Review: Phosphorus in Fish Nutrition

increas e urinary P. Boling et al. (2000) reported that both citric acid and sodium citrate (6%/diet) markedly
increas ed bone density, weight gain and feed intake (but not feed efficiency) of chicks fed P-defi cient corn-soybean
diet. The effect was absent with phytate-free (casein-dextrose) diet. Conversely, pigs fed P-deficient
corn-soybean diet increas ed neither bone density nor weight gain by dietary addition of citric acid; but greatly
improved feed effi ciency (gain/feed). Many researchers have postulated modes of action of citric acid or citrates
on phytate-P utilization. Based on available evidence, they argue that dietary citrates have an effect to chelate Ca
in the intestinal lumen and prevent precipitation of phytate-P (discussed in Boling et al. 2000a; see sections
“Mellanby’s toxamin” and “ Availability of phytate-P”). Ca-phytate is resistant to phytase hydrolysis, while soluble
phytates (phytic acid and sodium salts) are digestible by phytase (endogenous, bacterial and ingredient-derived).
Therefore, as Boling et al. (2000b) demonstrated, when a diet is high in Ca content, in the amount that overwhelms
the chelating capacity of citrates, then dietary addition of citrates has no effect on phytate-P utilization.
A recent study indicates that dietary acidification (with inorganic acid) inhibits gastric H+/K+ ATPase
(proton pump) and sodium bicarbonate cotransporter gene expressions in corpus stomach of rainbow trout. This
suggests that gastric acid secretion may be down-regulated by dietary exogenous acid intake by some feed-back loop
mechanism, which involves not only decreased proton secretion into the gastric lumen, but also corresponding
decrease of basolat eral bicarbonate exit (from the oxynticopeptic cells), leading to a deficit of bicarbonate needed
for gastric mucous cells as well as for neutralizing gastric chyme in the duodenum or pyloric ceaca. Thus, the
inhibition of proton pump by dietary inorganic acids could lead to an acid-base imbalance. Dietary acetic acid,
however, seems to be neutral in this regard.
In vitro estimation of P availability
Some earlier workers in the mining field apparently used simple laboratory techniques that could estimate P
"assimilability" of rock phosphates that was to be used as P supplements for animal feed (Raynolds et al. 1944).
The supply of bone meal, which was a common P supplement for animal feeds, was rather limited in those days.
The methods they used to evaluate the biological value of P in rock phosphates were based on the solubility of P in
dilute hydrochloric acid within the range of concentrations found in the stomach of animals. Reynolds et al.
(1944) and Hill et al. (1945) appears to be the first to conduct extensive investigations to estimate P availability in
various P supplements based on their solubility in dilute hydrochloric acid and other solvents. Gillis et al. (1948)
studied 19 P compounds in an in vivo chick bioassay (at 0.4% and 0.8% levels), and an in vitro 0.4%-HCl solubility
test. Correlations between in vitro and in vivo assays were low, which was probably because the study included
varieties of metaphosphates, pyrophosphates, phytate-phosphate and inorganic orthophosphates. Also, Day et al.
(1973) compared bioavailability of P compounds for chicks by an in vivo bone-ash assay and by in vitro solubility
tests using 0.4% HCl, 2% citric acid and neutral ammonium citrate. Satoh et al. (1986) and Shinma (1989)
reported in vitro methods for estimating phosphorus availability in various feed ingredi ents and commercial feeds
for fish based on differential solubility fractionation using distilled water, 80% acetic acid, and 0.9% hydrochloric
acid as the solvents. They noted that carp could utilize water-soluble fraction of dietary P, but trout could utilize all
three fractions of dietary P. Satoh et al. (1992) and Satoh et al. (1997) reported an in vitro technique to estimate
available P content in semi-purified diets, practical diets and commercial diets for carp and rainbow trout. The
values agreed well with those determined in in vivo feeding (digestibility) trials. Deionized water, 80% actic acid
and 0.25M-HCl were used to extract phosphorus from the testing materials. The extracted solutions were digested
with nitric-perchloric acid mixture. The authors did not discuss about the interference of phytate-P, which is highly
soluble in water and even more soluble in dilute acid (Jordan et al. 1906, Han 1988), but not available to fish.
Jahan et al. (2000) from the same laboratory also reported that the in vitro (solubility test) and in vivo (digestibility
trials) data of estimated P availability were in close agreement; however, the P-retention by the fish (determined
from body P content) was much lower than the digestibility or solubility data even in the group of fish fed a
P-deficient diet. The authors explained, "In this experiment, 61.6-70.5% of absorbed P was retained and the rest of
29.5 to 38.4% was probably returned into the digestive tract as observed in mammals or excreted through gills and
urine. . . . Furthermore, experiments will be needed to clarify the destiny of absorbed P in fish." The "absorbed P"
may include absorbed P by the fish, dissolved dietary P that was lost while the small carp was chewing the pellets,
and any soluble P in feces that was lost before fecal collection. The excretion of endogenous (absorbed) P back
into the digestive tract may not be a significant source of error in fish (thus, apparent availability of certain P
compounds can be close to 100% in fish even at high dietary levels). The apparent digestibility, which includes
endogenous P, does not explain the difference. Also, P-deficient fish do not excrete 33% of absorbed P via gills
and urine. Spencer et al. (2000) estimated bioavailability of P in ordinary and low-phytate corn based on peptic
and pancreatin digestion methods in vitro, and obtained similar values to those determined in vivo.
© 2000, 2005. Shozo H. Sugiura. All rights reserved.
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