Review: Phosphorus in Fish Nutrition

view of the evidence of interaction and balance among food constituents, . . . optimum Ca and P contents in diet
vary every time the other elements of diet are changed." Sugiura et al. (1998) calculated the interaction
(inhibition) potential of each feed ingredient by measuring mineral absorption when a semi-purified bas al diet was
fed alone and when it was mixed with one of test ingredients; thus the digestibility of minerals can be negative due
to the presence of interfering compounds in each feed ingredient.
Fecal endogenous losses may be estimated using a linear regression of the excretion (or ret ention) on the
intake, which is equal to the constant of the regression. Kienzle et al. (1998) estimated dietary P requirement for
adult cats based on a factorial procedure. The authors determined endogenous fecal loss, endogenous renal
excretion and true digestibility of P. A basal diet contained turkey meat (60%) as the major P source, which was
supplemented with P (source unknown) with varied amount of Ca (Ca/P ratios from 1/1-4/1). This procedure,
however, does not estimate the endogenous fecal P excretion accurately. When endogenous fecal P is to be
determined as the constant of a linear regression line, the P availability (absorption) of the diet has to be constant
throughout, i.e., constant regression coefficient.
True digestibility by differential approaches
The true digestibility of P may be determined without knowing the endogenous excretion. Steggerda & Mitchell
(1939) determined the true availability of Ca in human diets by giving the test materials at two levels of intake, one
very low and the other at about maintenance, and calculating the change in Ca retention relative to the change in Ca
intake (i.e., True availability = ∆Retention/∆Intake). A similar approach has been reported by Ammerman et al.
(1957) who determined the true absorption percentages of various P compounds by using the following equation;
True absorption (%) = {(Total P intake – P in basal ration) – (Total P exc. – Basal P exc.)}*100/(Total P intake – P in
basal ration). This formula may be simplified as True absorption = ∆Absorption/∆Intake. In this experiment, all
lambs received a basal ration containing 0.032%P for 4 weeks, and the fecal collections of 5-7d-duration were made
during the latter part of the 4wk-depletion period. The supplemental feeding of P lasted for 12d during which four
3-day collections were made. The supplemental rations contained 0.154-0.158%P, which are well below the
dietary requirement. Kleiber (1975) reported an equation to calculate "partial availability" as follows; Partial
availability = (∆Intake – ∆Feces)/ ∆Intake, which is the same as Ammerman's calculation. Hurwitz (1964)
reported a concept of "net P utilization". This method is also similar to the previously reported in that the Net P
utilization =(∆P body/∆P intake)*100 The slope of abscissa of the dose-response curve determined on various P
intakes will be (dP body/dP intake)*100. The author estimated total body P content from the tibia P-total body P
relationship that had been determined separately. The slope was determined in the liniar regression below the
minimum requirement. It should be noted that the term "net" normally indicates "apparent", but here the author
used the term as "true". Hurwitz et al. (1978) calculated "fractional absorption" to determine % absorption of a P
supplement at various Ca levels. The calculation is the same as Ammerman et al. (1957). Bondi (1987) reported
a "comparative bal ance technique" to determine percentage of utilization of a nutrient. The calculation is as
follows: Utilization (%) =∆Balance*100/∆Intake. Oldham & Emmans (1988) briefly discussed about absolute
response (= output/input), incremental response (=∆output/∆Input), marginal effici ency (= slope of the regression
line), and the differences between diminishing returns and the broken stick (line) responses. The differential
method of estimating the true digestibility or availability is based on the assumption that the endogenous P excretion
is constant at different dietary P intakes, the assumption of which has been questioned by Kleiber et al. (1951) as
mentioned above. Ammerman (1995) also discussed this point by using terms "Minimum endogenous"
(obligatory) and "Variabl e endogenous". When ∆intake is very small (i.e., similar levels of intake), the total
endogenous will be similar. Thus, the slope method reported by Kleiber and Hurwitz as discussed above is free
from such a compromise, but construction of a best-fit dose-response curve by measuring responses at various P
levels will be essential (thus more laborious than the differential method). The slope method also gives the
availability of a dietary nutrient at various fractions of dietary levels as reported by Gahl et al. (1995, 1996) for
lysine using pigs and rats. The authors defined that the marginal efficiency (d retention/d intake) is the effici ency
of retention or gain for a small increment of the amino acid added to the diet. The maximum marginal effici ency
occurred at about 40% of the maximum retention of lysine. They concluded that since diminishing returns affect
the upper 55-65% of the response range, this information has to be considered in formulating economically feasible
diets that would maximize economic returns rather than maximize animal growth. Rodehutscord et al. (2000)
reported similar results on P using rainbow trout.
© 2000, 2005. Shozo H. Sugiura. All rights reserved.
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