Methylmalonic Acid (MMA)

What is Methylmalonic Acid (MMA)?

Methylmalonic acid (MMA) is an organic acid that accumulates when the enzyme methylmalonyl-CoA mutase cannot function properly. This enzyme requires adenosylcobalamin — an active intracellular form of vitamin B12 — as a cofactor. When B12 is insufficiently available inside cells, methylmalonyl-CoA cannot be converted to succinyl-CoA in the citric acid cycle, and MMA rises in the blood and urine. MMA is therefore a direct functional reflection of B12 status at the cellular level, not in the bloodstream. Reference values for serum MMA in adults are typically below 0.27–0.40 µmol/L, depending on the laboratory. MMA rises specifically with vitamin B12 deficiency — unlike homocysteine, which also rises with folate and B6 deficiency. The combination of elevated MMA and elevated homocysteine points specifically to B12 deficiency; elevated homocysteine with normal MMA fits more with folate or B6 deficiency. This specificity makes MMA particularly valuable at borderline serum B12 levels (roughly 150–300 pmol/L), where the clinical question is whether a functional deficiency already exists. Beyond nutritional B12 deficiency, MMA rises with rare genetic enzyme deficiencies (methylmalonyl-CoA mutase deficiency, a serious metabolic disease that presents in early childhood) and with reduced kidney function — making interpretation in CKD especially careful.

Why is Methylmalonic Acid (MMA) relevant?

MMA is the most sensitive and specific functional marker for vitamin B12 deficiency at the cellular level, and is therefore most useful when serum B12 falls in the grey zone (150–300 pmol/L). In that range, the serum B12 value is insufficient to determine whether functional deficiencies already exist: the blood may show circulating B12 that is not biologically active (bound to haptocorrin, a less functional transport protein). MMA cuts through that ambiguity by measuring whether the cellular biochemistry is actually working. The clinical added value of MMA is greatest in people with symptoms consistent with B12 deficiency but borderline serum B12: unexplained fatigue, subacute combined degeneration (tingling, myelopathy), macrocytic anaemia, or cognitive changes. In such cases a normal serum B12 can mask a functional deficiency that MMA reveals. For groups at elevated risk of B12 deficiency — older adults, vegans, people after gastrectomy or bariatric surgery, long-term metformin users, people with IBD — MMA is a logical complement to routine B12 testing. MMA also has value for monitoring supplementation: with effective B12 treatment (oral or intramuscular), MMA typically falls to the reference range within 2–4 weeks, while homocysteine normalises somewhat more slowly and serum B12 with intramuscular supplementation can appear high without cellular status yet being restored. MMA is therefore an objective measure of therapeutic response.

Methylmalonic Acid (MMA) high or low — what it means

MMA is interpreted in combination with serum B12, holotranscobalamin (active B12), and homocysteine for a complete B12 profile. The key combinations: elevated MMA + elevated homocysteine + low/borderline B12 = cellular B12 deficiency; elevated homocysteine + normal MMA + low folate = folate deficiency; elevated MMA + low B12 but normal homocysteine = less common, possibly early B12 deficiency before homocysteine rises. Kidney function is a key confounder: with declining eGFR (< 60 mL/min/1.73m²), MMA accumulates independently of B12 status because the kidney contributes to its clearance. In patients with CKD, MMA is therefore less useful as a pure B12 marker — holotranscobalamin is a more reliable alternative in that group. Always check creatinine and eGFR when interpreting MMA. After starting B12 supplementation, re-testing MMA is most meaningful after 4–8 weeks: sufficient time for cellular recovery with oral supplementation. With persistent elevation despite adequate supplementation, non-compliance, malabsorption (pernicious anaemia with anti-intrinsic-factor antibodies), or an underlying genetic cause are to be considered. Urine MMA can be additionally diagnostically useful in ambiguous cases.

Educational information only — not medical advice. Consult a healthcare professional for clinical decisions.