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Methylmalonic acid (MMA) is a metabolic byproduct that accumulates in the blood and urine specifically when vitamin B12 is functionally deficient at the cellular level. MMA is a far more sensitive indicator of B12 deficiency than serum B12 itself — many people have a 'normal' B12 level yet elevated MMA, revealing tissue-level deficiency that causes neurological damage before anemia appears. It is particularly valuable in older adults and people taking metformin or acid-suppressing medications, where B12 deficiency is common but often missed on standard panels.
Also known as: Methylmalonate, Methylmalonic acid, MMA
High MMA is the most sensitive marker for functional B12 deficiency, elevated before nerve damage occurs. Low MMA confirms adequate B12 status.
Standard flag is above 378 nmol/L. Functional practitioners target below 250 nmol/L. MMA is superior to serum B12 for detecting true cellular deficiency.