Changes in serum levels of 1,25-dihydroxyvitamin D3, calcium and phosphorus with age and vitamin D status in chickens.

The effects of vitamin D3 (D3) on serum levels of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3), ionic calcium, total Ca and phosphorus in chicks were studied from the time of hatching until sexual maturity. Chicks fed on a diet low in D3 showed a serum level of 1,25(OH)2D3 higher than that in chicks on a normal-D3 diet, for both sexes and at any given age. A dramatic increase in the serum level of 1,25(OH)2D3 occurred in female birds approaching sexual maturity and in laying hens raised on the low-D3 diet the level was five times that of their counterparts raised on a normal-D3 diet. The serum 1,25(OH)2D3 level in adult males in the low-D3 groups was seven times that of those on the normal-D3 diet. The serum level of 25-hydroxyvitamin D3 remained relatively unchanged at weeks 2 and 15 in birds on a low D3 intake as well as in those fed on a normal-D3 diet. Nevertheless, the levels of 25-hydroxyvitamin D3 were different between the two groups. No significant change was observed in the level of ionized serum Ca in relation to dietary regimen, but there was an increase in total Ca concentration in females with the onset of reproduction. The serum P level decreased gradually with age, reaching a minimum value 3 and 8 weeks before laying commenced in the groups on low- and normal-D3 diets respectively. An increase was observed when the hens began laying.(ABSTRACT TRUNCATED AT 250 WORDS)