400
C. B. UOWEY AND J. R. SARQENT
sparing action occurred on the diet containing arginine but in this instance the action was even more marked. Part of the effect may be due
to a relative lack of arginine in the basal diet, the addition of this
essential amino acid enhancing growth. It is significant that a diet,
containing 20% of whole egg protein and the equivalent of 20%
protein in the form of a single amino acid, should result in growth
increments of about 80% of those occurring on a diet containing 40%
whole egg protein. It suggests that either arginine requirements have
been seriously underestimated or that some considerable quantity of
dietary protein may be spared if appropriate non-specific-nitrogenous
supplements can be found.
It has recently been shown (Vallet, 1970) that urea will exert a
sparing effect on dietary protein in mullet (Mugil sp.). Diets containing
(i) 4% protein nitrogen, (ii) 2% protein nitrogen + 2% urea nitrogen
and (iii) 4% protein nitrogen + 4% urea nitrogen were fed. Growth
rates on the first two diets were similar at 8 . 8 S ~ o and 8.36% of initial
weight per week, showing that in this fish, at a dietary protein intake of
2570, about half of the protein can be replaced by urea, the nitrogen
of which is used for amino acid synthesis. Growth rate on the third diet
was elevated to 10-1370 per week which, by comparison with diet (i)
again indicates utilization of dietary urea. The growth rate on this diet
was less than might have been expected from its nitrogen content (it
may be that the optimal dietary protein level was exceeded).
It is suggested by Vallet that the utilization of urea is rendered
possible by the action of intestinal bacteria and this appears the most
reasonable view. It has not yet been checked by, for example,
examining urea utilization of fish in which the intestinal flora had been
destroyed by incorporating antibiotics into the food.
Vallet emphasizes that optimal dietary urea levels for mullet, and
any long term effects of urea feeding, have not been evaluated. He points
out that in ruminants dietary urea levels greater than about 3% entail
the risk of hepatic and renal lesions. It should be noted that these
latter animals only metabolize urea because of the large numbers of
micro-organisms present in the rumen.
G. The biological value of food protein
It will be clear from the foregoing discussion that the utilization of
dietary protein is greatly dependent on the pattern of essential amino
acids which it provides. The more closely a food protein meets the
needs of the animal for essential amino acids the greater will
be its utilization. As accurate chemical methods are available for
analysing proteins, a first step in evaluating their nutritional value would
C. B. UOWEY AND J. R. SARQENT
sparing action occurred on the diet containing arginine but in this instance the action was even more marked. Part of the effect may be due
to a relative lack of arginine in the basal diet, the addition of this
essential amino acid enhancing growth. It is significant that a diet,
containing 20% of whole egg protein and the equivalent of 20%
protein in the form of a single amino acid, should result in growth
increments of about 80% of those occurring on a diet containing 40%
whole egg protein. It suggests that either arginine requirements have
been seriously underestimated or that some considerable quantity of
dietary protein may be spared if appropriate non-specific-nitrogenous
supplements can be found.
It has recently been shown (Vallet, 1970) that urea will exert a
sparing effect on dietary protein in mullet (Mugil sp.). Diets containing
(i) 4% protein nitrogen, (ii) 2% protein nitrogen + 2% urea nitrogen
and (iii) 4% protein nitrogen + 4% urea nitrogen were fed. Growth
rates on the first two diets were similar at 8 . 8 S ~ o and 8.36% of initial
weight per week, showing that in this fish, at a dietary protein intake of
2570, about half of the protein can be replaced by urea, the nitrogen
of which is used for amino acid synthesis. Growth rate on the third diet
was elevated to 10-1370 per week which, by comparison with diet (i)
again indicates utilization of dietary urea. The growth rate on this diet
was less than might have been expected from its nitrogen content (it
may be that the optimal dietary protein level was exceeded).
It is suggested by Vallet that the utilization of urea is rendered
possible by the action of intestinal bacteria and this appears the most
reasonable view. It has not yet been checked by, for example,
examining urea utilization of fish in which the intestinal flora had been
destroyed by incorporating antibiotics into the food.
Vallet emphasizes that optimal dietary urea levels for mullet, and
any long term effects of urea feeding, have not been evaluated. He points
out that in ruminants dietary urea levels greater than about 3% entail
the risk of hepatic and renal lesions. It should be noted that these
latter animals only metabolize urea because of the large numbers of
micro-organisms present in the rumen.
G. The biological value of food protein
It will be clear from the foregoing discussion that the utilization of
dietary protein is greatly dependent on the pattern of essential amino
acids which it provides. The more closely a food protein meets the
needs of the animal for essential amino acids the greater will
be its utilization. As accurate chemical methods are available for
analysing proteins, a first step in evaluating their nutritional value would