title: An extended model of phosphorus metabolism in growing ruminants.
abstract: A major objective of this study was to extend the Vitti-Dias model used to describe P metabolism in ruminants, by adding 2 new pools to the original model to represent the rumen and saliva. An experiment was carried out using 24 male sheep, initial BW of 34.5 kg, aged 8 mo, fed a basal diet supplied with increasing amounts of dicalcium phosphate to provide 0.14, 0.32, 0.49, and 0.65% P in the diet. Sheep were individually housed indoors in metabolic cages and injected with a single dose of 7.4 MBq of (32)P into a jugular vein. Feed intake and total fecal and urinary outputs were recorded and sampled daily for 1 wk, and blood samples were obtained at 5 min, and 1, 2, 4, 6, 24, 48, 72, 96, 120, 144, and 168 h after (32)P injection. Saliva and rumen fluid samples were taken on d 6, 7, and 8. Then, animals were slaughtered and samples from liver, kidney, testicle, muscle, and heart (soft tissue) and bone were collected. Specific radioactivity and inorganic P were then determined in bone, soft tissue, plasma, rumen, saliva, and feces, and used to calculate flows between pools. Increased P intake positively affected total P (r = 0.97, P < 0.01) and endogenous P excretion in feces (r = 0.85, P < 0.01), P flow from plasma to saliva (r = 0.73, P < 0.01), from saliva to rumen (r = 0.73, P < 0.01), and from lower gastrointestinal tract to plasma (r = 0.72, P < 0.01). Urinary P excretion was similar for all treatments (P = 0.35). It was, however, related to plasma P (r = 0.63, P < 0.01) and to net P flow to bone (accretion - resorption; r = -0.64, P < 0.01). Phosphorus intake affected net P flow to soft tissue (P = 0.04) but not net P flow to bone (P = 0.46). Phosphorus mobilized from bone was directed toward soft tissue, as suggested by the correlations between P flow from bone to plasma and net P flow to soft tissue (r = 0.89, P < 0.01), and P flow from plasma to soft tissue and net P flow to bone (r = -0.76, P < 0.01). The lack of effect of dietary P on net P accretion in bone suggests that P demand for bone formation was low and surplus P was partially used by soft tissue. In conclusion, the model resulted in appropriate biological description of P metabolism in sheep and added knowledge of the effects of surplus dietary P on P metabolism. Additionally, the model can be used as a tool to assess feeding strategies aiming to mitigate P excretion into the environment.
authors.affiliation: Centre for Nutrition Modelling, University of Guelph, Guelph, Ontario, Canada.
authors.forename: R S
authors.initials: RS
authors.lastname: Dias
authors.forename: S
authors.initials: S
authors.lastname: López
authors.forename: R M P
authors.initials: RM
authors.lastname: Patiño
authors.forename: T S
authors.initials: TS
authors.lastname: Silva
authors.forename: J C
authors.initials: JC
authors.lastname: Silva Filho
authors.forename: D M S S
authors.initials: DM
authors.lastname: Vitti
authors.forename: M R S R
authors.initials: MR
authors.lastname: Peçanha
authors.forename: E
authors.initials: E
authors.lastname: Kebreab
authors.forename: J
authors.initials: J
authors.lastname: France
mesh_terms: D000821:Animal Feed / Q000032:analysis; D000824:Animal Nutritional Physiological Phenomena; D000818:Animals; D004032:Diet / Q000662:veterinary; D004305:Dose-Response Relationship, Drug; D008297:Male; D008954:Models, Biological; D010758:Phosphorus / Q000378:metabolism*; D012680:Sensitivity and Specificity; D012756:Sheep / Q000254:growth & development* / Q000502:physiology*
chemical_list: D010758:Phosphorus