B12 for vegans: the supplement that isn't optional

If you're vegan and you take one supplement, it's B12. There is no reliable plant source. Subclinical deficiency is common, slow to develop, and has neurological consequences if ignored long enough.

Why no plant source

B12 is made by bacteria, not plants. Animals get it from contaminated water, soil, or other animals; modern farmed animals get it from feed supplementation. Modern hygiene removed the trace bacterial sources humans used to get accidentally.

Some claims about B12 in seaweed, mushrooms, fermented soy, and unwashed garden vegetables — none of these provide bioavailable B12 in reliable amounts. They contain B12 analogs that may even interfere with real B12 absorption.

The deficiency timeline

B12 deficiency is slow. Liver stores can last 3–7 years before depletion. Most newly-vegan adults won't see lab-detectable deficiency for years, which is why it's commonly missed.

The symptoms when they arrive:

• Fatigue (early)
• Tingling or numbness in hands and feet (peripheral neuropathy)
• Cognitive changes — brain fog, memory issues
• Megaloblastic anemia (late stage)
• Permanent nerve damage if untreated for years

The frustrating part: by the time you have symptoms, you may have already accrued partly-irreversible damage. The supplement is preventive, not corrective.

Dose

The RDA is 2.4 mcg/day, but absorption efficiency drops dramatically at high doses. The practical recommendations:

• Daily: 25–100 mcg of cyanocobalamin or methylcobalamin
• Weekly: 2,000 mcg (one large dose)
• Pregnant / nursing: higher; consult your provider

The two forms (cyano- and methyl-) are both fine for most people. Methyl- is preferred by some for theoretical reasons; cyano- is cheaper and just as effective in trials. A few people with specific genetic variants do better on methyl- — anecdotally, but the evidence is mixed.

The food sources for vegans

Fortified foods are reliable B12 vehicles:

• Nutritional yeast (fortified): 8–10 mcg per 1 tbsp
• Fortified plant milks: 1–3 mcg per cup
• Fortified breakfast cereals: 1.5–6 mcg per serving
• Some fortified meat substitutes

If you eat 2 tbsp of fortified nutritional yeast a day, you're covered. Most vegans don't, which is why supplementation is the simpler approach.

Testing

The blood tests for B12 status:

• Serum B12. First-line; can miss subclinical deficiency.
• Methylmalonic acid (MMA). More sensitive; elevated in true deficiency.
• Homocysteine. Elevated in B12 (and folate) deficiency.

If you're vegan and have been for 2+ years and never tested, get all three the next time you're at the doctor.

What to do if you're already deficient

Standard treatment is high-dose oral supplementation (1,000–2,000 mcg/day) for 1–3 months, with re-testing. Severe deficiency may require B12 injections initially (administered by your physician).

If your symptoms are neurological, treat aggressively. Time matters. The literature shows incomplete reversal of long-standing peripheral neuropathy.

What CalorieScan does for vegan users

In Settings → Diet → Vegan, the app:

• Highlights B12, iron, calcium, omega-3 ALA/EPA/DHA, zinc, and iodine in your daily summary
• Surfaces fortified foods in your search results
• Reminds you about supplementation if your tracked B12 from food is consistently below 2 mcg/day

We do not assume veganism is a problem. We do assume the planning matters.

The bottom line

A daily B12 supplement costs $5/month. The downside of taking it is essentially zero. The downside of not taking it is real and slow.

Take the supplement.

B12 isn't a vegan controversy. It's a closed scientific question. Take it.