Vitamin B12 (cobalamin) is the only nutrient the human body cannot produce or synthesize from sunlight: it comes exclusively from animal foods or supplements. A deficiency, common after age 60, among vegans, and under certain treatments, often builds up silently for months before the first symptoms appear.
In brief – Vitamin B12 is a water-soluble vitamin essential for making red blood cells, maintaining the nerve myelin sheath, and synthesizing DNA. The human body does not manufacture it: liver stores (2 to 5 mg) come solely from animal products (meat, fish, eggs, dairy) or supplements, and in theory last several years, which delays diagnosis of a deficiency. It mainly affects people over 60 (reduced stomach absorption), strict vegans and vegetarians, patients on metformin or proton pump inhibitors, and people after bariatric surgery. It shows up as fatigue, tingling, memory problems and, left untreated, anemia or nerve damage. The adequate intake set by French health authorities is 4 mcg a day for adults, more during pregnancy or breastfeeding.
Definition: what is vitamin B12?
B12 refers to a group of compounds called cobalamins, characterized by a central cobalt atom. It is the most complex of the B vitamins and the only one to contain a metal in its structure. It exists in several active forms in the body: methylcobalamin and adenosylcobalamin, while cyanocobalamin, the most stable form, is the one used in most supplements and fortified foods.
Unlike vitamin C or some B vitamins, B12 is stored in large amounts in the liver. This reserve of 2 to 5 mg covers needs for several years even with insufficient intake, which explains why a B12 deficiency often takes a long time to become clinically apparent.
Mechanism: what does B12 do in the body?
It takes part in two enzymatic reactions essential to cell function. It is required for cell division and for maintaining the nervous system, two roles that explain the wide range of deficiency symptoms.
- DNA synthesis and cell division: B12 is a cofactor for methionine synthase, an enzyme needed to make red blood cells in the bone marrow; its deficiency causes what is called megaloblastic anemia.
- Maintaining the myelin sheath: it helps build the myelin that insulates nerve fibers; a prolonged deficit can damage peripheral nerves and the spinal cord.
- Regulating homocysteine: together with vitamin B9 (folate), B12 converts homocysteine, an amino acid whose excess in the blood is linked to cardiovascular risk, into methionine.
- Energy metabolism: it helps convert fatty acids and certain amino acids into energy usable by cells.
What the science says about B12 deficiency
Vitamin B12 deficiency remains one of the most underdiagnosed vitamin deficiencies in adults, according to a landmark review published in the New England Journal of Medicine. It can cause megaloblastic anemia, demyelinating nerve damage, or both at once, sometimes before any abnormality shows up on a standard blood test (Stabler 2013).
Vegan and strict vegetarian diets are the main nutritional risk factor: a literature review covering several international cohorts found that vegetarians develop depleted or deficient B12 levels, regardless of age, sex, or how long they have followed the diet, in the absence of supplementation (Pawlak et al. 2013). According to Dr. Belghiti, “vigilance should focus as much on vegans as on older adults taking multiple medications, two groups in whom the deficiency progresses silently for months before the first clinical signs.”
A review published in Nutrients details the consequences of a chronic deficit: possible effects on pregnancy, vascular health, cognitive function, and bone density, with documented associations that do not, on their own, prove a direct causal link in every case (O’Leary & Samman 2010). French nutritional references set the adequate intake for vitamin B12 at 4 mcg a day for adults, a threshold established from several combined blood biomarkers (for a general overview, see the NIH Office of Dietary Supplements fact sheet on B12).
In practice: who is affected and how is B12 tested?
A B12 blood test is ordered for unexplained fatigue, neurological symptoms (tingling, loss of balance), or in at-risk groups. A clinical review published in American Family Physician recommends targeted screening in adults over 50, vegans, patients on long-term metformin or proton pump inhibitors, and after bariatric surgery, rather than routine screening across the general population (Langan & Goodbred 2017).
Recommended intakes vary by age and physiological status. The table below lists the reference values set for the French population.
| Population | Adequate intake of B12 | Note |
|---|---|---|
| Adult | 4 mcg/day | Covered by a varied omnivorous diet |
| Pregnant woman | 4.5 mcg/day | Increased need for fetal development |
| Breastfeeding woman | 5 mcg/day | Transfer through breast milk |
| Over 60 | 4 mcg/day | Stomach absorption often reduced: monitoring recommended |
| Strict vegan | 4 mcg/day | Not covered without supplements or fortified foods |
These reference values are part of UltraSante’s Diet pillar. To put these intakes in context, complementary lab monitoring fits into a broader approach to preventive blood markers, alongside ferritin for iron status. Vitamin B12, along with vitamin D, is among the most common nutritional deficiencies in France, though their at-risk populations differ markedly.
Protocol: how to correct a B12 deficiency
Correcting a B12 deficiency depends on its cause and severity, and should be supervised by a healthcare professional after a confirmed blood test. Prolonged self-treatment is not recommended when neurological symptoms are present.
- Diet: favor organ meats, fish, eggs, dairy products and, for vegans, fortified foods (certain plant-based drinks, nutritional yeast).
- Oral supplementation: suited to mild to moderate deficiencies and to prevention in vegans, using cyanocobalamin or methylcobalamin.
- Intramuscular injections: reserved for severe deficiencies, absorption disorders (pernicious anemia, digestive surgery), or nerve damage that has already set in.
- Lab follow-up: a blood check after a few months of treatment confirms normalization and allows the dose to be adjusted.
| Form of B12 | Origin | Common use |
|---|---|---|
| Cyanocobalamin | Synthetic, most stable form | Oral supplements, fortified foods |
| Methylcobalamin | Naturally active form | Oral supplements, sometimes preferred for prevention |
| Hydroxocobalamin | Reference injectable form | Intramuscular injections for severe deficiency |
Frequently asked questions about vitamin B12
What is vitamin B12 and what does it do?
Vitamin B12, or cobalamin, is a water-soluble vitamin found exclusively in animal-derived foods. It is essential for making red blood cells, maintaining the myelin sheath around nerves, and synthesizing DNA. The human body does not produce it and depends entirely on diet or supplementation to maintain its liver stores.
What are the signs of a B12 deficiency?
Early signs often include persistent fatigue, paleness, and shortness of breath on exertion, linked to megaloblastic anemia. Tingling in the hands and feet, balance problems, memory loss, or irritability can appear if the deficiency continues, due to nerve damage.
What is the recommended daily dose of B12?
French health authorities set the adequate intake at 4 mcg a day for an adult, 4.5 mcg during pregnancy, and 5 mcg while breastfeeding. These levels are covered by a varied omnivorous diet, but supplementation is needed for people following a strict vegan diet.
Who is most at risk of a B12 deficiency?
- People over 60, due to often reduced stomach absorption.
- Strict vegans and vegetarians, without fortified foods or supplements.
- Patients on long-term metformin or proton pump inhibitors.
- People who have had bariatric surgery or have pernicious anemia.
Cyanocobalamin or methylcobalamin: what is the difference?
Cyanocobalamin is a highly stable synthetic form, the most widely used in supplements and fortified foods; the body converts it into active forms. Methylcobalamin is already an active form, sometimes chosen for prevention, though no clear clinical advantage has been established between the two for most common deficiency cases.
Medical disclaimer. The information provided here is for informational purposes only and does not constitute medical advice. It does not replace a consultation. Ask a healthcare professional before changing your diet, taking dietary supplements or starting a new practice, especially if you have a medical condition, are pregnant or are under treatment. Dietary supplements do not replace a balanced diet or medical follow-up.
