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Waltham Center for Pet Nutrition, Melton Mowbray, Leics, LE14 4RT, UK
Dear Editor:
The letter from Drs. Rogers and Morris in response to the recently published study of cat protein oxidation (1
) reviews the current state of knowledge concerning feline protein catabolism. Application of such a variety of approaches to the subject of protein requirements should, however, be conducted cautiously because of the variety of diets (protein level, carbohydrate content) nutritional states (fed, food-deprived, time adapted to diet) and developmental ages (kitten and cat) used, and the relevance of each study considered carefully. In looking at the whole picture, there are several points that need clarifying that should throw light on how these findings relate to the feline protein requirement and indicate areas that require further research.
Cats (and other carnivores) apparently require more dietary protein than omnivores and herbivores, and it has been suggested that the reason for this is an inability of the cat to adapt its hepatic enzymes to dietary protein level (2
). What this meant in functional terms to the cat was unclear, with enzymes described in terms of activity. It was subsequently indicated by Drs. Rogers and Morris, and previously (1
), that such in vitro assays describe the maximal enzyme activity, i.e., equivalent to capacity in vivo and not flux through the enzyme pathways. In their recent letter, Drs. Rogers and Morris suggested that a permanently high hepatic catabolic capacity would confer a limited ability to adapt to a low protein diet, resulting in increased nitrogen loss and a high requirement for dietary protein. However, this theory may not be valid, as discussed below.
As pointed out by Drs. Rogers and Morris and previously (1
), it is necessary to test any theory of a lack of metabolic flexibility at very low protein intakes, below the minimum requirement for maintenance (10–11% dietary energy) (3
,4
). Unfortunately, this has not been satisfactorily achieved in previous studies including ours where the lowest dietary protein level was 20% energy (1
,5
), and the original study where the lowest level was 13% energy (2
). This means that all these results were obtained in cats fed adequate protein diets and so the whole question of adaptability at low protein intakes is unknown and requires further study.
This problem is difficult to address because, unlike omnivores and herbivores, adult cats are often unwilling to eat a very low protein diet (6
). It is certainly a challenge to produce a very low protein diet that will maintain adult cats in the long-term, with no loss of appetite or body weight. To our knowledge, there is no published evidence to the contrary. As well as being an indicator of diet acceptance, maintenance of body weight is critical in nitrogen balance (7
,8
) and other protein metabolism studies, because the associated catabolism of body tissues may distort the results. Unlike omnivores and herbivores, substantial body weight losses are often observed in cat studies and may contribute to overestimation of nitrogen requirements (6
,9
–11
) and increased obligatory nitrogen losses (12
). This may account for some of the apparently high protein requirements of this species.
The subject of adaptability of enzyme activity also deserves further thought. As pointed out by Drs. Rogers and Morris, there are at least four mechanisms available for altering hepatic enzyme activity, including up- and downregulation of the amount of ureagenic enzymes (changes in capacity). These latter changes in enzyme amount are readily demonstrable in vitro in rats (13
,14
) but not in cats (2
). However, it was recently suggested that such changes in enzyme amount in other species after a change in nitrogen intake may in fact be a result of, rather than a cause of, the observed changes in ureagenesis (15
). This means that the observed changes in enzyme capacity in other species may not be necessary for adaptation to diet, but rather, occur after the change in diet.
The calorimetric (1
) and urea kinetic (5
) data indicate that although enzyme capacity does not change in cats (2
), the absolute amount of protein catabolized does vary. Thus, the various mechanisms described by Drs. Morris and Rogers combine to give metabolic flexibility and adaptation so that cats can accommodate intakes where protein varies between 20 and 70% without having to alter total enzyme content (capacity). It remains possible that such an ability to handle high protein intakes (that would be problematic for omnivores and herbivores) has a penalty, in that a higher basal oxidation of protein occurs in cats. This really is a question of the limit of downregulation of enzyme activity rather than concerns about upregulation, i.e., the difference is then between adapting to a low protein diet rather than the ability to cope with a high protein diet.
The lower limit of adaptability to nitrogen intake is dependent upon the obligatory nitrogen loss, which is the essential background nitrogen metabolism that must continue even on a protein-free diet (and this is different from the situation during food deprivation). The obligatory nitrogen loss has been measured in cats with highly variable results (10
–12
), and certainly warrants further investigation. Thus, the "minimum level of catabolism" is defined by the nature of the metabolic pathways supplying nitrogen to the enzymes, and not the capacity of the enzymes themselves. This means that a permanently high catabolic capacity may not be the reason for cats’ high protein requirements.
In summary, for adult cats fed adequate protein diets, there is apparently no change in hepatic enzyme capacity (2
), but evidence of metabolic reaction (increased oxidation and ureagenesis) to increased dietary nitrogen intake (1
,5
). This suggests that cats may react, rather than adapt, to dietary protein intake but with the same net result. How these findings relate to the minimum protein requirement of cats remains a theoretical debate because the definitive study in cats successfully maintained on a very low protein diet has not been reported. The definitive study should carefully consider provision of dietary carbohydrate (to minimize gluconeogenesis) and provision of energy (to minimize weight loss), as well as using protein levels at the lower end of the range tolerated by omnivores and herbivores (e.g., 6% energy). Further work is required using very low protein diets to determine exactly why cats have an apparent need for so much protein.
Manuscript received 1 June 2002. Revision accepted 5 June 2002.
LITERATURE CITED
1. Russell, K., Murgatroyd, P. R. & Batt, R. M. (2002) Adaptation of net protein oxidation to dietary protein intake in the domestic cat (Felis silvestris catus). J. Nutr. 132:456-460.
2. Rogers, Q. R., Morris, J. G. & Freedand, R. A. (1977) Lack of hepatic enzymatic adaptation to low and high levels of dietary protein in the adult cat. Enzyme 22:348-356.[Medline]
3. Burger, I. H., Blaza, S. E., Kendall, P. T. & Smith, P. M. (1984) The protein requirement of adult cats for maintenance. Feline Practice 14:8-14.
4. National Research Council (1986) Nutrient Requirements of Cats 1986 National Academy Press Washington, DC. .
5. Russell, K., Lobley, G. E., Rawlings, J., Millward, D. J. & Harper, E. J. (2000) Urea kinetics of a carnivore, Felis silvestris catus. Br. J. Nutr. 84:597-604.[Medline]
6. Zentek, J., Dekeyzer, A. & Mischke, R. (1998) Influence of dietary protein quality on nitrogen and some blood parameters in cats. J. Anim. Physiol. Anim. Nutr. 80:63-66.
7. Hegsted, D. M. (1976) Balance studies. J. Nutr. 106:307-311.
8. Young, V. R. (1986) Nutritional balance studies: indicators of human requirements or of adaptive mechanisms?. J. Nutr. 116:700-703.
9. Allison, J. B., Miller, S. A., McCoy, J. R. & Brush, M. K. (1956) Studies on the nutrition of the cat. North Am. Vet. 37:38-43.
10. Miller, S. A. & Allison, J. B. (1958) The dietary nitrogen requirements of the cat. J. Nutr. 64:493-501.
11. Greaves, J. P. & Scott, P. P. (1960) Nutrition of the cat: protein requirements for nitrogen equilibrium in adult cats maintained on a mixed diet. Br. J. Nutr. 14:361-369.
12. Hendriks, W. H., Moughan, P. J. & Tarttelin, M. F. (1997) Urinary excretion of endogenous nitrogen metabolites in adult domestic cats using a protein free diet and the regression technique. J. Nutr. 127:623-629.
13. Das, T. K. & Waterlow, J. C. (1974) The rate of adaptation of urea cycle enzymes, aminotransferases and glutamic dehydrogenase to changes in dietary protein intake. Br. J. Nutr. 32:353-373.[Medline]
14. Schmike, R. T. (1962) Adaptive characteristics of urea cycle enzymes in the rat. J. Biol. Chem. 237:459-468.
15. Waterlow, J. C. (1999) The mysteries of nitrogen balance. Nutr. Res. Rev. 12:25-54.
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