Efficacy of 1-α-Hydroxycholecalciferol Supplementation in Young Broiler Feed Suggests Reducing Calcium Levels at Grower Phase

Abstract

Increasing biopotency of cholecalciferol (D₃) from vitamin sources is essential in the poultry industry to meet nutritional demands and counter stressors. D₃ exhibits hormonal traits and is responsible for calcium (Ca) absorption. 1-α-Hydroxycholecalciferol (1α) is a synthetic form of D₃ that has equal efficacy and is cheaper to synthesize than 1,25-dihydroxycholecalciferol (active form of D₃), on broilers. However, 1α bypasses a critical regulatory point, the kidney, and may consequently lead to toxicity levels of Ca via Ca absorption. This study examined 1α supplementation in broiler diets with different Ca inclusion levels to determine if 1α at higher Ca levels caused Ca toxicity at starter and grower phases with Ross 708 male broiler chicks. In Experiment 1 (1-15 days of age), chicks were assigned to one of 10 treatment starter diets with five levels of Ca inclusion (0.80, 0.95, 1.10, 1.25, and 1.40%) with or without 1α supplementation (5 μg 1α/kg in feed) and eight replicate cages per treatment. In Experiment 2, chicks were fed common starter diet until 16 days of age, and then they were assigned to one of eight treatment diets with four levels of Ca inclusion (0.54, 0.76, 0.98, or 1.20%) with or without 1α supplementation (5 μg 1α/kg in feed). At the end of both experiments, blood was collected from broilers to determine blood chemistry, including concentrations of vitamin D metabolites. Intestinal tissues were also collected to examine gene expression. In Experiment 1, broilers not fed 1α exhibited a quadratic effect in ionized blood Ca (iCa) as dietary Ca inclusion levels increased; 1α-fed broilers displayed an increase in iCa as Ca inclusion levels increased (p = 0.0002). For Experiment 2, 1α-fed broilers displayed a decrease in 25-hydroxycholecalciferol plasma concentration as dietary Ca inclusion levels increased (p = 0.035); also, increasing Ca inclusion in diets increased expression of duodenal sodium phosphate cotransporter type II b (NPTIIb, p = 0.03). Our findings imply that inclusion of 1α was beneficial because 1α enhanced Ca absorption during the starter phase; however, to avoid potential Ca toxicity or antagonism while using 1α during the grower phase, there should be consideration with reducing dietary Ca levels.

Keywords: 1-α-hydroxycholecalciferol; 25-hydroxycholecalciferol; blood chemistry; broiler; calbindin d28k; calcium; sodium phosphate cotransporter type IIb; vitamin D3.

Figure 1

Metabolic pathway of cholecalciferol (vitamin D₃) and 1-alpha-hydroxycholecalciferol (1α) to 1,25-dihydroxycholecalciferol (1,25-(OH)₂-D₃). Vitamin D₃ is converted to 25-hydroxycholecalciferol (25-OH-D₃) in liver, then 25-OH-D₃ is converted to 1,25-(OH)₂ D₃ in Kidney. 1α travels to liver to be converted to 1,25-(OH)₂-D₃. Green circle highlights C-25 of D3; orange circle highlights C-25 with hydroxyl group for 25-OH-D₃; blue circles denote C-1 and C-25 hydroxyl groups of 1,25-(OH)₂-D₃; purple circle highlights C-1 hydroxyl group of 1α.

Figure 2

Ionized blood calcium of 15 d broiler chickens fed different levels of calcium with or without 1-alpha hydroxycholecalciferol supplementation (D₃ + 1α; D₃, respectively). Line graphs show means ± standard error means (n = 8). Interaction effect observed between calcium inclusion and 1α supplementation [General linear model (GLM), *p < 0.05].

Figure 3

Selected blood chemistry of 35 d broiler chickens fed different levels of calcium with or without 1-alpha-hydroxycholecalciferol supplementation (D₃ + 1α; D₃, respectively). (A) Ionized blood calcium (B) Base excess of extracellular fluid (BEecf) (C) Blood bicarbonale (HCO₃). Line graphs show means ± standard error means (n = 5). Calcium inclusion effect observed for ionized blood calcium; 1α supplementation effect with BEect [General linear models (GLM), p < 0.05].

Figure 4

Vitamin D metabolite plasma concentrations of 35 d broiler chickens fed different levels of calcium with or without 1-alpha-hydroxycholecalciferol supplementation (D₃ + 1α;D₃, respectively). (A) 24,25-dihydroxycholecalciferol (24,25-(OH)₂-D₃) (B) 25-hydroxycholecalciferol (25-OH-D₃) (C) Cholecalciferol (Vitamin D₃) (D) Comparison of relative concentration between each vitamin D₃ metabolite between D₃ and D₃ + 1α groups; diagonal patterned bars denote D₃ and solid bars denote D₃ + 1α. 1α supplementation effect with 25-OH-D₃. Interaction between calcium inclusion and 1α supplementation with plasma vitamin D₃. Line graphs show means ± standard error means [n = 3; General linear models (GLM), *p < 0.05; ***p ≤ 0.0001].

Figure 5

Relative gene expression in duodenal tissue of 35 d broiler chickens fed different levels of calcium with or without 1-alpha-hydroxycholecalciferol supplementation (D₃ + 1α; D₃, respectively). (A) Calbindin d28k (CALB) (B) Mucin 2 (MUC2) (C) Vitamin D receptor (VDR) (D) Sodium-phosphate cotransporter type II b (NPTIIb). Duodenal tissue was analyzed using qPCR normalized against glyceraldehyde phosphate dehydrogenase (GAPDH; housekeeping gene) expression (n = 5). [General linear models (GLM). *p < 0.05].

Figure 6

Hypothetical model on comparing how dietary vitamin D₃ and 1-α-hydroxycholecalciferol (1α) are converted to water soluble calcitroic acid to be excreted. Black arrows denote vitamin D₃'s pathway to calcitroic acid and orange arrows denote 1α 's pathway. Dashed arrows signify 24-hydroxylation step. Multiple arrows between 1,24,25-(OH)₃-D₃and calcitroic acid denote number of conversion steps. 25-OH-D₃ is 25-hydroxycholecalciferol; 24,25-(OH)₃-D₃ is 24,25-dihydroxycholecalciferol; 1,25-(OH)₃-D₃ is 1,25-dihydroxycholecalciferol; and1,24,25-(OH)₃-D₃ is 1,24,25-trihydroxycholecalciferol.

Figure 7

Inverse relationship between duodenal calbindin d28k (CALB) and sodium-phosphate cotransporter type IIb (NPTIIb) in broiler chickens relative to calcium availability. As duodenal calcium concentration increases, then CALB expression decreases and NPTIIb expression increases to potentially maximize phosphate absorption because of the potential excessive calcium binding to phosphorus to form tricalcium phosphate and making phosphorus unavailable.