Taking calcium and vitamin D supplements does not reduce the chances of developing breast cancer in a U.S. study of women's health, according to findings released Monday, but some women may benefit and more study was needed to confirm the findings.

"We can't yet make a general recommendation about how much calcium and vitamin D individuals should take each day as supplements," said Dr. Rowan Chlebowski, the study's lead author.

One nutrition expert said the new results were unreliable, in part because women were allowed to take supplements besides what they were given as part of the study.

Earlier research had suggested that vitamin D and calcium supplements may protect women from breast cancer, which is expected to kill about 41,000 U.S. women this year.

The findings came from a look at 36,282 U.S. post-menopausal women in the long-term Women's Health Initiative study.

Chlebowski, a cancer specialist at the Los Angeles Biomedical Research Institute at Harbor-UCLA Medical Center, and his team measured breast cancer incidence among 18,176 women randomly assigned to take 1000 milligrams of calcium carbonate and 400 international units of vitamin D daily. They compared the cases to another group of 18,106 who got placebo pills.

After about seven years, breast cancer incidence did not differ significantly between the two groups overall. About 3 percent of women in each group were diagnosed with the disease.

Breast cancer patients in the calcium and vitamin D group did have smaller tumors, the study found.

For one set of women, breast cancer risk was reduced. Women who did not report any supplement use at the time they entered the study had an 18 percent lower risk of breast cancer if they took vitamin D and calcium during the study, compared with others who had no prior supplement use but took placebos.

Dr. Walter Willett, an epidemiologist and nutritionist at Harvard, said the study was flawed because all of the women were allowed to continue personal use of calcium and vitamin D supplements besides those provided in the study.

"I think it's uninterpretable," he said.

Sixty-nine percent of women in the study said they took calcium supplements on their own, and about 30 percent took multi-vitamins containing vitamin D, he said.

The women also may have been consuming calcium in milk or other foods in which it is added.

The findings of the study, which is ongoing, were presented at the annual meeting of the American Society of Clinical Oncology.

Natural sources of vitamin D, which also helps to prevent fractures, include sun exposure and oily fish, but most people do not get enough from those sources, experts say.

The Institute of Medicine advises most adults to get at least 400 international units per day, and 600 international units for people 70 and older.