Of the seven gaps, this is the only one that food alone cannot close. Vitamin B12 is made by bacteria, not by plants, and the traces that turn up on unwashed vegetables, in spirulina or in tempeh are either unreliable or the wrong form. Every vegan body runs down its stores eventually; the only question is how long the liver’s reserve lasts, which is usually two to five years, and whether you notice the slide before it costs you.
That is also what makes it the most satisfying gap to close. The fix is cheap, exact, and you feel it.
The signs that B12 is already low
B12 runs the machinery that makes red blood cells and insulates nerves, so the early signs are vague and then suddenly not.
- Tiredness that eight hours of sleep does not touch, and breathlessness on stairs, because the red cells being made are too few and too large.
- Pins and needles in the feet or hands, a numb patch, a sense that your balance is slightly off in the dark.
- A sore, smooth, unusually red tongue and cracks at the corners of the mouth.
- A mind that mislays words and names, low mood, irritability.
- Pale or slightly yellow skin.
The nerve signs are the ones that matter. Anaemia from low B12 reverses. Nerve damage that has gone on for months may not fully reverse, which is why no one with tingling feet should wait for a “natural” solution.
The blood test, and what the numbers mean
Ask for these three, together. A single serum B12 can mislead.
| Test | Low | Comfortable | Note |
|---|---|---|---|
| Serum B12 | below 150 pmol/L (about 200 pg/mL) | above 300 pmol/L (about 400 pg/mL) | the grey zone between is where most tired vegans sit |
| Methylmalonic acid (MMA) | above 0.37 µmol/L | low | rises when cells are starved of B12; the functional test |
| Holotranscobalamin (active B12) | below 35 pmol/L | above 50 pmol/L | the fraction your cells can use |
Two traps. A high folate intake, which every vegan has, can keep the blood count looking normal while the nerves run short, so never let anyone reassure you from a full blood count alone. And if you have been eating spirulina or algae products, serum B12 can read falsely high because the test counts inactive analogues; MMA is the number to trust.
How much you need
| Reference | Amount |
|---|---|
| EFSA adequate intake, adults | 4 µg a day |
| US RDA, adults | 2.4 µg a day |
| Pregnancy and breastfeeding | 4.5 to 5 µg a day (EFSA) |
Those are intakes from food, absorbed through the normal pathway, which can only take up about 1.5 to 2 µg per sitting. Everything above that is absorbed passively at about one percent. That single fact explains supplement doses, below.
Foods that carry B12, per serving
Only fortified foods count. Read the label every time; brands change.
| Food | Typical B12 | What it covers |
|---|---|---|
| Fortified soy or oat milk, 250 ml | 0.4 to 1.0 µg (often 0.38 µg per 100 ml) | a quarter of the day |
| Fortified nutritional yeast, 1 tablespoon (5 g) | 2 to 8 µg, check the label | half a day to two days |
| Fortified breakfast cereal, 40 g | 0.6 to 2.5 µg | a quarter to a half |
| Fortified plant yoghurt, 150 g | 0.4 to 0.8 µg | a quarter |
| Fortified marmite-style yeast extract, 8 g | about 1.9 µg | half |
| Tempeh, nori, spirulina, mushrooms | unreliable or inactive analogues | do not count them |
To reach 4 µg from food, you need fortified foods at three separate meals, every day, because of the absorption ceiling per sitting. Most women manage that on weekdays and lose it at weekends. Hence the supplement.
The supplement: form, dose, timing
- Cyanocobalamin is the best studied, the most stable and the cheapest. It is the form used in nearly every trial that showed correction of deficiency. Methylcobalamin is fine but less stable in the bottle and not better in the blood for most people.
- Daily: 25 to 100 µg once a day, chewed or dissolved under the tongue or simply swallowed.
- Or twice weekly: 1,000 µg, two days apart. The high dose works through the passive one percent route.
- If already low or with any nerve signs: 1,000 µg a day for eight weeks, then the maintenance dose above, and a repeat test at twelve weeks. Nerve symptoms that are not clearly improving at eight weeks belong with a clinician, who can give injections.
- There is no upper limit set for B12 because no toxicity has been found; the excess leaves in urine.
Take it at any time of day, with or without food. It does not matter. Consistency does.
The one-week fix
- Buy one bottle of cyanocobalamin, 1,000 µg. Put it next to the kettle.
- Take one on Monday and one on Thursday. Set two phone alarms that repeat forever.
- Switch the household milk to a fortified one and keep a jar of fortified nutritional yeast on the table; two teaspoons over anything savoury.
- Book the three blood tests above for twelve weeks from today, so the result arrives after the correction has had time to show.
What the research says
- Pawlak and colleagues, Nutrition Reviews, 2013, systematic review of 18 studies in vegetarians and vegans of all ages, with and without supplements: B12 depletion or deficiency ranged from 0 to 86.5 percent across the adult study groups, higher in vegans than lacto-ovo vegetarians, lowest where supplements or fortified foods were used. The contrary finding: the range is wide because studies used different cut-offs and markers, and supplemented groups in several studies showed little or no deficiency. doi.org/10.1111/nure.12001
Read that again from the other side: the women who supplemented were, in several studies, indistinguishable from omnivores. The gap is not a property of veganism. It is a property of not supplementing.
Questions women ask
Do vegans need B12 even if they feel fine? Yes. The liver’s store can carry you for years while the markers drift; by the time you feel it the numbers are already poor.
Can I get enough from nutritional yeast alone? Only if it is fortified, you eat it daily, and you read the label; unfortified yeast has none. Treat it as a bonus on top of a supplement, not the plan.
How long until I feel better? Energy often lifts within two to four weeks of a corrected dose. Nerve signs take longer, sometimes months, and need the higher dose.