Review: Phosphorus in Fish Nutrition

maturing salmon. Chang et al. (1960) in his paper neither discussed about their data of low P levels in skeletal
tissues nor compared their values with data reported by others (e.g., Shearer, 1984; Poston & Ketola, 1989). The
data showed that the total P concentration (µg P/g tissue) of the head (with skin, bones and tail) was only about 1/10
to 1/5 of other parts of the body (flesh, gonads, GI tract, liver, or kidney). In humans, skeleton is the storage site of
P and Ca, and skin seems to serve as an irretrievable store of Pi (Anderson & Barrett 1994). Satoh et al. (1984)
analyzed the content of ash, Ca, P and other minerals in the whole body of tilapia during 82 days of starvation (at 25
and 15°C). During this period, fish weight decreased from 43.8 to 37.5 g, body protein from 16.5 to 13.6%, body
fat from 8.3 to 2.9%, while body moisture increased from 70 to 77%. Ash, Ca and P contents (per wet whole body)
all increased, since the fish became skinny or bony on starvation. The Ca/P ratio in the whole body increased
during starvation, suggesting that fish lost muscular tissues that contained P but only trace amounts of Ca.
The body fat content is quite variable depending on the nutritional status and also on the age of the animal,
but total of fat and water contents in body is relatively constant since the amount of fat is inversely related to the
amount of water in the body (Kleiber 1975; Papoutsoglou & Papoutsoglou 1978; Bondi 1987; Shearer 1994).
Body mineral contents, therefore, should be expressed on a wet body basis (not dry body basis). Shearer (1984)
showed that whole body P content of rainbow trout of various sizes (life stages) is much more constant when
expressed as wet body basis than as dry body basis. A similar observation was reported by Satoh et al. (1987) with
rainbow trout. They also reported that Ca and P contents in fish body were higher when diets were not
supplemented with Zn than when they conteined supplemental Zn. In fish, body P concentration ranges between
3.8 and 4.5 g per kg body weight with a few data below 3.0 and above 5.0 depending on the fish P status (reviewed
in Wiesmann et al. 1988, Lall, 1991). Satoh et al. (1996) showed that whole body P of rainbow trout increased
from 0.41 to 0.50%, which corresponded to the dietary P level (5 graded levels from 1.2% to 3.0% P in mostly
available form; Note that the dietary requirement is only ~0.6%). In contrast, the vertebral P content decreased
from 10.3 to 8.7% (fat-free dry basis) as the dietary P level increased. According to Bondi (1987), the proportion
of each mineral, when expressed as percentage of “ fat-free dry body substance”, seems to be very similar among
species in adult mammals.
Factorial approach
The factori al approach has been discussed by some workers to 'estimate' the dietary P requirement in fish. Pfeffer
& Pieper (1979) derived the requirement value from: Gross requirement = (Amount retained + Endogenous
loss)*100 / Availability. Shearer (1995) considered Dietary requirement = Gross requirement / Feed consumed.
Nakashima & Leggett (1980) used the following formula to estimate P surplus (=urinary P excretion): P surplus = P ingestion
(food) – P growth – P egestion (feces) – P maintenance. From this, the dietary P requirement (P ingestion when P surplus is 0) =
P growth + P maintenance + P egestion. The amount retained by the fish and the availability can easily be found by analyzing
body P content and by measuring the absorption. The most uncertain factor in the above formulas is the
endogenous loss or the maintenance requirement. The first researchers measured the endogenous loss using
starving fish with some reservation that the data of starving fish might be too low due to "sparing mechanisms" by
fasting. The second researcher used the fi rst researchers' data. The third researchers used data of Kitchell et al.
(1975) who determined the maintenance requirement of P for juvenile bluegills to be 5.4mg P/g body P per day.
The calculated values by the third authors for juvenile yellow perch, however, showed that the requirement for
maintenance was higher than the requirement for growth (ret ention). This is unlikely if fish are fed normally and
growing normally. P-requirement for maintenance (endogenous obligatory loss) may not be determined on
starving fish since starvation is a catabolic state. Growing fish require minerals, but starving fish do not. The
obligatory loss of P is lower during growth than during fast, which is generally known in higher animals and humans
(see Part 1, Endogenous loss). The obligatory nonfecal P loss by rainbow trout fed P-deficient diet was report ed
undetectabl e (Sugiura et al. 2000). Theoretically, fish lose P in an amount that corresponds to obligatory N loss
from the wasting muscle during starvation. Fecal obligatory loss is also negligible compared with dietary P intakes
becaus e the apparent availability of highly available P sources such as sodium or potassium phosphates is 95-98%.
If the obligatory fecal loss is significant, the apparent P availability cannot be high. The above figures indicate that
only up to 2-5% of dietary P (at near requirement level) may be attributed to the obligatory endogenous loss. This
level will be lower if the true absorption of the P compound is lower than 100%. If the apparent absorption is used
in the above formulas to calculate the requirement, the fecal obligatory loss must be ignored and only the nonfecal
loss will be the factor of argument, which is, as mentioned above, close to zero. Schwarz (1995) wrote that the
factori al deduction of requirements for minerals can provide only approximate requirement values, and that the
dose-respons e technique is the best option to estimate requirements of minerals.
© 2000, 2005. Shozo H. Sugiura. All rights reserved.
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