B12 is the most absorption-fragile nutrient in the diet. Your stomach has to make acid to pry it off food and a protein it rides through the gut on; break any step and uptake stops even when intake looks fine 1. Acid drops with age, so about 15% of over-60s have the atrophic gastritis that strips out the acid cells 2. Heartburn pills (Prilosec, Nexium) suppress that acid over years 3, and metformin jams the intestinal step: a 13% higher deficiency risk per year of use 4. None of it feels like anything. It just empties the tank.
The at-risk groups almost never know they're in one. Roughly half of unsupplemented vegans test subnormal 5, and about 1 in 6 over-60s can no longer pull B12 off the steak they've always eaten 6. The blood test lags too: about a quarter with real tissue deficiency show a normal serum value 1. What's at stake is the nerves, which B12 insulates; when that fails the spinal cord and long nerves slowly lose it 7.
The whole decision is one question: are you in a group, or not?
Even pernicious anemia responds to pills now: about 1% of a big oral dose crosses by diffusion, so a 2,000 ยตg tablet clears the broken pathway without an injection 8.
If you were truly deficient, recovery is fast and staged. The marrow makes normal red cells within the first week 9; fog and tongue soreness ease by week two or three; the anemia clears by two months. Nerve symptoms mend slower, over 3 to 12 months, and whatever loss remains at a year tends to stay 7. If you're supplementing to prevent deficiency you feel nothing, and that is the point: no slide through tiredness, no pins and needles, no shuffle at 70.
Read next: folate (B9), the other half of the same enzyme pathway; pernicious anemia as its own autoimmune diagnosis.
The fine print โ when to skip it, and what people get wrong
"B12 gives you energy": only if you were low; at normal levels it doesn't move fatigue 10. "Methyl beats cyano": both land in the same pool, and cyano is cheaper 1.
Folate hides the damage: high-dose folic acid fixes the anemia while nerve loss keeps progressing, so B12 gets checked first. Quitting once you feel better restarts the slide; for pernicious anemia or post-surgery it's a lifelong baseline.
- 1Green et al. (2017). Vitamin B12 deficiency. Nature Reviews Disease Primers. link
- 2Andres et al. (2004). Vitamin B12 (cobalamin) deficiency in elderly patients. Canadian Medical Association Journal. link
- 3Lam et al. (2013). Proton pump inhibitor and histamine 2 receptor antagonist use and vitamin B12 deficiency. JAMA. link
- 4Aroda et al. (2016). Long-term metformin use and vitamin B12 deficiency in the Diabetes Prevention Program Outcomes Study. Journal of Clinical Endocrinology & Metabolism. link
- 5Niklewicz et al. (2023). The importance of vitamin B12 for individuals choosing plant-based diets. European Journal of Nutrition. link
- 6NIH Office of Dietary Supplements (2024). Vitamin B12 โ Fact Sheet for Health Professionals. link
- 7Stabler SP (2013). Vitamin B12 deficiency. New England Journal of Medicine. link
- 8Kuzminski et al. (1998). Effective treatment of cobalamin deficiency with oral cobalamin. Blood. link
- 9Carmel R (2008). How I treat cobalamin (vitamin B12) deficiency. Blood. link
- 10Schloss et al. (2018). Surplus vitamin B12 use does not reduce fatigue in patients with irritable bowel syndrome or inflammatory bowel disease: a randomized double-blind placebo-controlled trial. Clinical Nutrition ESPEN. link
Related in the handbook (9)
- โ Low B12 doesn't just cause fatigue and tingling; in older or at-risk people it accelerates hearing decline too.
- โ Long-term metformin is one of the named reasons your B12 quietly runs out; it puts you in the at-risk group.
- โ Long-term acid-blockers stop you absorbing the B12 in food; that's why PPI users are an at-risk group.
- โ Folate and B12 are linked; high folate can hide a B12 shortfall, so they're worth thinking about together.
- โ If you're vegan and watching B12, put iodine on the same list; plant diets run low on both.
- โ Large red cells on a blood test are a classic B12 (or folate) clue worth chasing down.
- โ An MTHFR result doesn't change your B12 needs; check B12 on its own merits, not because of a gene variant.
- โ If you eat organ meats weekly you're already covered for B12; the supplement is for vegans and the over-60s.
- โ Zinc and B12 run low in the exact same people โ vegans, the over-60s, anyone on long-term acid-blockers โ so if you check one, check both.
Vitamin B12
A year of tablets costs about the same as one decent coffee.
One small pill, swallowed or chewed. That's the whole protocol.
Decades of clinical trials and consistent guideline backing for treating deficiency. The data is just as solid that it does nothing for people who weren't low.
If you're actually low โ older, vegan, on metformin or acid suppressants โ repletion clears the fog and weakness in weeks. If you're not low, nothing changes.
Fixes deficiency-related fatigue completely. Won't lift your energy if you weren't low to begin with โ the "B12 shot for energy" is not a thing for replete people.
Reverses the brain fog and memory slips that come with real deficiency. In people who aren't deficient, supplementation does nothing for focus.
Deficiency-driven low mood, irritability and apathy lift on repletion. No mood lift if your levels were already fine.
Catching deficiency early prevents spinal-cord and nerve damage that becomes permanent after a year. The supplement isn't a longevity tool for the rest.