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The effect of increasing phosphorus (P) intake on P utilization was investigated in balance experiments using 12 Saanen goats, 4 to 5 mo of age and weighing 20 to 30 kg. The goats were given similar diets with various concentrations of P, and 32P was injected to trace the movement of P in the body. A P metabolism model with four pools was developed to compute P exchanges in the system. The results showed that P absorption, bone resorption, and excretion of urinary P and endogenous and fecal P all play a part in the homeostatic control of P. Endogenous fecal output was positively correlated to P intake (P < .01). Bone resorption of P was not influenced by intake of P, and P recycling from tissues to the blood pool was lesser for low P intake. Endogenous P loss occurred even in animals fed an inadequate P diet, resulting in a negative P balance. The extrapolated minimum endogenous loss in feces was .067 g of P/d. The minimum P intake for maintenance in Saanen goats was calculated to be .61 g of P/d or .055 g of P/(kg(.75) x d) at 25 kg BW. Model outputs indicate greater P flow from the blood pool to the gut and vice versa as P intake increased. Intake of P did not significantly affect P flow from bone and soft tissue to blood. The kinetic model and regressions could be used to estimate P requirement and the fate of P in goats and could also be extrapolated to both sheep and cattle.

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RESUMO -A pesquisa destinou-se a simular modelos matemáticos relacionados ao fluxo de fósforo (P), partindo-se das hipóteses de que o P endógeno do trato digestivo foi absorvido na mesma proporção do P alimentar e o fluxo de entrada e de saída desse mineral nos compartimentos foi constante. Foram avaliadas as variáveis absorção, retenção, P endógeno que retorna ao trato gastrintestinal, P dietético absorvido, incorporação, reabsorção e balanço de P no osso e nos tecidos moles e P proveniente do osso, dos tecidos moles e do total absorvido, que retorna ao trato gastrintestinal, em suínos em crescimento, alimentados com dietas contendo P de diferentes fontes de fosfato. O modelo usado foi determinístico e compartimental, em que o trato gastrintestinal (C1), os ossos (C2) e os tecidos moles, fígado, coração, rim e músculo, estudados em conjunto (C3), representaram os compartimentos. Foram utilizadas informações sobre metabolismo e cinética do fósforo em tecidos, obtidas pela técnica da diluição isotópica. Constatou-se que as hipóteses estabelecidas no modelo estão coerentes e os valores obtidos se ajustam ao fluxo do fósforo em suínos; as variáveis P consumido, P endógeno, que retorna ao trato gastrintestinal, P absorvido de origem dietética, retenção de P, incorporação, reabsorção e balanço de P do osso e dos demais tecidos e fração de P do total absorvido que retorna ao trato gastrintestinal não foram influenciadas pelas fontes suplementares de fósforo; a origem do fósforo dietético pode interferir na distribuição do P nos tecidos, após o processo de absorção desse mineral; e o P proveniente do absorvido que retorna ao trato gastrintestinal foi proporcional ao P consumido.Palavras-chave: absorção real, cinética, incorporação de fósforo, metabolismo mineral, perdas endógenas, radiofósforo Model of the Biological Flow of Phosphorus from Phosphate Sources in Pig, Using 32 P as MarkerABSTRACT -The experiment was designed to simulate the mathematical models that relate to the phophorus (P) flow, considering the hypotheses that the endogenous phosphorus from digestive tract was absorbed in the same proportion of the dietary phosphorus and that the input and output flow of this mineral in the compartments was constant. The variables absorption, retention, endogenous P that returns to the gastrointestinal tract (GIT), dietary absorbed P, accretion, reabsorption and balance of P in bone and in soft tissues and P from bone and soft tissue and from the total absorbed that returns to GIT, in growing pigs fed diets containing P from different phosphate sources were evaluate. The model was deterministic and compartmental, in which the GTI (C1), bones (C2) and soft tissues, liver heart, kidney and muscle, (C3) represented the compartments. The information on metabolism and kinetics of the P in tissues obtained by isotopic dilution technique was used. It was concluded that: the hypotheses established in the model were coherent and the obtained values were adjusted to the P flow in pigs; the variables P intake, endogenous P that ...

The objective of the study was to revise a model of P kinetics proposed by Vitti et al. (2000) and extend its use to study Ca flows in growing sheep. Twelve Santa Ines male sheep, 8 mo of age, with average BW of 31.6 kg were injected with 32P and 45Ca to trace the movement of P and Ca in the body. The original model had 4 pools representing the gut, plasma, soft tissues, and bone. In the revised model, instantaneous values rather than averages for pool derivatives were incorporated, and the model was extended to represent absorption and excretion of phytate P explicitly. The amendments improved the model, resulting in higher flows between plasma and bone than between plasma and tissue and, therefore, a more accurate representation of P metabolism. Phosphorus and Ca metabolism were then assessed conjointly using the revised model. The results showed that P and Ca metabolism are closely related as evidenced by the ratio of these minerals in the bidirectional flows between plasma and bone and between plasma and tissue. Phytate P digestibility was 47%, and P retention was negative (-1.4 g/d), suggesting that a feed characteristic impaired P utilization and led to P deficiency. The revised model provides an improved prediction of P and Ca metabolism that can be used to assess mineral requirements and to estimate losses to the environment.