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The most sensitive sign of a B12 shortage inside the cells — measured before the first dose of B12, and read together with the homocysteine.
Also known as: methylmalonic acidemia · elevated methylmalonic acid (MMA)
Open in the tool — with search and all the links →This compilation is for information and replaces neither medical diagnosis nor treatment. If you have symptoms, see a doctor.
Two very different things hide behind this name. One is a rare inherited metabolic disease in which an enzyme is missing; it shows itself in the first days or weeks of life with poor feeding, vomiting, acidosis and listlessness, is looked for by newborn screening and accompanied lifelong with a special diet and an emergency plan. The other is the reason this value appears in everyday life at all: methylmalonic acid rises with a beginning vitamin B12 deficiency, and does so before the B12 in the blood becomes abnormal. It is thus the most sensitive available sign that B12 is missing inside the cells and not merely looking low in the blood. It is especially valuable in the grey zone, where the B12 value is neither clearly low nor clearly normal. Together with homocysteine it also gives a clean distinction: both raised speaks for B12, homocysteine alone for folate. Two pitfalls belong with it — reduced kidney function and bacterial overgrowth of the small bowel also lift the value. And it is measured before the first dose of B12, because afterwards it falls quickly.
After an injection or a few tablets the value falls quickly and no longer answers the question. Treating first and measuring afterwards leaves one knowing nothing.
The pair separates B12 from folate: both raised speaks for B12, homocysteine alone for folate. Read singly, either value occasionally misleads.
A limited kidney lifts the methylmalonic acid without any B12 shortage. Without the kidney value beside it this test produces false diagnoses.
Bacteria in the small bowel use up B12 and at the same time produce methylmalonic acid. Bloating and changing stools make this explanation likely.
The value normalises within a few weeks if B12 was the reason. If it stays high, the explanation was a different one — and that is itself important information.
A permanent supplement is not a precaution here but part of the diet. The methylmalonic acid shows reliably whether the chosen amount is enough.
A raised methylmalonate shows a true B12 deficiency. A rare but fully treatable cause is a tapeworm from raw freshwater fish that takes the vitamin in the small bowel.
High-dose B12 works in the form that responds to it — which one that is, a trial over some days with measurement of methylmalonic acid shows. In the non-responsive form it is about amount: a low-protein diet with special foods, plus carnitine, which binds the accumulated acid and carries it out. What no treatment does: prevent the metabolic crisis when infection or fasting comes on top — there, early carbohydrate matters, often into the vein.
About one in fifty to a hundred thousand newborns is affected; in Switzerland the newborn screening looks for it. The first crisis often comes in the first days or weeks of life, triggered by protein intake. Whether a form responds to B12 is shown by a trial over ten to fourteen days with measurement of methylmalonic acid. With infection or fasting the requirement rises at once — here hours count, not days.
Neither clearly low nor clearly normal, together with complaints that would fit. Exactly here the methylmalonic acid decides — otherwise one stays in guesswork.
Numb feet, an unsteady gait and forgetfulness with an unremarkable B12 are the classic occasion for this value. The nerves suffer earlier than the blood count.
Without supplementation a shortage over the years is the rule, not the exception. Methylmalonic acid shows it earlier than any other value.
All three reduce B12 uptake over years. With them a check is worthwhile before complaints appear.
The rare inherited form shows itself in the first days of life. It is looked for by newborn screening and followed in specialised centres.
Rises only with B12 deficiency, not with folate deficiency. That is exactly what makes it the decisive value when the blood count alone cannot tell the two apart.
The familiar value, but unreliable in the grey zone: part of the measured B12 is bound to a protein the cells cannot use at all.
The share of B12 that actually reaches the cells. It falls earlier than the total B12 and complements the methylmalonic acid well.
Rises with both B12 and folate deficiency. Only paired with methylmalonic acid does it become a distinction: both high speaks for B12, this one alone for folate.
The main pitfall. With declining kidney function the methylmalonic acid rises without any B12 deficiency — so this value always belongs read beside it.
The metabolism goes off the rails with infections, fasting and fever — exactly when the body breaks protein down. This is why these children have an emergency plan, and why with an infection getting energy in counts more than rest.
In an emergency the point is to stop the breakdown of the body’s own protein — with sugar through the vein, if need be together with insulin. That is the opposite of what one otherwise does with a sick child, and the reason for the emergency card.
B12 is mainly in animal foods.
Methylmalonic acid rises when B12 is missing in the tissue — a more sensitive value than B12 in blood itself. In the inherited metabolic form this does not apply; that belongs in specialised care.
AWMF 027-067 · S3 · 2025-04 — in force
Methylmalonazidämien und Propionazidämien
checked 2026-09