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OMAHA, Neb. - Building hope for one pill to prevent many cancers, vitamin D cut the risk of several types of cancer by 60 percent overall for older women in the most rigorous study yet.

The new research strengthens the case made by some specialists that vitamin D may be a powerful cancer preventive and most people should get more of it. Experts remain split, though, on how much to take.

"The findings ... are a breakthrough of great medical and public health importance," declared Cedric Garland, a prominent vitamin D researcher at the University of California-San Diego. "No other method to prevent cancer has been identified that has such a powerful impact."

While the most reliable yet, the study does have drawbacks. It was designed mainly to monitor how calcium and vitamin D improve bone health, and the number of cancer cases overall was small, showing up in just 50 patients.

"It's a very small study," said Dr. Edward Giovannucci, who researches nutrition and cancer at the Harvard School of Public Health. "I don't think it's the last word."

In either case, the study takes an important step in extending several decades of research that began with observations that cancer rates among similar groups of people were lower in southern latitudes than in northern ones. Scientists reasoned that had to do with more direct sunlight in southern regions.

The skin makes vitamin D when exposed to sunlight's ultraviolet rays. This study used that same form of the vitamin, known as D3 or cholecalciferol. Multivitamins usually carry a much weaker variant known as D2, but D3 is available in stand-alone dietary supplements.

Earlier research has shown that vitamin D helps regulate cell growth, a fundamental biological process that goes haywire in cancer. Most other supplements have tended to target specific types of disease in early testing, like selenium or vitamin E for prostate cancer.

This study, published Friday in the American Journal of Clinical Nutrition, is the first time that researchers significantly boosted -- and measured -- blood levels of vitamin D and then followed identical groups of patients from start to finish.

That's why, despite its modest size, the research was generating excitement. Nearly all other work has compared disparate groups of patients.

The researchers at Creighton University in Omaha focused on 1,179 seemingly healthy women with an average age of 67. The women were divided into three groups: 446 got calcium and vitamin D3 supplements, a similar number got calcium alone, and 288 took dummy pills.

The research team gave 1,000 daily international units of vitamin D, more than current guidelines calling for 200 to 600 units depending on a person's age.

The researchers intended to check mainly for the effects of calcium on bone health. Their interest in cancer risk was secondary.

But the lower cancer risk stood out. Only 13 women, or 3 percent, developed cancer over four years of calcium and vitamin D supplements. With calcium alone, 17 women, or 4 percent, got cancer. With dummy pills, cancer appeared in 20 women, or 7 percent.

That shows a 60 percent lower cancer risk over four years in the group taking both supplements, compared to patients taking placebos. And when the first-year cancers were excluded -- the ones mostly likely present before the study began -- the findings were stronger still: a 77 percent lower risk for the combo group.

While the calcium-only group lowered its four-year cancer risk by 47 percent compared to the untreated group, it did no better when early cancers were excluded. That suggests calcium alone may have done little in this experiment, the researchers said.

Experts reviewing the study focused on vitamin D as the powerful agent in the combo group, but it can't be ruled out that calcium might somehow amplify the effect of vitamin D.

While numbers were limited, these women developed a broad range of cancers, including disease of the breast, colon, lungs and blood. Dr. Michael Holick, of Boston University Medical Center, who sat on the professional panel that issued the 1997 guidelines for vitamin D, said this study shows that enough vitamin D "markedly reduces the risk of developing the most serious deadly cancers."

He supports raising the recommended amount of the vitamin and said 1,000 daily units of vitamin D3 would now be reasonable for most people.

On the other hand, Dr. Michael Thun of the American Cancer Society favors keeping the current recommendation of 200 to 600 IUs for now. And he cautioned that more than 2,000 units is viewed in the guidelines as potentially dangerous.

Joan Lappe, the study's lead researcher, said it "just adds to the great bunch of evidence that we need to have better vitamin D nutrition." Some foods carry the vitamin, like salmon, tuna and fortified milk, but diet accounts for little of the vitamin circulating in the body. Overexposure to the sun can cause skin cancer.

Still, people should consult their doctors before boosting their vitamin dosage, several experts also warned.

More study is needed to determine if the effects in this study hold true for large groups of people and men as well as women.