Methylmalonic Acid, Serum or Plasma

Methylmalonic acid (MMA) is a substance that builds up in your blood when your body cannot properly use vitamin B12. Measuring MMA in your blood is one of the best ways to detect a vitamin B12 deficiency, sometimes even before symptoms like fatigue, anemia, or nerve problems appear. High MMA levels most often mean your body is not getting or using enough vitamin B12, but they can also be caused by kidney problems or, rarely, an inherited metabolic condition. Your doctor will look at this result alongside other tests, including your vitamin B12 level and kidney function, to understand what the result means for you. This test alone does not diagnose a condition or determine treatment.

When elevated: Elevated MMA suggests functional vitamin B12 deficiency, inherited methylmalonic acidemia, or renal insufficiency. Persistent elevation in the context of adequate B12 intake may indicate an inborn error of cobalamin metabolism or methylmalonyl-CoA mutase deficiency. Elevated MMA has been associated with neurologic dysfunction, cognitive impairment, and hematologic abnormalities when related to B12 deficiency. When low: Low or undetectable MMA levels are generally not clinically significant and are consistent with adequate vitamin B12 status and normal organic acid metabolism. There are no well-established adverse clinical associations with low MMA values.

MMA is not a routine performance marker, but relevant to endurance and strength athletes who may have restricted diets (vegan, plant-based, or low-animal-protein) that increase B12 deficiency risk. B12 insufficiency impairs energy metabolism and red blood cell formation, potentially limiting oxygen delivery and recovery; elevated MMA can catch functional deficiency before fatigue or performance drops become obvious.