Vitamin B12: what it actually does, the dose, and whether it is worth it

B12 is one of the few supplements where the story is refreshingly simple: if you are short on it, fixing that makes a big, noticeable difference; if you are not, you will feel exactly nothing. The problem is that marketing turned it into a universal energy powder, so people eating 200 g of meat a day still buy it. Here is what B12 really does, how much you need, and how to check whether you are in the group it helps.

What B12 actually does

Vitamin B12 (cobalamin) acts as a cofactor for exactly two enzyme reactions in the human body. The first is methionine synthase, which converts homocysteine into methionine and drives the methylation cycle — DNA synthesis depends on it, and therefore so does red blood cell production. The second is methylmalonyl-CoA mutase; without B12, methylmalonic acid builds up and over time damages the myelin sheath around your nerves.

Translated into gym language: without enough B12 you get megaloblastic anaemia (red cells become oversized and carry oxygen poorly) plus neurological symptoms — tingling hands and feet, weakness, balance problems. B12 is not a stimulant and does not produce energy directly. It lets the system run normally. When the system is already running normally, adding more changes nothing.

Who is genuinely at risk

Deficiency is not random bad luck. It shows up in predictable situations:

• Vegans and strict vegetarians. B12 in nature comes only from animal foods and fortified products. Liver stores last roughly 2 to 5 years, so the problem arrives quietly and late.
• People over 50. In 10 to 30 percent of them, stomach acid production drops enough that B12 no longer separates properly from food protein.
• Metformin for more than 4 months and proton pump inhibitors for more than 12 months measurably lower blood levels.
• Stomach or bowel surgery, Crohn's disease, coeliac disease, pernicious anaemia (autoimmune loss of intrinsic factor).
• Regular heavy drinking and very low calorie diets under 1,200 kcal run for months.

If you are in none of these groups, you statistically probably do not have a deficiency — and a tablet buys you nothing but more expensive urine.

How much you actually need

The recommended daily intake for adults is 2.4 micrograms (mcg), 2.6 in pregnancy and 2.8 while breastfeeding. Those are absurdly small numbers compared with what is printed on the bottles.

Here is what ordinary food delivers: 100 g of beef about 2.5 mcg, 100 g of salmon about 5 mcg, 100 g of canned tuna about 2 mcg, one egg about 0.5 mcg, 250 ml of milk about 1.2 mcg, 30 g of hard cheese about 0.5 mcg. Beef liver is the outlier at over 70 mcg per 100 g. In practice: 150 g of meat or fish plus two eggs a day already puts you at 4 to 6 mcg, double what you need.

So why do tablets contain 1,000 mcg? Because intrinsic-factor absorption saturates at roughly 1.5 to 2 mcg per meal. Everything above that enters by passive diffusion, and that pathway captures only about 1 percent of the dose. From a 1,000 mcg tablet you realistically absorb 5 to 10 mcg. The big number is not hype, it is compensation for poor absorption.

Doses that make sense

1. Vegan with no confirmed deficiency: 50 to 100 mcg daily, or 2,000 mcg once a week. Both regimens work equally well.
2. Confirmed deficiency, oral route: 1,000 mcg daily for 8 to 12 weeks, then retest and drop to maintenance.
3. Malabsorption or pernicious anaemia: injections, typically 1,000 mcg every other day for the first 1 to 2 weeks, then monthly. That is a doctor's call, not yours.

The form (cyanocobalamin versus methylcobalamin) makes a negligible practical difference in healthy people; cyanocobalamin is cheaper and best studied. No upper limit has been set because the excess is simply excreted — which is still not a reason to take 5,000 mcg for no reason.

How to check

Do not guess from symptoms, because fatigue has a hundred other causes. Get a serum B12: below 148 pmol/L (about 200 pg/ml) is deficiency, and 148 to 221 pmol/L is a grey zone worth investigating further. The best confirmatory test is methylmalonic acid — a value above roughly 0.4 µmol/L means B12 is genuinely short at the cellular level. On a full blood count, watch MCV: above 100 fL is a warning sign.

High-dose folic acid can normalise the blood count and mask the anaemia while nerve damage quietly progresses. That is why folate and B12 are checked together.

What B12 will not do

It will not add kilograms to your squat or shave seconds off your 5 km if you are not deficient. It will not burn fat. The energy injection at the wellness studio is a placebo if your level is normal. And it will not compensate for 5 hours of sleep, training with no rest days, and 60 g of protein a day at 85 kg bodyweight.

When to see a doctor

Book an appointment, and do not put it off, if you have:

• tingling, pins and needles or numbness in hands and feet, or unsteady walking;
• unexplained fatigue and breathlessness lasting more than 4 weeks, pallor, a sore smooth tongue;
• memory loss or confusion in someone over 60;
• a vegan diet for more than 2 years with no supplement at all;
• a plan to start 1,000 mcg daily — test first, then supplement, because after supplementing the result no longer tells the truth.

The short version

• B12 does exactly two jobs: it enables DNA synthesis (red blood cells) and protects nerve myelin. It is not an energy source.
• You need 2.4 mcg a day; 150 g of meat or fish plus two eggs covers that twice over.
• Real risk sits with vegans, people over 50, and anyone on metformin, proton pump inhibitors, or with bowel disease.
• Sensible doses: 50 to 100 mcg daily as prevention, 1,000 mcg daily for 8 to 12 weeks for confirmed deficiency.
• Test first (serum B12, plus methylmalonic acid if borderline), supplement second — the other order just blurs the picture.