitamin D could be photosynthesized
safely by spending 10 to 15 minutes in the noontime sun
on a clear day with 50 percent of skin area exposed to
the sun. Darker skinned individuals may require more time
in the sun, such as 25 minutes. For people with photosensitivity
disorders, or anyone with a personal or family history
of nonmelanoma skin cancer, any amount of extra sun exposure
would be inadvisable.
The meta-analysis on colorectal cancer includes data
from the Women's Health Initiative, which had shown in
2006 that a low dose of vitamin D did not protect against
colorectal cancer within seven years of follow-up. However,
the researchers wrote, the meta-analysis indicates that
a higher dose may reduce its incidence.
"Meta-analysis is an important tool for revealing
trends that may not be apparent in a single study,"
said co-author Sharif B. Mohr, M.P.H. "Pooling of
independent but similar studies increases precision, and
therefore the confidence level of the findings."
The authors recommend further research to study individuals
for the effect of vitamin D from sunlight, diet and supplements
on the risk of cancer.