Vitamin D metabolism and calbindin (calcium-binding protein) in aged laying hens.

Calbindin (calcium-binding protein) concentration, vitamin D metabolism and shell quality were investigated in young (7- to 9-mo-old) and aged (19- to 21-mo-old) laying hens consuming normal or low levels of calcium (Ca). Although egg weight and percent of cracked eggs were higher and egg production and shell density (mg/cm2) were lower (significantly, P less than 0.01) in aged hens, shell weight, plasma Ca and duodenal and egg shell gland calbindin were similar to those of young hens. Dietary Ca restriction reduced shell weight, shell density and structural bone and plasma Ca in both the young and aged birds. The production of 1,25-dihydroxycholecalciferol [1,25(OH)2D3] and its concentration in the plasma were higher in hens fed low dietary levels of Ca than in hens fed normal Ca levels only in the younger hens. However, a slightly higher production of 1,25(OH)2D3 and concentration of duodenal calbindin were also observed in severely Ca-restricted (1.4% Ca for 19 d) aged hens than in the younger hens. The results suggest that the aged hen loses its ability to adapt to changes in Ca intake or needs through mechanisms involving modulation of vitamin D metabolism.