B12 keeps your nerve insulation and DNA chemistry running. When it runs short, two compounds pile up in your blood: methylmalonic acid (MMA) and homocysteine. They climb while total B12 still sits in the normal range, because the standard number can't see the active fraction dropping. The lab looks fine while the deficiency is real.

The nerves can go first and the blood second, sometimes never. Of 141 patients with B12-deficiency nerve and psychiatric symptoms, 28% had no anemia at all ¹. Roughly 3 to 6% of adults run biochemical deficiency on metabolite testing with a "normal" serum number ². A borderline result plus a high MMA is the fingerprint ³.

Five groups carry most of the risk. Strict vegans, since animal foods are the only real natural source. Adults over 60, whose stomachs release less B12 from food. Long-term metformin users. Long-term acid-blocker users. Anyone with pernicious anemia or gut surgery. If you're in one with unexplained fatigue, tingling, or fog, serum B12 alone is the wrong test.

The pathway is a test, then a pill.

When the diagnosis is right, the fix is one of the cleanest in medicine. Fresh red cells flood out of the marrow within a week ⁵. The afternoon crash softens by week two. The pins-and-needles retreat over the next month, and the low mood lifts alongside ¹. Cognitive fog is slower. At the year mark, most people say the same thing: they hadn't realised they'd been running on fumes.

Catch it early, because late doesn't fully reverse. Recovery scales with how long symptoms ran: treated within weeks, people recover fully; treated past a year, many keep a residual deficit for good ⁶. The spinal-cord tract that tells you where your foot is angled repairs slowly, if at all.