The Definitive B12 Deficiency Test
MMA is a metabolite that accumulates when B12 is deficient. While serum B12 levels can be borderline or misleading (normal B12 doesn't always mean adequate B12 function), MMA is elevated only when there's true functional B12 deficiency at the cellular level. It's the most sensitive and specific test for B12 deficiency.
What is Methylmalonic Acid (MMA)?
MMA is a dicarboxylic acid that requires B12 (as adenosylcobalamin) for its conversion to succinyl-CoA by methylmalonyl-CoA mutase. Elevated MMA is specific for B12 deficiency (unlike homocysteine which rises with both B12 and folate deficiency). Sensitivity >95% for B12 deficiency.
↑ What High Methylmalonic Acid (MMA) Means
Functional B12 deficiency—even if serum B12 looks "normal." B12 is needed to convert MMA to succinyl-CoA. Without enough B12, MMA accumulates. Also elevated in kidney disease (reduced clearance).
Common symptoms:
B12 deficiency symptoms: numbness/tingling (peripheral neuropathy), fatigue, weakness · Macrocytic anemia (large red blood cells) · Cognitive changes, difficulty walking (subacute combined degeneration) · Glossitis (swollen tongue)
↓ What Low Methylmalonic Acid (MMA) Means
Not clinically significant.
Common symptoms:
Not significant
Why It Matters
When normal:
Most sensitive test for B12 deficiency
Detects functional deficiency even with "normal" serum B12
Specific for B12 (not elevated in folate deficiency)
Resolves borderline B12 levels
Risks if abnormal:
Elevated: true B12 deficiency requiring treatment
False positive: kidney disease (reduced clearance)
Untreated B12 deficiency: irreversible neuropathy
What Can Cause Abnormal Levels?
B12 Deficiency
75% likelyDietary (strict vegan), pernicious anemia (autoimmune), malabsorption (celiac, Crohn, gastric surgery, metformin).
Kidney Disease
20% likelyReduced renal clearance of MMA. False positive.
Pernicious Anemia
Autoimmune destruction of gastric parietal cells → no intrinsic factor → can't absorb B12.
Metformin Use
Metformin reduces B12 absorption in 10-30% of users.
Elderly/Atrophic Gastritis
Reduced acid production impairs B12 liberation from food.
What You Can Do
If MMA elevated: B12 supplementation regardless of serum B12 level
Impact: Elevated MMA = functional deficiency. Treat it. \u00B7 Timeline: 4-8 weeks
B12: 1000-2000mcg daily sublingual or oral
Impact: High-dose oral can overcome even absorption issues \u00B7 Timeline: 4-8 weeks
If lifestyle changes aren't enough:
B12 injections (1000mcg IM) if severe deficiency or neurological symptoms
Impact: Bypasses absorption entirely \u00B7 Timeline: Weekly x4, then monthly
Recheck MMA 2-3 months after starting B12
Impact: Should normalize if B12 was the cause \u00B7 Timeline: 2-3 months
Recommended retest: 2-3 months after starting B12 supplementation
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