PTHrP expression results from a local serotonin signaling system and is highly induced in response to alveolar dilatation due to milk secretion and provides export of large amounts of calcium and  phosphorous. There is a four-fold greater calcium loss during 6 months lactation than whole pregnancy.

American Journal Physiol Endocrinol Metab 2012,302  E1009-E1015  Hernandez L et al.

Formula does not provide physiological protection for mother’s calcium loss during pregnancy leading to  a 50% increased risk of femur neck fracture in post menopausal women who have formula fed.