[Determination of metabolism-oriented methionine requirements using 35 S methionine]

Experimental rats (weighing 50-100 gm) received semisynthetic diets containing 8%, 10%, or 12% of crude protein (Soya protein). These were supplemented with graded amounts of L- or DL methionine. After a 5-day feeding period the rats were injected 35S methionine. Subsequently, the levels of urinary 35S excretion were determined over a period of 4 days after methionine injection. The level of urinary 35S excretion was found to be clearly increased if methionine supplementation exceeded the methionine requirements of the animals. Supplementation with 0.15% methionine was just enough for diets containing 8-10% crude protein. 0.2% methionine had to be supplemented to meet the methionine requirements of the animals if the diet contained 12% crude protein. Requirements for the content of sulfur-containing amino acids in the protein were shown to be independent of the protein content of the diet, and were found to vary between 4.4% and 4.7% of the crude protein. The needs for methionine supplementation were independent of the fact whether L methionine or DL methionine was added. It is definite advantage of the present method that methionine demands are determined in close correlation with metabolic processes, including the maintenance metabolism.