ANN ARBOR—University of Michigan researchers have determined the surprising mechanism that moves calcium and phosphate from a nursing mother’s bones to her breast milk so that her nursing infant has the minerals to build strong bones. The mechanism is PTHrP (parathyroid-hormone-related peptide), a peptide previously known to be associated only with a disease- related process—the buildup of excess calcium in the lymph systems of breast cancer patients and in other types of cancer patients.

“Our study shows that the peptide, typically associated with hypercalcemia in malignancy, has a normal—and very important— physiological function,” says MaryFran Sowers, professor of epidemiology at the U-M School of Public Health and lead investigator for the study, which will appear in the Aug. 21 issue of the Journal of the American Medical Association (JAMA).

The study reports that elevated PTHrP levels were significantly associated with elevated levels of prolactin, the hormone that stimulates milk production, and lower levels of estradiol, a hormone related to menstruation. It also was strongly associated with declining levels of bone mass density and elevated levels of bone turnover markers, osteocalcin, alkaline phosphatase and n-teleopeptide, which rise when bone tissue is being resorbed and replaced.

The researchers note that bone mass density is restored as PTHrP diminishes and nursing tapers off. “This new finding provides, for the first time, an explanation for how minerals can be moved from mother to infant during breastfeeding,” Sowers says.

The study included 115 nursing mothers who participated in a series of bone mass density measurements and PTHrP analyses from two to 18 months after the birth of the infant. The same research group has previously shown that nursing mothers typically lose about 5 percent of their bone mass density in the first six months of nursing.

On average, a nursing mother produces approximately 600 milliliters of milk and 168 milligrams of calcium per day at three months following the infant’s birth, and a liter of milk and 280 milligrams of calcium per day at six months after birth. “The calcium concentration of milk is regulated and appears to be rather constant, even when maternal calcium intake varies,” Sowers says.

Sowers’ U-M colleagues included Mary Crutchfield and Mary Russell-Aulet, research associates, Department of Epidemiology; M. Anthony Schork, professor of biostatistics; graduate students Carol Janney and Daowen Zhang of the Department of Biostatistics; Dr. Brahm Shapiro, Department of Internal Medicine; and Dr. John F. Randolph, Department of Obstetrics and Gynecology.

Dr. Bruce Hollis of the Department of Pediatrics, Medical University of South Carolina, and Dr. Frank Stanczyk, Department of Obstetrics and Gynecology, University of Southern California School of Medicine, Los Angeles, also participated in the study.

Journal of the American Medical AssociationM. Anthony Schork