Two studies have found that higher blood levels of vitamin D are linked to lower rates of breast cancer and colorectal cancer cases.
"It has more potential than any other vitamin or micronutrient that we know of to prevent cancer," Dr. Cedric Garland of the University of California, San Diego, told 愛污传媒.
"This is a powerful vitamin."
The breast cancer study, published online in the current issue of the Journal of Steroid Biochemistry and Molecular Biology, found that individuals with the highest blood levels of vitamin D had the lowest risk of breast cancer.
The work was conducted by a core team of cancer prevention specialists at the Moores Cancer Center at University of California, San Diego as well as colleagues from both coasts.
The researchers divided the 1,760 records of individuals in the two studies into five equal groups, from the lowest blood levels to the highest.
"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 Dr. Cedric Garland, professor at University of California, San Diego Department of Family and Preventive Medicine.
"The serum level associated with a 50 per cent 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 Feb. 6 in the American Journal of Preventive Medicine, is a meta-analysis of five studies that explored the association of blood levels of vitamin D with risk of colon cancer.
All of the studies involved blood collected and tested for vitamin D levels from healthy volunteer donors who were then observed for up to 25 years for development of colorectal cancer.
As with the breast cancer study, data on the nearly 1,500 individuals were organized by order by vitamin D blood levels and then divided into five equal groups.
"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, assistant professor of family and preventive medicine.
"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 sun."
Previous research has suggested a possible role for vitamin D in the fight against cancer. It's been observed, for example, that death rates due to breast, colon, ovarian and prostate cancer are lower in sunnier climes.
The "sunshine vitamin" has also been shown to kill some cancer cells in laboratory experiments.
While the natural form of the vitamin, known as D3, is normally produced in the skin after exposure to sunlight, it can also be obtained from dietary sources, such as oily fish, eggs and meat.
But the researchers underscored the importance of limiting sun exposure.
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 per cent of skin area exposed to the sun.
Darker skinned individuals may require more sun exposure, such as 25 minutes.
However sun exposure would be inadvisable, researchers said, for people with photosensitivity disorders, or a history of nonmelanoma skin cancer.
The analysis of data on colorectal cancer includes research 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.
But the researchers wrote that 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, of the University of California, San Diego.
"Pooling of independent but similar studies increases precision, and therefore the confidence level of the findings."
Meta-analysis is a sophisticated form of examination in which data from multiple reports is combined.
The authors recommend further research to the effect of vitamin D on the risk of cancer.
Researchers have warned in the past that a high dose of vitamin D has its own risks.
More than 2,000 IU a day can make the body absorb too much calcium, and damage the liver and kidneys.
With a report from CTV's Avis Favaro and Elizabeth St. Philip
