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Improving dietary phosphorus (P) absorption and retention has been intensively studied in pig and poultry due to the low digestibility of inositol phosphate (IP), global mineral P scarcity and increasing public concerns over environment pollution. The main aim of this thesis was to provide novel insights into calcium (Ca) and P absorption, metabolism and retention in pigs and poultry.The impact of dietary Ca content with or without microbial phytase supplementation on Ca and P absorption/solubilization in different digestive tract segments was studied in pigs. Increasing dietary Ca content reduced apparent P digestibility in all digestive tract posterior to stomach, and this impact was greater for the phytase supplemented diets in the distal ileum. Moreover, incremental dietary Ca content reduced P solubility, accompanied with a reduced IP6 degradation in the distal ileum. Thus dietary Ca reduced P absorption via reducing IP6 degradation and precipitation of P, which was more pronounced in microbial phytase included diets. Furthermore, a high dietary Ca content reduced mRNA expression of Ca-related transporters in the colon and jejunum, while it increased mRNA expression of Ca-related claudins in the jejunum. These results indicated that increasing dietary Ca supply switched Ca absorption from transcellular to paracellular pathway. In addition to that, dietary Ca reduced mRNA expression of Ca-related transporters and claudins in the kidney, suggesting that kidney played a central role in Ca and P homeostasis.In contrast to pigs, the negative impact of dietary Ca on P absorption was less pronounced in broilers fed diets supplemented with microbial phytase. Microbial phytase supplementation also increased Ca absorption particularly in the low-Ca diets. However, this positive impact was insufficient to restore the negative effect of Ca deficiency on growth performance or tibia breaking strength. Moreover, incremental dietary Ca supply reduced mRNA expression of both Ca-related transporters and claudin in the duodenum, while upregulated their expression in the kidney. These results suggested that kidney might not play a central role in Ca and P homeostasis.The impact of limestone particle size and inclusion level on Ca and P absorption was studied in broilers. Fine was more digestible than coarse limestone in the jejunum, but this difference disappeared in the ileum particularly at a low Ca intake. Irrespective of limestone particle size, minimal requirement of Ca/P was 1.00-1.25 to optimize Ca and P digestibility without compromising tibia breaking strength. Moreover, P digestibility in the ileum was gradually decreased with incremental Ca intake. The P solubility in the crop, gizzard and jejunum, however, was not affected by limestone particle size or inclusion level. Furthermore, mRNA expression of P-related transporter in both duodenum and jejunum was downregulated with increasing Ca intake. Thus Ca-induced downregulation of P transporter was an important mechanism by which dietary Ca reduced P absorption. Apart from that, mRNA expression of Ca- and P-related transporters in the duodenum was downregulated by dietary Ca intake in a limestone particle size dependent manner.In conclusion, in pigs incremental dietary Ca content reduces intestinal P absorption via hampering IP degradation and precipitation of P, with a larger impact in microbial phytase-supplemented diets. Increasing dietary Ca content shifts intestinal Ca absorption from transcellular to paracellular pathway, and kidney plays a central role in whole body Ca homeostasis. By contrast, in broilers incremental dietary Ca content reduces intestinal P absorption, which was more pronounced in phytase-free diets. Furthermore, Ca-induced downregulation of P transporters is an important mechanism by which dietary Ca reduces intestinal P absorption. Overall, insights obtained in poultry studies cannot be extrapolated to pigs and vice verse when it comes to Ca and P homeostasis.