Vitamin B12维生素 B12

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只有细菌会造 · 需要胃里出门票 · 让神经有外套，让 DNA 能复印

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B12 (cobalamin) is the largest and most complex of all the vitamins — at its core is a cobalt (Co) ion wrapped by a corrin ring, structurally homologous to heme (which holds Fe) and chlorophyll (which holds Mg).

One key fact: only prokaryotes (bacteria and archaea) have the enzymes to synthesize B12. Plants, fungi, and animals cannot make it — animals accumulate B12 by eating microbe-laden forage or via gut microbial symbionts, and then concentrate it in liver, muscle, eggs, and milk. So dietary B12 comes almost entirely from animal sources:

Clams: ~99 µg / 100 gBeef liver: ~70 µg / 100 g (off the charts)Salmon, tuna: ~4-7 µg / 100 gEggs: ~0.6 µg per eggMilk: ~1.3 µg per cup
The RDA is only 2.4 µg/day — a small number. But plant sources are almost all unreliable:

Nori / seaweed contain pseudo-B12 analogs — structurally similar but with zero biological activity, and they actually interfere with real B12 absorptionUnfermented legumes and nuts: noneFortified foods (fortified breakfast cereal, plant milks) contain real B12 — for vegans, this is the only reliable plant-based dietary source of B12
So strict or near-vegan eaters typically need fortified foods plus a supplement.

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本站提供科普与一般性建议，不替代医师诊断或处方；有症状或正在服药请咨询合格医师。 Educational content, not medical advice — consult a clinician for diagnosis or treatment.

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Vitamin B12维生素 B12

Place · Level 3

只有细菌会造 · 需要胃里出门票 · 让神经有外套，让 DNA 能复印

Food

Stop 1/6

食物

B12 (cobalamin) is the largest and most complex of all the vitamins — at its core is a cobalt (Co) ion wrapped by a corrin ring, structurally homologous to heme (which holds Fe) and chlorophyll (which holds Mg).

One key fact: only prokaryotes (bacteria and archaea) have the enzymes to synthesize B12. Plants, fungi, and animals cannot make it — animals accumulate B12 by eating microbe-laden forage or via gut microbial symbionts, and then concentrate it in liver, muscle, eggs, and milk. So dietary B12 comes almost entirely from animal sources:

Clams: ~99 µg / 100 gBeef liver: ~70 µg / 100 g (off the charts)Salmon, tuna: ~4-7 µg / 100 gEggs: ~0.6 µg per eggMilk: ~1.3 µg per cup
The RDA is only 2.4 µg/day — a small number. But plant sources are almost all unreliable:

Nori / seaweed contain pseudo-B12 analogs — structurally similar but with zero biological activity, and they actually interfere with real B12 absorptionUnfermented legumes and nuts: noneFortified foods (fortified breakfast cereal, plant milks) contain real B12 — for vegans, this is the only reliable plant-based dietary source of B12
So strict or near-vegan eaters typically need fortified foods plus a supplement.

Read story

本站提供科普与一般性建议，不替代医师诊断或处方；有症状或正在服药请咨询合格医师。 Educational content, not medical advice — consult a clinician for diagnosis or treatment.