ORIGINAL ARTICLE Inter-related effects of dietary fat and protein level on growth performance in rabbits A. Alhaidary, H. E. Mohamed and A. C. Beynen Department of Animal Production, College of Food and Agricultural Sciences, King Saud University, Riyadh, Saudi Arabia Introduction The isoenergetic replacement of dietary carbohy- drates by corn oil has been found to enhance growth in rabbits fed restricted, isoenergetic amounts of feed (Van Manen et al., 1989). It would appear that iden- tical amounts of metabolizable energy in the form of either corn oil or carbohydrates have a differential effect on growth efficiency. Possibly, dietary corn oil is more effective than carbohydrates in reducing pro- tein catabolism and thus sparing it for growth. In rabbits with free access to feed, the isoenergetic sub- stitution of corn oil for carbohydrates in the diet also stimulated weight gain, whereas the effect of corn oil was greater than that of coconut fat (Alhaidary et al., 2010). The idea that dietary corn oil spares protein for growth more effectively than do carbohydrates has a biochemical basis. Corn oil is rich in the omega-6 polyunsaturated fatty acid, linoleic acid. The feeding of the polyunsaturated fatty acid, eicosapentaenoic acid, to tumour-bearing mice suppressed protein catabolism in muscle through an inhibition of the ATP-ubiquitin-dependent proteolytic pathway (Whitehouse et al., 2001). A protein sparing effect of polyunsaturated fatty acids has also been demon- strated in burned rats fed enterally (Selleck et al., 1994) and in surgical infants fed intravenously (Pierro et al., 1988). The inhibitory effect of fatty acids on proteolysis may be specific for polyunsaturated fatty acids. The saturated fatty acid, palmitic acid and the monounsaturated fatty acid, oleic acid, have been shown to enhance protein degradation by activating the E3 ubiquitin ligases (Zhou et al., 2007). This would explain that additional corn oil induced greater weight gain in rabbits than did coconut fat which is rich in saturated fatty acids (Alhaidary et al., 2010). Keywords rabbits, corn oil, casein, growth performance, nitrogen retention Correspondence Ahmed Alhaidary, PhD, Department of Animal Production, College of Food and Agricultural Sciences, King Saud University, PO Box 2460, Riyadh 11451, Saudi Arabia. Tel: +966 14678475; Fax: +966 14678474; E-mail: ahaidary@ksu.edu.sa Received: 3 January 2010; accepted: 17 May 2010 Summary Isoenergetic substitution of dietary corn oil for dietary carbohydrates enhances growth in rabbits. It was hypothesized that identical amounts of metabolizable energy in the form of corn oil are more effective than those of carbohydrates in reducing protein catabolism and thus sparing it for growth, which would imply that the fat effect is greater on a mar- ginal than normal protein diet. Young growing rabbits were fed semi- purified diets either relatively high (21.6 energy % protein) or low in casein (13.0 energy % protein) to which extra corn oil (21.1 instead of 5.3 energy %) was added at the expense of an isoenergetic amount of corn starch and dextrose. The addition of corn oil to the diet with 21.6 energy % protein indeed increased weight gain, but the addition to the diet with 13.0 energy % protein left weight gain unchanged. These results refute our hypothesis, because the low-protein intake was not limiting growth. The enrichment of the high-protein diet with extra corn oil did not affect nitrogen retention, whereas urinary nitrogen excretion was increased. These observations also are at variance with the idea that additional fat would spare nitrogen for growth. DOI: 10.1111/j.1439-0396.2010.01044.x Journal of Animal Physiology and Animal Nutrition 95 (2011) 223–227 ª 2010 Blackwell Verlag GmbH 223