Increasing dietary crude protein does not increase the essential amino acid requirements of kittens*

Abstract

Essential amino acid (EAA) requirements of omnivores and herbivores (e.g. chicks, lambs, pigs and rats) are directly related to the concentration of dietary crude protein (CP). When an EAA is limiting in the diet, addition of a mixture of EAA lacking the limiting one (which increases dietary CP) results in a decrease in food intake and weight gain. This interaction has been referred to as an AA imbalance and has not been studied in depth in strict carnivores. The objectives of these experiments were to examine the effects on growing kittens (2-week periods) of the addition to diets of a mixture of AA lacking the limiting one. The control diets were at the requirement of the respective limiting EAA (or about 85% of the 1986 National Research Council requirement). In experiment 1, with the dietary EAAs at the minimally determined requirements, the concentration of the essential or dispensable amino acids was increased to determine if CP or an EAA was limiting. Results of growth rates (n = 12) and plasma AA concentrations indicated that tryptophan was limiting, but increased body weight gain also occurred when the concentration of CP was increased as dispensable amino acids without additional tryptophan. Experiment 1 was repeated in experiment 2 using a crossover design. Again, when tryptophan was limiting additional concentrations of dispensable AAs increased body weight gain. This response is the opposite of that in herbivores and omnivores. Experiment 3 consisted of 10 separate crossover trials, one for each of the 10 EAA and examined the effect of two concentrations of dietary CP (200 and 300 g CP/kg diet) on body weight gain of kittens (n = 8) offered diets limiting in each respective EAA. Body weight gain was numerically greater when diets contained 300 g CP/kg than 200 g CP/kg for eight of 10 EAAs (p < 0.05 for only isoleucine and threonine) when each amino acid was limiting. This response is the reverse of that which occurs in chicks, lambs, pigs and rats when an EAA is limiting and dietary CP lacking the limiting EAA is increased. These results indicate that the EAA requirements of kittens are not positively correlated with dietary CP concentrations.

Increasing dietary crude protein does not increase the methionine requirement in kittens*†‡

“…It is important from a metabolic point of view, that as the CP in the diet decreases below the requirement, the EAA requirements decrease proportionately. Previous studies in our laboratory indicated that this may not be true for dogs (Delaney et al., 2001) or for kittens (Strieker et al., 2006a) for certain EAA. Thus, the response of carnivores when compared with ominvores or herbivores appears to differ.…”

“…If an EAA is limiting and dietary CP is increased without the limiting amino acid, an amino acid imbalance occurs, and weight gain is depressed, primarily as a result of decreased food intake (Harper et al., 1970). Previous experiments with kittens (Strieker et al., 2006a) showed that when isoleucine, leucine, methionine (met) or threonine were in turn limiting, increasing dietary CP did not create an imbalance, but rather increased weight gain. These observations contradicted those found in other species and suggested that the EAA requirements of kittens may not be positively correlated with the concentration of dietary CP.…”

“…The objectives of this study were to examine the performance of kittens given diets containing increasing concentrations of met along with increasing concentrations of CP, and to determine whether the met requirement of kittens was correlated with the concentrations of dietary CP. Met was chosen for this study, because it is most likely to be limiting in commercial cat foods and also the weight gain response to the addition of CP to the diet was greater when met was limiting than for the other amino acids studied (Strieker et al., 2006a).…”

Protein Requirements

“…The ability to upand down-regulate these catabolic enzymes provides a mechanism by which potentially toxic amino acids can be catabolized when animals are consuming highprotein diets. 42,43 In other animals, but not in cats, increasing dietary crude protein causes a concomitant increase in essential amino acid requirements. 35 The activities of three urea cycle enzymes and seven nitrogen catabolic enzymes in the liver were then measured.…”

Cats Select for Adequate Methionine but Not Threonine

“…The minimal Met requirement for adult cats to maintain nitrogen balance is ;2 g/kg of diet (19), but it is not known whether this quantity optimizes all metabolic needs for Met. However, with 6 g of Cys/kg of diet, the diet containing 4 g of Met/kg of diet provides plenty of total sulfur amino acids to sustain maximal nitrogen retention in growing kittens (20,21). To obtain information on these possibilities, we offered the adult cats a ''fully adequate'' dietary concentration of protein (41% crude protein that provided ;2.5 g of Cys and 9 g of Met/kg diet) with and without the addition of 2 g of free Met/kg of diet.…”