Bypass methionine supplements are about a 50:50 mixture of D-methionine and L-methionine.
Although it has been known for years that L-methionine is the one that cows use for protein synthesis, it’s been difficult to separate them out, chemically — which, if possible would give cows more of the L-methionine isomer and less of the D-methionine isomer. Researchers from Canada now have found that much of the D-methionine in these supplements is transformed into L-methionine by the cows, so D-methionine is available to the dairy cow, as well. The cow’s utilization of D-methionine is slower than utilization of L-methionine, but that could be turned into a plus, as well. Assuming that D-methionine makes it through the rumen and to the small intestine, the longer shelf life of D-methionine could be useful as it offers the opportunity to delay the clearance of the absorbed methionine and act as potential reservoir for L-methionine protein synthesis, say the Canadian researchers.