Dietary super-doses of cholecalciferol fed to aged laying hens illustrates limitation of 24,25-dihydroxycholecalciferol conversion

ABSRACT Background Older humans who take high levels of vitamin D supplementation for a prolonged time may be at risk of vitamin D toxicity. It is unclear how dietary super-doses (10,000x greater than requirement) can affect vitamin D status in aged animals. Aged laying hens could be a model to compare vitamin D supplementation effects with women in peri-or postmenopausal stages of life. Objective We investigated dietary super-dose impacts of cholecalciferol (vitamin D 3 ) on vitamin D status in aged laying hens in production. Methods Forty-eight 68 wk old Hy-Line Brown laying hens were individually housed in cages with eight hens per dietary treatment for eleven weeks. Hens were randomly assigned to one of six treatment groups of dietary vitamin D 3 supplementation and fed ad libitum . Supplementation levels were 400, 800, 7,400, 14,000, 20,000, and 36,000 IU D 3 /kg of feed. At the end of the study, all hens were euthanized and tissue samples and feces were collected. Plasma and egg yolk vitamin D metabolites, calcium and phosphorus composition of multiple sites, and tissue gene expression levels were measured. Results We observed that increasing dietary vitamin D 3 increased plasma vitamin D 3 and egg yolk vitamin D 3 (p < 0.0001 for both sites). We also observed an increase in plasma 24,25-dihydroxycholecalciferol as dietary vitamin D 3 levels increased (p < 0.0001). The plasma 25-hydroxycholecalciferol:24,25-dihydroxycholecalciferol ratio exhibited an asymptotic relationship starting at the 14,000 IU/kg D 3 treatment. Conclusions Dietary super-doses of vitamin D 3 led to greater plasma and egg yolk vitamin D levels to show that aged laying hens can deposit excess vitamin D in egg yolk. We suggest future research should explore how 24-hydroxylation mechanisms are affected by vitamin D supplementation. Further understanding of 24-hydroxylation can help ascertain ways to reduce risk of vitamin D toxicity.