Vitamin B12 — Cobalamin

B12 is one of the most important vitamins in the body and one of the most commonly deficient — not because it’s hard to find in food, but because of specific circumstances that make it hard to absorb. It’s involved in red blood cell production, nervous system function, and DNA synthesis, and it works closely with folate in several of those processes. When B12 is low for an extended period, the effects on the nervous system can become serious and, in some cases, irreversible if left unaddressed long enough.

The absorption story is what makes B12 unique. Unlike most vitamins, B12 requires a protein produced in the stomach called intrinsic factor to be absorbed properly. As people age, production of intrinsic factor tends to decline, which is why B12 deficiency becomes increasingly common in older adults even when dietary intake is adequate. Certain medications — most notably metformin, commonly prescribed for type 2 diabetes, and proton pump inhibitors used for acid reflux — also interfere with B12 absorption over time.

The other major context for B12 deficiency is plant-based eating. B12 is found almost exclusively in animal foods — meat, fish, eggs, and dairy. It does not exist in meaningful amounts in plant foods, and claims about certain plant sources like spirulina or nutritional yeast providing adequate B12 are not well supported by the evidence. People eating a fully plant-based diet need to supplement B12 or consume fortified foods consistently. This is not optional or negotiable — B12 deficiency on a vegan diet is a real and serious risk that develops slowly and can be well advanced before symptoms become obvious.

For everyone else eating animal products regularly, B12 status generally takes care of itself.

Reference Card

Vitamin type: Water-soluble Pillar: Nourish

What it does for you

Essential for red blood cell production
Supports nervous system function and myelin maintenance — the protective coating around nerve fibers
Required for DNA synthesis
Works with folate to manage homocysteine levels

Where to get it

Beef, clams, oysters, salmon, tuna, eggs, dairy, fortified plant milks and cereals

Considerations

Water-soluble but unlike other B vitamins, the body stores B12 in the liver — deficiency develops slowly, sometimes over years
Absorption requires intrinsic factor produced in the stomach — older adults are at higher risk of absorption issues regardless of dietary intake
Fully plant-based eaters must supplement B12 or consume fortified foods consistently — this is non-negotiable
Certain medications affect B12 absorption over time — worth discussing with a doctor if you’re on long-term medication and haven’t had B12 status checked

Signs your intake might be low

Fatigue and weakness
Tingling or numbness in the hands and feet
Difficulty with memory or concentration
Low mood or depression
In advanced deficiency — balance problems and more serious neurological symptoms

Common myths

Plant-based eaters can get enough B12 from spirulina or fermented foods — the evidence does not support these as reliable B12 sources; supplementation is necessary on a fully plant-based diet
B12 deficiency develops quickly — the body stores B12 in the liver and deficiency can take years to develop, which is part of why it often goes undetected until it’s already significant
Only vegans need to worry about B12 — older adults and people on certain medications are also at meaningful risk regardless of diet