Review: Phosphorus in Fish Nutrition

4.4, 4.1 and 3.3. The acidified diet was fed to rainbow trout (95-130 g in body wt) for 140 days either solely or in
combination with the control diet (non-acidified, pH 6.6) at various ratios. These authors found that 1.5-2.5%
dietary acid levels depress ed feed intake and growth of the fish. At 1.25%-level of acidi fication, the dietary pH
was 5.3 (hydrochloric acid), 4.6 (formic acid), and 4.3 (sulfuric acid). The feed intake of fish did not differ, but
feed conversion was low in formic acid and sulfuric acid groups. Fish growth was also low in formic acid and
sulfuri c acid groups in the first month compared with non-acidified and hydrochlori c acid groups; however, all
groups showed similar growth during the second and third months. Asgard & Austreng (1981) reported that
formic acid alone or in combination with sulfuric acid is, from practical and economical reasons, best suited to make
acid silage of fish offal. Hardy et al. (1983) studied the effects of acidi fication. Acidified fish meal and acidi fied
fish silage both slightly reduced growth and feed effi ciency of rainbow trout in 84 days of feeding compared with
those neutralized with Ca(OH) 2. Those wet materials were acidifi ed with 2% sulfuric acid and 0.75% propionic
acid, which after drying and mixing with other materials concentrated to 5.9% of the diet. Thus, the diet acidity
seems to be very high compared with other acidification studies especially because strong acids like sulfuric acid are
almost completely dissociated. This study indicated that the fish fed the acidi fied (non-neutralized) diets had
higher P and Ca contents in the body than the other fish fed neutralized diets (though the authors did not mention
this), suggesting acidification increased availability of dietary P. This also suggests that the silage may be used
without neutralization, and the diet may be prepared as moist pellets and stored at ambient temperature.
The advantage of dietary acidi fication is that the acid can increase P digestibility, which was not a matter of
interest in the past. Vielma & Lall (1997) reported that rainbow trout increased absorption of P, Ca and Mg when
a fish meal-based diet was supplemented with formic acid (0, 4, 10 mL/kg). However, the level of increase was
small (70% P-absorption in non-acidifi ed diet vs 75% in acidified diet). The dietary pH was 6.3 and 5.3 in
non-acidi fied and acidified diets, respectively. However, the pH of the intestinal contents (both proximal and
distal) of the fish fed the acidi fi ed diet was higher than that of fish fed the nonacidi fied diet. Vielma et al. (1999)
fed rainbow trout (initial wt 3 g) for 6 weeks with casein-based diets containing varied levels of bone meal with or
without supplemental citric acid. The diet contained total P in the amount of 0.5-0.6%; and citric acid was added in
the amounts of 0, 0.4, 0.8 and 1.6%; which resulted in the dietary pH of 6.0, 5.7, 5.4 and 5.0, respectively. Dietary
supplementation of citric acid increased whole-body concent rations of ash, P, and Ca, but only modestly (0.297 vs
0.313%P in non-acidified vs the most-acidified diets, respectively). Sugiura et al. (1998) reported that
supplementing a fish meal-based diet with citric acid at a level of 5% increas ed P absorption from 75% (non-acid
diet) to 87% in rainbow trout. However, sodium citrate had no effect, and sodium bicarbonate decreased the
apparent P absorption. Fecal P content of fish fed acidifi ed diets was about 1/3 of the non-acidified group.
Supplementing a high-ash diet (2.0%P) with citric acid increased urinary P and decreased fecal P, with no change in
body P retention in fish. Thus, fecal P can be reduced by increasing P availability (by acidification), while urinary
P must be reduced by reducing P content in the diet. Inorganic acids also increased availability of P in fish
meal-bas ed diet from 71% (non-acidi fied, pH 6.1) to 95% (sulfuric acid, pH 2.4) or 88% (hydrochloric acid, pH2.0).
The fish accept ed the acidic diets during 23 days of satiation feeding with slight (sulfuric acid) or some
(hydrochloric acid) reduction of feed intake. Adding citric acid to a soybean meal-based diet containing no fish
meal had without effect on P availability. However, P availability increased markedly when both citric acid and
phytase were added compared with when only phytase was added to the diet. Radcliffe et al. (1998), however, did
not see any synergistic interaction between dietary phytase and citric acid in corn-soybean meal-bas ed diets fed to
weaning pigs. Also, the addition of citric acid alone had little effects on performance and Ca digestibility. Some
workers observed a reduction of intestinal pH in pigs fed acidi fied diets (Scipioni et al. 1978, Burnell et al. 1988),
which may prevent minerals from being precipitated and phytase from being inactivated in the intestinal lumen.
Thus, low intestinal pH may confer favorable luminal environment for P absorption. However, intestinal
sodium-phosphate cotranspoters (type-II) have a strong pH preference. In mammals, they are generally more
active at acidic pH, whereas in fish they are much more active at alikaline pH (see “P transport” section). The
intestinal pH of fishes is strongly alkali (especially of seawater fishes) compared with homeotherms due presumably
to low bacterial fermentation in the intestinal lumen of fishes (Wilson et al. 2002). In fish intestine, therefore, the
NaPi transporter is very active, whereas at the same time P tends to precipitate. However, high bicarbonate
concentrations in the lumen of fish intestine may protect some phosphatre salts from being precipitated. Maier &
Tullis (1984) noted that lowering dietary pH (by inclusion of acetic acid) or raising the pH (by ammonium
hydroxide) of an algae-based diet had a significant effect on the gut pH of tilapia. Chonan et al. (1998) reported
that the absorption of dietary Ca and P increased in gastrectimized rats when lactic acid was added to the diet.
Jongbloed et al. (2000) studied the effect of lactic acid (1.6 or 3.2% /diet) and formic acid (0.8 or 1.6%)
supplementation to a pig diet with or without microbial phytase. Both phytase and acids increased P digestibility;
however, there was no interaction. The acid level did not affect the P-digestibility. The acidification did not
© 2000, 2005. Shozo H. Sugiura. All rights reserved.
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