Physiological and skeletal responses to graded dietary non-phytate phosphorus reduction in broiler chicks

In this study, we aimed to investigate the physiological and skeletal responses of broiler chicks to graded dietary reductions in dietary non-phytate phosphorus (NPP) at constant calcium (Ca) levels. The chicks were fed a control diet until day 7 and then fed (days 8–21) either a control diet (NPP 0.48%) or NPP-deficient diets (0.37, 0.26, or 0.15%) with a constant Ca content (0.96%). Serum Ca, phosphorus (P), and 1,25-dihydroxyvitamin D [1,25(OH)₂D₃] levels were analyzed; intestinal and renal P- and Ca-related transporter gene expression was quantified; and tibial ash content, strength, and growth plate (GP) histology were evaluated. The results revealed that graded reductions in dietary NPP at a constant Ca level were associated with reduced serum P levels and increased serum Ca and 1,25(OH)₂D₃ levels. The mRNA expression of CALB1, OCLN, CLDN12, SLC34A2, and SLC34A1 was upregulated; however, despite the upregulation of these mineral transport-related genes, serum P concentrations remained low under severe dietary NPP restriction. The tibial size, strength, and ash content decreased, and thickening of the GP, indicative of hypophosphatemic rickets, was observed. The serum Ca:P ratio increased with the dietary Ca:NPP ratio and appeared to plateau under severe NPP restriction, suggesting a threshold-like response within the range of dietary conditions tested. In conclusion, graded reductions in dietary NPP were associated with increased circulating 1,25(OH)₂D₃ concentrations and upregulation of Ca- and P-transport-related genes, while systemic P availability remained low under severe NPP restriction. The serum Ca:P ratio may have potential as an exploratory marker of dietary mineral imbalance in broilers.