BALTIMORE (AP) _ About one out of three calcium supplements, which many women take to strengthen their bones in later years, pass through the stomach before they can take effect, a university study has concluded.

Of 35 calcium carbonate tablets tested in the University of Maryland School of Pharmacy laboratories over the last year, 11 or more took so long to disintegrate they could not be sufficiently used by the human body before being eliminated, said the study.

Calcium deficiencies have been linked to osteoporosis, a loss of bone tissue that makes makes bones fragile and vulnerable to fractures, which particularly afflicts post-menopausal women. Physicians recently have been prescribing calcium supplements to fortify women's bones.

Only 17 products tested disintegrated in 30 minutes, the Food and Drug Administration standard for drugs, and only 14 disintegrated in 10 minutes or less, a time considered to be efficient, said Dr. Ralph F. Shangraw, chairman of the school's department of pharmaceutics.

Quick disintegration is particularly important for elderly users who often have less stomach acid and are less able to digest the calcium supplements than the average user, Shangraw said.

The other form of calcium supplement, tri-based calcium phosphate, is even more resilient to digestion, he said. It was not part of the study.

The study did not attempt to address whether calcium supplements, if digested, help in fortifying bones. Shangraw and his associates said they were attempting to address the issue of quality among products on the market.

''If calcium has a role, we should see that they (senior citizens) get good calcium,'' said Dr. Peter P. Lamy, whose Center for the Study of Pharmacy and Therapeutics for the Elderly at the university partially funded the study.

''The problem as far as the elderly are concerned is they spend billions of dollars on these things that may not be effective,'' Lamy said Wednesday.

The study was published in the February issue of American Pharmacy magazine.

The FDA last year said calcium, in conjunction with exercise and estrogen supplements, appears to help deter osteoporosis, but, Lamy said, ''We don't know yet how, why and to what extent.''

Calcium is virtually unregulated because it is classified as a nutritional supplement by the FDA, not a drug, said Shangraw.