The research suggests that taking at least 1,000 international units (IU) of vitamin D daily can slash the risk of breast, colon, and other cancers, says Cedric Garland, DrPH, of the University of California, San Diego.
His new study, presented here at the annual meeting of the American Association for Cancer Research, suggests that increasing intake of vitamin D can lower the chance of developing breast cancer by 10% to 50%.
Other research, also presented at the cancer meeting, suggests that consuming adequate amounts of vitamin D during adolescence and young adulthood is particularly important for lowering breast cancer risk.
Yet another study, also out today, suggests that consuming 1,500 IU of vitamin D can cut a man's risk of developing any cancer by almost one-fifth.
Diet Alone Doesn't Do the Job
Current recommendations call for people between the ages of 1 and 50 to consume 200 IU of vitamin D daily, with 400 IU recommended for those between the ages of 51 and 70. After age 70, 600 IU of vitamin D are recommended each day.
But given widespread vitamin D deficiency, those recommendations are way too low, Garland tells WebMD.
It's nearly impossible to get enough vitamin D in your diet alone, Garland says. An 8-ounce glass of milk contains only 100 IU of vitamin D. A serving of cereal has 20 IU.
By comparison, someone who spends 10 to 15 minutes in the sun on a sunny day without sunscreen can absorb 2,000 to 5,000 IU of vitamin D if 40% of the body is exposed, he says.
But come winter, supplements are really the only way to go, he says.
The Role of Supplements
Not all vitamin D supplements are created equal, Garland says.
Most use an old form -- D-2 -- that is far less potent than the more desirable D-3. Multivitamins typically contain only small amounts of D-2 and include vitamin A, which offsets many of vitamin D's benefits, he tells WebMD.
"It may be hard to find and expensive to find it, but you want to take vitamin D-3," Garland says. At the same time, researchers warn against taking large amounts of vitamin D without medical supervision. High doses can cause some side effects, including a dangerous buildup of calcium in the blood.
Results showed that women with the highest blood levels of vitamin D were 50% less likely to develop breast cancer than those with the lowest levels. But very few women achieved that.
But even women in the second lowest quintile were 10% less likely to develop breast cancer than those with the lowest levels.
Sun Exposure and Diet
In a separate study, Julia A. Knight, PhD, of the Mount Sinai Hospital in Toronto, and colleagues conducted telephone interviews with 576 women who had been diagnosed with breast cancer and about 1,000 women without cancer. The women, aged 20 to 59, were asked about sun exposure and diet throughout their lives.
Ever working in a job outdoors, thereby exposing women to sunlight, resulted in an estimated 40% reduced risk of breast cancer for women, while frequent outdoor activities between ages 10 to 29 lowered breast cancer risk by an estimated 35%.
Also, taking vitamin D-rich cod liver oil between ages 10 to 19 reduced breast cancer risk by about 35%, and consuming at least nine glasses of milk every week between the ages of 10 to 29 reduced the risk by 25%.
"Current thinking is that exposures during adolescence or before a full-term pregnancy may have a greater effect, as that is when breast tissue is going through the most rapid development," Knight tells WebMD.
High Intake Protects Men, Too
The third study, which appears in the current issue of the Journal of the National Cancer Institute, examined the association between vitamin D and cancer among 47,800 men in the Health Professionals Follow-up Study.
Between 1986 and 2000, 4,286 of the men developed cancer and 2,025 of them died from it. According to Harvard researcher Edward Giovannucci, MD, ScD, men whose vitamin D levels reflected an increased increment (that could be provided by at least 1,500 IU daily) were 17% less likely to develop cancer and 29% less likely to die of the disease.
High levels of vitamin D were particularly protective against digestive system cancers (which includes colon cancer), with 1,500 IU daily associated with a 43% reduction in risk of developing such tumors and a 45% lower risk of dying of them.
William G. Nelson, MD, PhD, of Johns Hopkins University, says he agrees that men and women should get at least 1,000 IU of vitamin D a day.
"Higher amounts may eventually prove better, but for now that amount is likely to be safe and have a protective effect," he tells WebMD.
Nelson moderated a news conference to discuss the findings.