Leave this vitamin gap long enough and the nerve damage becomes permanent

Vitamin B12 deficiency does not announce itself. The body stores between 1,000 and 2,000 times a typical day’s intake, mostly in the liver, so a shortfall can take years to turn into a symptom anyone would notice. By the time it does, the first signs are ordinary enough to miss: fatigue, light-headedness, a racing heart, a sore red tongue. Dr. Howard LeWine, chief medical editor at Harvard Health Publishing, writes that a deficiency left uncorrected long enough can cause irreversible damage to nerve cells — a distinct, later-stage consequence separate from the anemia most people associate with the vitamin.

That distinction matters because the two problems arrive on different timelines and do not always overlap. Megaloblastic anemia, the blood disorder that gives vitamin B12 deficiency its clinical name, responds to treatment within days once diagnosed. Nerve damage develops more slowly, after low B12 levels persist for an extended stretch, and it is the symptom category most likely to still be present after the anemia itself has been corrected.

Pernicious anemia is the most common cause, and it clusters by ancestry and age

Harvard Health Publishing’s overview of the condition identifies an absent intrinsic factor, the protein the stomach secretes to carry B12 into the intestine for absorption, as the most common cause of vitamin B12 deficiency, a condition also called pernicious anemia. It usually traces back to atrophic gastritis, a thinning of the stomach lining that is more common in older adults of African American or Northern European descent, in whom pernicious anemia tends to appear around age 60. A rare inherited version produces juvenile pernicious anemia in children younger than 10.

Pernicious anemia also clusters with other autoimmune conditions, including Graves’ disease, an underactive thyroid, thyroiditis, vitiligo and Addison’s disease, a pattern consistent with an immune-system origin rather than a simple dietary gap. Stomach or intestinal surgery, and gut conditions that let bacteria overgrow and consume B12 before the body can absorb it, account for most of the remaining non-dietary cases.

Vegans rarely run short quickly, because the liver banks years of supply

Diet-related deficiency is the exception rather than the rule. The National Institutes of Health’s Office of Dietary Supplements notes that the body stores 1,000 to 2,000 times as much vitamin B12 as a person typically eats in a day, so someone who stops eating meat, fish, eggs and dairy without supplementing can take years to become deficient. The same agency estimates that vitamin B12 deficiency still affects somewhere between 3% and 43% of older adults, a wide range that reflects how differently studies define a low level, and advises that people over 50 get most of their B12 from fortified foods or supplements rather than meat and dairy, since the crystalline form in those products skips the stomach-acid step that food-bound B12 requires.

Older adults make up a disproportionate share of cases for a separate reason: reduced stomach acid production lowers how efficiently the body releases B12 from food in the first place, independent of any autoimmune process. Mayo Clinic’s own guidance suggests older adults may need to consume 10 to 12 micrograms of vitamin B12 daily, well above the standard adult recommendation of 2.4 micrograms, to compensate for that drop in absorption efficiency.

LeWine’s symptom list only appears once the body’s reserve runs out

Early deficiency produces symptoms that could describe dozens of unrelated conditions: weakness and fatigue, light-headedness, heart palpitations, shortness of breath, a sore tongue with a red, beefy appearance, nausea, weight loss, diarrhea and a yellowish tinge to the skin and eyes. Dr. Howard LeWine’s overview lists a separate, later set of symptoms that only shows up once low B12 levels have persisted long enough to damage nerve cells directly: numbness and tingling in the hands and feet, difficulty walking, muscle weakness, irritability, memory loss, dementia, depression and psychosis.

Harvard’s overview flags that second list specifically as potentially irreversible. The nerve symptoms can overlap with ordinary aging or with unrelated neurological conditions, which is part of why LeWine’s guidance calls for blood testing rather than diagnosis by symptom alone.

The testing itself runs through a short battery: a standard blood count that flags unusually large red blood cells, a direct measurement of B12 and folate, a methylmalonic acid test that rises specifically when B12 is short, and an antibody test that catches most people missing intrinsic factor. A bone marrow biopsy is reserved for cases where those results still leave the diagnosis unclear.

Injections work within days, but Harvard’s own prognosis carries a caveat

Treatment for confirmed deficiency is not complicated once intrinsic factor is the problem: regular B12 injections, sometimes five to seven of them in the first week for people with severe symptoms, followed by maintenance doses every one to three months for as long as the underlying absorption problem persists. A typical response — a surge in new red blood cell production — shows up within 48 to 72 hours of the first injection. Mayo Clinic notes that certain medications interfere with absorption, including the acid-reducing drugs omeprazole and lansoprazole and the diabetes drug metformin, and that even high-dose vitamin C supplements can lower how much B12 the body has available, pushing a borderline level into a deficient one without anyone changing their diet.

Harvard’s own summary calls the outlook excellent for the anemia itself, which typically responds within days of starting treatment. The caveat sits in the same paragraph: nerve cell damage can be permanent, and some residual damage to the nervous system may remain in people who did not start treatment until late in the illness.

More from Morning Overview

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.