2.4 New Paradigm for Optimal Vitamin D Status Physiologically, when blood calcium falls, PTH is released to promote hydroxylation of 25(OH)D to 1,25(OH)2D. Both PTH and 1,25(OH)2D mobilize of calcium (Ca) from bone and enhance absorption of Ca from the diet and glomerular filtrate. Elevated PTH levels characteristically seen in individuals with inadequate vitamin D status can result in the long latency disease of osteoporosis (27). A high PTH can also result from low dietary Ca, but for infants, breast milk and formula provide ample Ca.