Nitrogen Balance Studies with Dogs on Casein or Methionine-Supplemented Casein

For these studies the protein reserves of dogs were brought to a low level by feeding them limiting amounts of casein as their nitrogen source. Such dogs were maintained in nitrogen equilibrium with (median values) 139 mg of casein N, 102 mg of 1% methionine-supplemented casein N, or 72 mg of 3% methionine-supplemented casein N per kilogram per day. Since others have demonstrated an equal doubling in the biological usefulness of casein N with somewhat more methionine, it is suggested that a 3% methionine supplement imparts maximal usefulness to casein. That slightly less methionine might be equally effective is not ruled out by these data. Calculation of the sulfur amino acid intakes of the dogs indicates them to have been maintained in nitrogen equilibrium with about 30 mg of sulfur amino acids per kilogram per day, 11% or less having been contributed by cystine.