Essentiality of Vitamin D3and its Metabolites in Poultry Nutrition: A Review

Abstract

SummaryVitamin D (cholecalciferol) is one of the most important dietary factors responsible for normal growth, egg production, shell quality and reproduction in fowls. Unlike other vitamins, 7-dehydrocholesterol in the skin is converted to vitamin D3 in the presence of ultra-violet radiation. In the modern poultry industry where the birds are raised mostly indoors, the essentiality of vitamin D is important for profitable egg and meat production.Cholecalciferol undergoes two sequential hydroxylations; the first in the liver to form 25 hydroxyvitamin D3, and the second in the kidney to form the active metabolite 1,25 dihydroxyvitamin D3. The latter metabolite is a steroid hormone which plays a major role in bone mineral mobilization and active intestinal absorption of calcium and phosphorus, accommodating the increased calcium demands by maintaining calcium homeostasis through the vitamin D endocrine system in the laying hen.Due to the continuing problems of poor egg shell quality in the poultry industry, scientists have been investigating the value of feeding vitamin D3 and its metabolites. Most research supports the National Research Council's recommended laying hen requirement of 500 ICU of vitamin D3/kg of diet. However, evidence suggests that the 25 hydroxyvitamin D3 metabolite is 2.0 to 2.5 times more active than vitamin D3 for normal egg production and shell quality. The most active 1, 25 dihydroxyvitamin D3 metabolite has an antirachitic activity 10 times that of vitamin D3. However, feeding laying hens 1, 25 dihydroxyvitamin D3 as their sole source of vitamin D produced eggs that had a high incidence of embryonic mortality prior to hatching. Improvements in hatchability of these eggs were observed when several forms of vitamin D3 were injected into the eggs. It appears that 1, 25(OH)2 D3 is not transferred or stored in the egg yolk or the vitelline membrane may be lacking specific receptors for 1,25 (OH)2 D3. Which of these possibilities is correct will undoubtedly become apparent with continued investigation.