1. 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.2. Chicks fed on a diet low in D3showed a serum level of 1, 25(OH)2D3higher than that in chicks on a normal-D3diet, for both sexes and at any given age.3. A dramatic increase in the serum level of 1, 25(OH)2D3occurred in female birds approaching sexual maturity and in laying hens raised on the low-D3diet the level was five times that of their counterparts raised on a normal-D3diet.4. Theserum 1, 25(OH)2D3levelin adultmalesin thelow-D3groups wasseven timesthatofthoseon thenormal-D3diet.5. The serum level of 25-hydroxyvitamin D3remained relatively unchanged at weeks 2 and 15 in birds on a low D3intake as well as in those fed on a normal-D3diet. Nevertheless, the levels of 25-hydroxyvitamin D3were different between the two groups.6. 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.7. 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-D3diets respectively. An increase was observed when the hens began laying.8. Chicks adapted to a low-D3diet by elevation of their plasma level of 1, 25(OH)2D3. The mechanism by which this is achieved is not known, but the results suggest that parathyroid hormone, Ca and P are unlikely to play roles in the adaptive increase in the level of 1, 25(OH)2D3in the blood of chicks given a minimal amount of D3. The possibility that the rate of degradation of 1, 25(OH)2D3is greatly reduced under these conditions cannot be excluded and this could account for the level of this metabolite in those birds.
Age and seasonal variations in the serum levels of 25-hydroxyvitamin D and 24,25-dihydroxyvitamin D in normal humans.
