How much vitamin B12 do you need per day?

The RDA for vitamin B12 is 2.4 mcg a day for adults, 2.6 mcg in pregnancy and 2.8 mcg when breastfeeding.

These are the reference amounts used on US food labels and in dietary guidance, not personal targets. What you need shifts with age, pregnancy, medication and health conditions. The % Daily Value shown on every food page here is calculated against the FDA Daily Value for vitamin B12, which is 2.4 µg a day.

What this means for you

Two point four micrograms is a tiny amount. If you eat animal products you almost certainly get it without thinking. A palm-sized piece of salmon is about a day. A tin of tuna is more than a day. Mussels are in another league, with eight days' worth in one small portion. One egg gives you a fifth of the day.

That is the whole story for most people. B12 is a nutrient you can either ignore completely or need to take seriously, and there is not much in between. Four things put you in the second group.

You eat no animal products. More greens and more fermented food will not fix this. The one plant food that has been shown to move the blood markers is argued out at the bottom of this page, and it does not change the advice. On a vegan diet a supplement or a reliably fortified food is part of the diet rather than an optional extra.

You are over about fifty. Stomach acid falls with age, and acid is what releases B12 from food. Somewhere between 3 and 43 per cent of older adults living independently are deficient, depending on how you measure it. No reading of that range is comforting. It is also why supplements absorb about half again as well as food does. They arrive already free.

You take metformin, or long-term acid-reducing medication. Both interfere with absorption, and both are usually taken for years. This is a conversation with whoever prescribes them, not a change to make on your own.

You have had stomach or bowel surgery, or a condition that affects absorption. Same logic. The food may be fine and the uptake is not.

Established. The damage from being low is to nerves, and it can arrive with no anaemia to warn you. Numbness and tingling in hands and feet, and further along, changes in memory and mood. Caught late, some of it does not reverse. Meanwhile no upper limit has been set, because no harm has been found at high intakes.

The number itself is under question

Two point four micrograms is what most countries settled on, and it was worked out from what it takes to keep blood markers normal. There is another way to do the sum. Measure how much B12 the body loses in a day, then ask how much you would have to eat to replace it.

Unsettled. Done that way the answer lands between 3.8 and 20.7 micrograms a day, against the 1.4 to 3 micrograms official recommendations use. The width of that range tells you as much as the numbers do. It rests on 52 people across five studies of daily losses and 83 people across eight studies of how much B12 gets absorbed from real food, and none of it is a randomised trial. The researchers who ran the sum did not ask for the recommendation to be changed. They said nobody has yet tied these markers to what happens to a person over a lifetime.

Treat the number on the label as a working figure that nobody has shown to be wrong, and nobody has shown to be right either. It is still not an argument for eating eight times as much, and the next part is why.

Why eating more moves the needle so little

Blood B12 follows what you eat, and it follows it weakly. Double your intake and the level in your blood rises by about 11 per cent. Thirteen per cent in older people, eight in younger adults. Methylmalonic acid, the marker that shows whether your cells are actually short, drops by 7 per cent for that same doubling.

The reason sits in the gut. Given 2.5, 5 or 10 micrograms in one go, healthy adults took up roughly the same absolute amount every time, between 1.16 and 1.39 micrograms. The percentage collapsed from about 51 to about 15 as the dose grew, because the amount getting through hardly moved at all. Past roughly 2.6 micrograms in a single dose, about one per cent of the extra is absorbed.

One useful thing falls out of that. If you are covering B12 from food and fortified products, spreading it across the day beats one large hit at dinner.

What that asymmetry means for you

Too little can cost you permanently. Too much costs you nothing. That is unusual and it is the whole argument for testing or supplementing if you are in one of the four groups above, rather than waiting to see how you feel.

Not for everyone. A daily number assumes your gut works the way the textbook says. In 100 older people already low on B12, eight weeks of 500 micrograms a day left 8 per cent with a blood level still abnormal and 12 per cent still abnormal on holotranscobalamin. Between 15 and 25 per cent still showed a metabolic shortage on methylmalonic acid. Ten micrograms a day was enough to move the markers for most of the group. For the rest, no dose in that trial finished the job, and the trial could not say why. If you are being treated for a deficiency, the repeat blood test is the part that matters, not the size of the tablet.

If none of those apply to you and you eat meat, fish, eggs or dairy, you can stop reading here with a clear conscience.

Two more things 2.4 does not settle

Unsettled. Europe and the United States do not agree on the number. The 2.4 micrograms on this page is the American figure. Europe's food safety authority set an adequate intake of 4.0 micrograms a day for adults aged 18 and over. Neither body has shown the other wrong. If you have been told you fall short, it is worth knowing which of the two you were measured against.

The second thing is who the vegan warning belongs to. Pooling studies of 1,994 Seventh-day Adventists eating plant-based diets against omnivore controls, serum B12 came out the same, a difference of 9.85 pmol/L with an interval running from 45.64 below to 25.94 above. Daily intake matched as well. Fortified food and supplements are ordinary in that population, which is the point of the result rather than a footnote on it. Heterogeneity between the pooled studies was high, so read it as proof that the gap can be closed rather than as a measurement of any particular diet.

One food has now been tested head on. Five grams of nori a day, about four sheets, took vegetarians from 0.3 to 2.0 micrograms of intake, roughly four fifths of the American figure, and eight grams a day did no better than five. The full argument about what that does and does not change is on the foods page.

Written from the evidence cards and the data on this page, then checked back against the sources. Bioma Learn has no human medical reviewer at this time and we say so rather than invent one. Information, not medical advice. How we verify.

Foods highest in vitamin B12 on this site

• Mussel, blue, cooked — 20.4 µg per 3 oz (850% DV)
• Herring, Atlantic, cooked — 18.8 µg per 1 fillet (783% DV)
• Mackerel, Atlantic, cooked — 16.7 µg per 1 fillet (697% DV)
• Chicken, liver, all classes, cooked — 14.3 µg per 3 oz (597% DV)
• Sardine, Atlantic, canned in oil, drained solids with bone — 13.3 µg per 1 cup, drained (555% DV)

All vitamin B12 sources ranked →

Full guide: Vitamin B12: who actually needs to worry, and why the damage can be permanent