Vitamin D 3 and fertility in poultry: balancing benefits and risks

SUMMARYVitamin D3 (VD3) has traditionally been acknowledged for its very pivotal impact on calcium (Ca) and phosphorus (P) balance; however, growing evidence highlights its widespread impact on reproductive biology. Fertility is a complex trait regulated by interactions between the gonadal tissues, endocrine system, and immune responses, and VD3 has emerged as an important key modulator in these processes. Poultry, with its short reproductive cycle and is considered a well-established nutritional model, provides an ideal platform to explore both the advantages and drawbacks of VD3 on fertility traits. On the beneficial side, VD3 enhances ovarian follicle development, steroid hormone synthesis, and eggshell quality in female birds through vitamin D receptor (VDR)-mediated mechanisms and regulation of intracellular Ca signalling. In male birds, VD3 has been associated with improved sperm motility and testicular function via similar molecular pathways. It also exerts antioxidant and immunomodulatory effects that support reproductive performance under environmental stress. In contrast, excessive supplementation may disrupt endocrine balance, provoke hypercalcaemia, and impair hatchability, underscoring the narrow margin between sufficiency and toxicity. Case studies in roosters and laying hens provide strong evidence of these dual impacts, making poultry an important translational model for livestock and human fertility studies. This review critically discusses the mechanisms through which VD3 influences female and male fertility, summarises experimental results in poultry, and focuses on both the potential benefits and adverse outcomes of its supplementation. Future perspectives emphasise omics-driven approaches, precision nutrition, and the development of safer VD3 analogs to optimise reproductive health while minimising risks.