Methionine requirement of kittens given amino acid diets containing adequate cystine

Six purified amino acid diets containing 6.0 g cystine/kg and the following levels of L-methionine (g/kg diet): 2.1, 2.7, 3.3, 3.9, 4.5, 9.0 were presented to twelve weanling kittens (six male and six female) for six periods of 10 d each. Kittens were assigned to the diets in accordance with a 6 x 6 balanced Latin-square design. Body-weight gains of males and females attained apparent plateaux at 3.3 g methionine/kg diet and were respectively (mean +/- SEM) 22 +/- 4 and 18 +/- 2 g/d. Daily food intakes attained apparent plateaux at 2.7 g methionine/kg diet for male and female kittens and were 63 +/- 10 and 49 +/- 4 g/d respectively. Nitrogen retentions (calculated as dietary-N intake minus faecal- and urinary-N excretion) attained apparent plateaux at 3.9 g methionine/kg diet for both male and female kittens and were 0.85 +/- 0.15 and 0.65 +/- 0.05 g/d respectively. Previous work has shown that the kitten's L-methionine requirement, in a diet lacking cystine, is 7.5 g/kg diet. Our results indicate that the kitten's L-methionine requirement is 3.9 g/kg diet when 6.0 g cystine/kg is provided, thus approximately 50% of the animal's sulphur amino acid requirement can be met by cystine.