[2] The starting sources for this are the following with the respective approximate contributions to whole body propionate metabolism in brackets:[3] The propionate derivative, propionyl-CoA, is converted into D-methylmalonyl-CoA by propionyl-CoA carboxylase and then converted into L-methylmalonyl-CoA by methylmalonyl-CoA epimerase.

[5][2] The enzyme acyl-CoA synthetase family member 3 (ACSF3) is in turn responsible for the conversion of methylmalonic acid and CoA to methylmalonyl-CoA.

It has lower specificity since 20–25% of patients over the age of 70 have elevated levels of methylmalonic acid, but 25–33% of them do not have B12 deficiency.

[13] Bacterial overgrowth in the small intestine can also lead to elevated levels of methylmalonic acid due to the competition of bacteria in the absorption process of vitamin B12.

[16] It has been shown that in these cases, methylmalonic acid levels returned to normal with the administration of metronidazole.