Effects of excess dietary l-methionine and N-acetyl-l-methionine on growing rats.

We have determined the effects of excessive dietary intakes of L-methionine and N-acetyl-L-methionine by measuring parameters which are known to be affected by excess methionine. Male Sprague-Dawley weanling rats were fed complete diets containing 10% protein supplied as isolated soybean protein and graded levels of supplemental L-methionine (from 0.30% to 5.0%) or equimolar levels of N-acetyl-L-methionine. The basal diet without supplemental methionine contained 0.15% methionine. Rats fed diets containing 0.3% supplemental L-methionine or the equivalent levels of supplemental N-acetyl-L-methionine grew best. Higher dietary levels of either L-methionine or N-acetyl-L-methionine caused progressive decreases in weight gain. However, L-methionine at 1.8% and above tended to more severely depress growth than did equivalent amounts of N-acetyl-L-methionine. L-Methionine at levels of 1.2% and above, or equivalent levels of N-acetyl-L-methionine caused comparable hypertrophy of the spleen and comparable increases in spleen iron levels. Hematocrits were not affected by either L-methionine or N-acetyl-L-methionine at the levels used in this experiment. We have concluded that N-acetyl-L-methionine is no more detrimental than L-methionine and is less detrimental at very high levels as evidenced by weight gain.