Methionine Requirement of Kittens Given Amino Acid Diets Containing Adequate Cystine

1. Six purified amino acid diets containing 6 0 g cystine/kg and the following levels of L-methionine (g/kg diet): 21, 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. 2. 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 e methionine/ke diet for both male and female kittens and were 0–85 + 0*15 and 0*65 + 0*05 e/d resnectivelv. 3. 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.