Methylmalonic acid is a by-product of the propionate metabolism pathway.[2] The starting sources for this are the following with the respective approximate contributions to whole body propionate metabolism in brackets:[3]
Increased methylmalonic acid levels may indicate a vitamin B12 deficiency. The test is highly sensitive (those with vitamin B12 deficiency almost always have raised levels) but not very specific (those that do not have vitamin B12 deficiency may have raised levels too).[10] Methylmalonic acid is elevated in 90–98% of patients with vitamin B12 deficiency. 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. For this reason, the testing of methylmalonic acid levels is not routinely recommended in the elderly.[11]
If elevated methylmalonic acid levels are accompanied by elevated malonic acid levels, this may indicate the metabolic disease combined malonic and methylmalonic aciduria (CMAMMA). By calculating the malonic acid to methylmalonic acid ratio in blood plasma, CMAMMA can be distinguished from classic methylmalonic acidemia.[12]
Cancer
Moreover, methylmalonic acid accumulation in the blood with age has been linked with tumour progression in 2020.[13]
Bacterial overgrowth in the small intestine
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.[14][15] This is true of vitamin B12 from food and oral supplementation and can be circumvented by vitamin B12 injections. It is also hypothesized from case studies of patients with short bowel syndrome that intestinal bacterial overgrowth leads to increased production of propionic acid, which is a precursor to methylmalonic acid.[16] It has been shown that in these cases, methylmalonic acid levels returned to normal with the administration of metronidazole.[16][17]
^Giannella RA, Broitman SA, Zamcheck N (February 1972). "Competition between bacteria and intrinsic factor for vitamin B 12 : implications for vitamin B 12 malabsorption in intestinal bacterial overgrowth". Gastroenterology. 62 (2): 255–260. doi:10.1016/s0016-5085(72)80177-x. PMID4629318.