Review: Phosphorus in Fish Nutrition

organic forms through 4 days after feeding, however, at the 4th day, predominant portions (ca. 92%) of 32 P in muscle
were still in the inorganic fraction rather than phospholipid, nucleic acid, or protein fractions. The muscle
contained 1.40 mg Pi, 0.18 mg phospholipid-P, 0.25 mg nucleic acid-P, and 0.10 mg phosphoprotein-P per gram.
Hurwitz et al. (1978) determined the effect of dietary Ca levels on the apparent absorption of P by measuring P and
radioactive yttrium content in the feed and feces of turkey. Dietary Ca intake over 440mg/d reduced the fractional
absorption of supplementary P in a linear manner. Nakamura (1982) fed carp for one week with
egg-albumin-bas ed diets of varied Ca content (0.1, 0.4, 0.7, 1.3 and 2.6%) but fixed P content (0.64%). Ca-lactate
and KH 2PO 4 served the sources of Ca and P in the diets, respectively. Feces were collected by stripping. The
highest absorption of P (98.1%) was observed with a diet of the lowest Ca content (0.1%). The P absorption
decreased as the dietary Ca level increased. Vielma & Lall (1998b) fed Atlantic salmon (initial wt 42 g) for 15
weeks with a low-P diet (3.1 g P, 1.3 g Ca /kg diet) with or without Ca (as CaCO 3) and/or P supplementation. The
P in the basal (low-P) diet was supplied solely from casein, while inorganic P supplied a large portion of P in the
high-P diet. Thus, the sources of P in the low-P diet and high-P diet were different, and so does their interaction
with Ca. The vertebral P content of fish after 15 wk of feeding did not differ between fish fed the low-P (basal)
diet (3.1 g P/kg diet) and fish fed high-P diets (8.3 g P/kg diet) when the diets were supplemented with Ca. This
indicates that the dietary P requirement was about satisfied at 3.1 g P/kg diet based on bone P content. The
authors found that supplementing the low-P diet with Ca greatly increased bone calci fication, but decreased weight
gain. Retention and intestinal absorption (digestibility) were not determined. They wrote, ". . . from the practical
diet formulation point of view, too high of a dietary Ca content per se seems not to be of a concern (for dietary P
utilization)." This conclusion, however, seems to disagree with the other studies discussed above. Walter et al.
(2000) fed rats on corn-soybean diet. Increasing the supplementary level of calcium carbonate proportionately
reduced apparent P absorption by the rat. The apparent Zn absorption and femur zinc concentration were also
decreased with calcium supplementation to the diet. The reduced P absorption might be due to the formation of
phytate-Ca that precipitates in the intestinal lumen and is neither absorbed not digested by luminal phytases.
Adverse effects of excess P
Numerous studies in animals and fishes showed that the absorption or bioavailability of dietary trace minerals (e.g.,
Zn, Mn) is decreased by excess Pi in the diet. Milby (1933) and Hammond (1936) noticed and Wilgus et al.
(1937) and Wiese et al. (1938) confirmed that dietary excess calcium phosphate induces perosis in the chick.
Wedekind et al. (1991) showed that excess P per se is the antagonizing factor of Mn absorption. Baker & Oduho
(1994) repoted that excess dietary P is anorectic unless Ca is added to the diet to keep the ratio adequate. Hossain
& Furuichi (2000a; 2000bc) reported that scorpion fish and Japanese flounder required calcium in diet for optimum
growth. The basal diet contained about 1.0% available P supplied from casein and monosodium phosphate, which
is considerably high compared with the dietary requirement of P (about 0.6% for most fishes). Interestingly, the
fish fed diets low in Ca, though growth was suboptimal, had similar body and bone P and Ca contents. Hossain &
Furuichi (1998) reported similar results with puffer fish. If a high level of available P in diet has any adverse
physiological effect, an increas ed growth rate by calcium supplementation should be attributed to as due to reduced
intestinal P absorption rather than to the nutritional essentiality of calcium. Since the capacity of renal P handling
is limited, high intakes of dietary P could lead to hyperphosphatemia, unless P is precipitated by Ca in the intestinal
lumen. Dietary Ca might effectively reduce the toxicity of excess dietary (availabl e) P to the organism. Dietary
calcium or antacid is a means to reduce renal burden. Also, the seawater is abundant in Ca that fish can absorb.
Thus, the Ca supplementation might be unnecessary if the basal diet is normal in P content. In rainbow trout,
Sugiura et al. (2000) showed that when dietary available P was higher than the requirement level in a semi-purified
low-calcium diet, the N retention-- i.e., growth rate-- of the fish was depressed. This subject has been discussed in
Ketola (1979), Gatlin & Wilson (1984), Richardson et al. (1985), Hardy & Shearer (1985), Satoh et al. (1987,
1989, 1992, 1996, etc.) for fish, and in Ammerman et al. (1995), Mertz (1987), and other textbooks for animal
species.
Physiology of P transport
Dietary P absorption occurs in the upper part of the small intestine in humans, many animals and birds (Cross et al.
1990, Breves & Schroder 1991, Danisi & Murer 1991, Schroder et al. 1996), whereas the P transporter protein
seems to be most abundant in the ileum (discuss later). This discrepancy is due to paracellular diffusion that is
dominant in the duodenum. Murdoch (1927) and Warkany (1928) report ed that infants were able to bring about
© 2000, 2005. Shozo H. Sugiura. All rights reserved.
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