Canada and the United States could cut their annual incidence of colon and breast cancer, two of the most common and deadly malignancies, by about 25 per cent if everyone took vitamin D supplements of 2,000 International Units a day, according to a new study.
The recommended dose - which corresponds to the maximum safe level set by Health Canada - comes from a team of researchers at the Moores Cancer Center at the University of California, San Diego, who said in the study that they believe there are "no unreasonable risks" from taking that amount.
Increasing vitamin D intake is "a strong means of prevention," said Cedric Garland, a professor of family and preventive medicine and lead author of the study, which appears today in the Annals of Epidemiology.
He also said that those who already have the cancers could improve their odds of beating the diseases by taking vitamin D as they undergo chemotherapy and radiation treatments, or while dealing with metastases.
"I think it offers the hope of pulling back from the extremely toxic doses of chemotherapy that we think we need right now, and ultimately I think it offers a way of reducing cancer death," he said.
The call by a leading U.S. team researching vitamin D to pop more of the sunshine vitamin comes as the nutrient is emerging as one of the most debated topics in cancer prevention and treatment.
Medical findings that some cancers and other chronic diseases may be linked to having too little of it have become so frequent that the Canadian and U.S. governments have ordered an expert panel to assess whether current intake advice needs to be revised.
The panel is expected to complete its work next year, and in the meantime the governments say people need take only 200 to 600 IU a day, depending on age. This recommendation, set in 1997, is based on bone health and isn't linked to any of the purported anti-cancer benefits.
The new study is one of the most detailed to date to look at the association between blood levels of Vitamin D and the odds of developing colon or breast cancer in the future.
When doctors have conducted these kinds of blood surveys, and followed up years later to see who developed the cancers, they've noticed a striking pattern: Those with the most vitamin D generally have about half the risk of those with the least.
The University of California researchers estimated how much the two cancer rates would fall if everyone raised their blood levels by taking 2,000 IU a day, or the amount in two of the highest-dose supplement pills allowed on the Canadian market.
They selected the 2,000 figure because it's the current government maximum for those not under doctor's care, but Dr. Garland said further research might lead to a higher figure.
The dosage exceeds the 1,000 IU a day recommendation issued in 2007 by the Canadian Cancer Society, the first major public-health group in the world to call for taking the vitamin to cut cancer risk.
The cancer society said non-whites should take 1,000 IU a day year-round, and whites during fall and winter, when a lack of sunlight makes it impossible to make the vitamin the natural way.Not all medical experts are jumping on the vitamin D bandwagon.
The International Agency for Research on Cancer, a United Nations affiliate, called last year for a major drug-style trial to prove the safety and efficacy of vitamin D before health authorities recommend population-wide dosing. Such a trial would likely cost more than $100-million (U.S.) and take years to complete.
The IARC cautioned that high vitamin D levels in blood samples may have little to do with lowering cancer risk, and might just be a sign that people have a healthy lifestyle involving summer exercise and sun exposure.
Although the IARC found the evidence linking low levels of the nutrient to colon cancer persuasive, it also warned that supplements have a long history of not living up to their initial hype. Some have eventually been found to be harmful, such as beta carotene, it said.
John McLaughlin, vice-president of population studies and surveillance at Cancer Care Ontario, said his organization is "very interested" in vitamin D because of its intriguing link to prevention of the disease. But he added that more research needs to be done to zero in on the most appropriate dose. The organization currently calls for taking between 400 and 800 IU daily.
Dr. Garland said he believes the scientific case is strong enough to take action now, and his study estimates that increasing vitamin intake would reduce the total number of North Americans contracting breast and prostate cancer by more than 100,000 a year.
"Clinical trials take a long time," he said. "Do we want another four years or five years before we move ahead ... I would say no."
A quicker and less costly way to verify the efficacy of vitamin D would be to include it in the control groups of some of the many drug trials undertaken each year for new cancer medicines, he said.
About 3,000 studies have been published in the medical literature investigating the association between vitamin D and all types of cancer, according to Dr. Garland. He said a preponderance, although not all, have found the vitamin plays a beneficial role.
According to the literature, other cancers linked to low vitamin D status include those of the ovary, kidney and uterus. For prostate cancer, the picture is mixed. Some research has associated low blood levels of the nutrient to more aggressive prostate cancer, but others found no correlation. While Dr. Garland contends that low vitamin D levels are a cancer risk, some researchers believe huge seasonal fluctuations in blood levels of the nutrient may also be a problem.
In high-latitude countries such as Canada, vitamin D levels rise sharply in summer when people make the nutrient in skin exposed to strong sunlight. Levels then plunge in winter.
Reinhold Vieth, professor of nutritional sciences at the University of Toronto, says humans evolved near the equator where vitamin D would be high year-round with little seasonal variation. Although he recommends taking vitamin D, he says its link to cancer may be "more complicated" because both low blood amounts and wide annual fluctuation "may be a problem."
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Binding cells together
The reason vitamin D is being investigated for anti-cancer properties is that almost every cell in the body, including those that are malignant, have receptors for the nutrient and need it for proper growth.
Researchers have determined that the vitamin is involved in everything from telling cells when to die, a process known as apoptosis, to the production of proteins that suppress tumours.
One theory is that vitamin D may be able to prevent cancers, or slow down their development, because it is important in producing E-cadherins, a kind of biological mortar that binds cells tightly together and limits their ability to grow out of control.
Researchers are focusing on this aspect of vitamin D because normal cells are supposed to be tightly packed together, much like the bricks in a wall, a configuration that limits their ability
to subdivide and proliferate. If there isn't enough E-cadherins, cells aren't as tightly packed in together.
Some scientists believe that these slightly aberrant cells eventually go on to produce what we recognize as tumours when there are enough of them. Even if these cells only have a 2 per cent advantage in reproduction compared with normal cells, after 9,000 generations - or about 20 years - they will completely dominate an area.
Advocates for vitamin D as a therapy say that at any point in the development of cancer at its initial site, or after it spreads in a process known as metastasis, the nutrient is able to coax cells into proper behaviour.
Those who are recommending vitamin D say it may stop cells from proliferating into full-blown tumours, or keep tumours under better control.
Martin Mittelstaedt
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Bricks in a wall
Vitamin D may help prevent cancer by keeping cells tightly packed together.
SOME RESEARCHERS BELIEVE TAKING 2000 IU OF VITAMIN D DAILY WILL STALL CANCER AT ANY STEP IN THIS PROCESS:
Normal
When cells are tightly packed, like bricks in a wall, they are not prone to cancer
Low Vitamin D
A shortfall in the nutrient leads to a sligh separation between each cell.
Mutation
The weaker cell structure sets the stage for cancer to begin
Natural selection
Cancerous cells dividing at an accelerated rate will dominate nearby tissue.
Overgrowth
These cells compete for nutrients, breaking through membrane walls and into the lymphatic system
Metastasis
Malignant cells begin to colonize new locations within the body
TONIA COWAN/THE GLOBE AND MAIL
SOURCE: ANNALS OF EPIDEMIOLOGY AND STAFF