Vitamin B12: When deficiency threatens and how to recognise it

Vitamin B12 is essential for the nervous system and blood formation — and a deficiency can develop gradually with non-specific symptoms. Who is most at risk and what a blood test can reveal.

What vitamin B12 does in the body

Vitamin B12 (cobalamin) is involved in two vital processes: the synthesis of myelin (the protective sheath around nerve fibres) and the formation of blood cells in the bone marrow. Without enough B12, red blood cells cannot divide properly — a condition known as megaloblastic anaemia, where the cells become too large and dysfunctional. At the same time, myelin breakdown can cause neurological symptoms.

B12 is also involved in the methionine cycle — a metabolic pathway that breaks down homocysteine. Elevated homocysteine is considered a risk factor for cardiovascular disease and cognitive decline. A B12 deficiency therefore also leads to a rise in homocysteine levels in the blood.

Who is particularly at risk

B12 is found almost exclusively in animal foods — meat, fish, eggs, dairy. People who eat a vegan diet take in little or no B12 through food and are at risk over time without supplementation. This is not a matter of opinion but medical consensus: a vegan diet without B12 supplementation is not safe long-term.

Vegetarians who avoid dairy and eggs, and older adults, are also at risk. From around age 50, production of the so-called intrinsic factor in the stomach declines — a protein essential for B12 absorption in the small intestine. This reduces absorption capacity even when dietary intake is adequate. People taking metformin (a diabetes medication) also have an increased risk of B12 deficiency.

Which symptoms point to a deficiency

B12 deficiency develops gradually, as the body stores B12 in the liver for two to five years. Early symptoms are often non-specific: persistent fatigue, difficulty concentrating, mild low mood and a pale complexion. Later, numbness or tingling in the hands and feet may appear — signs of early nerve damage.

The tricky part: neurological damage from B12 deficiency can be irreversible if the deficiency goes undetected for a long time. That's why early testing is more important than waiting. Anyone in a risk group should check their B12 status at least once a year via a blood test — ideally not just serum B12, but also holotranscobalamin (active B12) and methylmalonic acid.

Food sources and what they provide

Beef liver is by far the richest source: 100 g contains over 60 µg of B12 — many times the daily requirement of 4 µg. Mackerel, sardines, salmon, beef and pork, as well as chicken and duck eggs, are other good sources. Milk and cheese contain B12 in lower concentrations but are relevant with regular consumption.

Plant-based foods do not contain reliable B12 — including fermented products such as tempeh or nori algae, which are sometimes cited as sources. The B12 analogues (pseudovitamins) present in these foods can actually block the absorption of true B12. Anyone eating a plant-based diet cannot avoid a supplement.

Supplementation: forms and dosage

B12 supplements are available in several forms: cyanocobalamin is the most common and most affordable, well-documented and stable. Methylcobalamin and adenosylcobalamin are active forms that can be used directly — they may be advantageous for people with certain genetic variants (MTHFR polymorphism).

For vegans, the German Nutrition Society (DGE) recommends at least 5 µg daily from fortified foods or supplements, or a higher weekly dose (250 µg). Since passive absorption (without intrinsic factor) kicks in at high doses, weekly high doses are also an option. Important: do not rely on fortified foods alone if you need to be certain your needs are covered.

Keeping track of your levels with YUTHI

In the YUTHI app, you can log your serum B12 value and other blood markers and track them over months. This is especially useful if you belong to a risk group: plant-based diet, age 50+, taking metformin. A blood test brings clarity — and the app helps you make sense of the results over time.

Sources

• Watanabe F. et al. Vitamin B12-containing plant food sources for vegetarians. Nutrients. 2014;6(5):1861–1873.
• Pawlak R. et al. How prevalent is vitamin B12 deficiency among vegetarians? Nutr Rev. 2013;71(2):110–117.
• Deutsche Gesellschaft für Ernährung. Referenzwerte für die Nährstoffzufuhr: Vitamin B12. DGE. 2019.
• Carmel R. How I treat cobalamin (vitamin B12) deficiency. Blood. 2008;112(6):2214–2221.