Effect of dietary contents of cholecalciferol, 1α,25‐dihydroxycholecalciferol and 24,25‐dihydroxycholecalciferol on blood concentrations of 25‐hydroxycholecalciferol, 1α,25‐dihydroxycholecalciferol, total calcium and eggshell quality

1. Withdrawal of cholecalciferol (D3) supplement from a layers diet drastically reduced blood 25-hydroxycholecalciferol (25-OH-D3), 1 alpha,25-dihydroxycholecalciferol (calcitriol) and egg specific gravity (SG) within two weeks, followed by a decrease in blood total calcium (Ca). 2. Doubling the D3 supplement in the control diet (27.5 micrograms or 1100 IU/kg) almost linearly increased the circulating concentration of 25-OH-D3 without raising the concentration of calcitriol, Ca, or egg SG. 3. Replacing D3 by the optimal concentration of calcitriol (5 micrograms/kg diet) improved egg SG after 21 weeks of treatment without increasing blood calcitriol or total Ca. 4. By itself, 24,25-dihydroxycholecalciferol [24,25-(OH)2D3] was unable to maintain normal blood levels of calcitriol, Ca or egg SG and, when added together with calcitriol in the diet, tended to elevate blood Ca but suppress the beneficial effect of calcitriol on shell quality, with little or no effect on blood calcitriol.