Vitamin B12: who runs short and why it’s not just vegetarians

Vitamin B12 is unusual: its usual sources are animal foods only — meat, milk, eggs, fish and shellfish (Watanabe, 2007). That’s why it mostly comes up when people talk about vegetarianism. But B12 deficiency also happens in people who eat meat every day — just for other reasons.

Vegetarians and vegans

A review of 18 studies that measured B12 deficiency with sensitive blood markers showed how common it is among vegetarians: 62% of pregnant women, from 25% to almost 86% of children, 21–41% of adolescents and 11–90% of older people. Deficiency was more common in vegans than in vegetarians, and in those vegetarian from birth than in those who switched later. The authors’ conclusion is clear: vegetarians need regular B12 supplements (Pawlak et al., 2013).

What about “plant sources”? Spirulina and other blue-green algae sold as supplements contain mostly pseudovitamin B12, which is inactive in humans, so they aren’t a suitable B12 source, especially for vegans. Dried nori seaweed is the exception: it contains a meaningful amount of real B12. Fortified foods are a valuable source for vegans and older people (Watanabe, 2007).

After 50: enough in the food, not enough absorbed

B12 deficiency is estimated to affect 10–15% of people over 60. The main cause is atrophic gastritis, common at this age: the stomach produces little acid and pepsin, so B12 is poorly released from food proteins. Meanwhile, the pure vitamin — from supplements and fortified foods — is absorbed as well as in youth. So the review’s authors advise older people to get B12 from supplements or fortified foods. The classic anaemia is often absent, making the deficiency easy to miss (Baik, Russell, 1999).

Milk is a good source for people whose stomach works normally. When 16 healthy people over 60 were given labelled B12 in water, milk or fortified bread, they absorbed 55%, 65% and 55% — with no significant difference (Russell et al., 2001).

Medicines that lower B12

Metformin is the most widely used medicine for type 2 diabetes. In a randomised trial, 390 patients took it or a placebo for over four years. On metformin, B12 levels fell by 19% on average, and the risk of deficiency was 7.2 percentage points higher: one extra case of deficiency for every 14 patients. The authors recommend checking B12 regularly during long-term treatment (de Jager et al., 2010).

Heartburn medicines that suppress stomach acid act similarly. Comparing almost 26,000 patients with B12 deficiency to 184,000 without it showed that taking proton pump inhibitors (omeprazole, pantoprazole and others) for two years or more was linked to 65% higher odds of deficiency, and H2 blockers to 25% higher odds. The higher the dose, the stronger the link (Lam et al., 2013).

Where B12 is in food

The adult target is 2.4 µg a day, so 100 g of herring or mackerel covers it several times over. But absorption has its quirks. The mechanism that takes up B12 in the gut saturates at about 1.5–2 µg per meal, so relatively less is absorbed from a big portion. On average 42% of B12 is absorbed from fish, 56–89% from lamb, 61–66% from chicken — and less than 9% from eggs (Watanabe, 2007). So eggs, despite the figure in the table, are a poor source of B12.

Why pharmacy tablets contain such high doses

In a study by Eussen and colleagues (2005), 120 older people with mild B12 deficiency took 2.5 to 1,000 µg a day for 16 weeks. Normalising their blood markers took 647–1,032 µg a day — more than 200 times the daily target. So the high doses in B12 tablets aren’t a mistake: once deficiency is present, small doses barely help. In that case, the dose is chosen by a doctor.

The calculator counts B12 in your diary, taking cooking into account, and the richest sources are listed on the Foods rich in vitamin B12 page. But the diary shows how much B12 you ate, not how much you absorbed: if you’re in a risk group, it’s no substitute for a blood test.