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 has 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.

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.

This study, published today 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.

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.

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.