Vitamin B12

Vitamin B12 is a water-soluble vitamin your body uses to make red blood cells. It also keeps the myelin sheath around your nerves intact and runs the methylation reactions that turn homocysteine into methionine.^[1] B12 is made by bacteria. It is found naturally only in animal foods like meat, fish, eggs and dairy. Plants do not contain it.

Deficiency is more common than most women realise. It is higher in vegetarians. It is near universal in long-term vegans who do not supplement.^[6] Two medications are also strong drivers. Metformin used for type 2 diabetes or PCOS reduces B12 absorption in the small intestine. Proton pump inhibitors taken for reflux suppress the stomach acid your body needs to release B12 from food. Women taking both at once carry the highest risk.^[5]

Forms of vitamin B12

Supplements use four chemical forms. They differ in how the body uses them and in how stable they are on a shelf.

Methylcobalamin is one of the two bioactive forms your body uses directly. No conversion is required. It is the form involved in homocysteine methylation. This is the form used across all three phases of nōuxx Cycle Routine.^[4]

Cyanocobalamin is synthetic. It is the cheapest and most stable form. The body must strip off a cyanide group then convert it to a bioactive form. Conversion efficiency varies between people.^[4]

Hydroxocobalamin is the form your gut bacteria produce. It is used in injections because it stays in circulation longer than cyanocobalamin.^[4]

Adenosylcobalamin is the second bioactive form. It works inside mitochondria where energy is produced. It is less common in oral supplements because it is harder to stabilise.^[4]

How much you need

The European Food Safety Authority sets the adequate intake for vitamin B12 at 4 µg per day for adult women. The EU Nutrient Reference Value used on supplement labels is older and lower at 2.5 µg. This is why a 25 µg supplement reads as 1001% NRV.^[1]

The number looks alarming. It is not. B12 has no known toxicity at oral doses. Absorption through the active intrinsic factor pathway is capped at around 1.5 to 2 µg per dose. Higher amounts are absorbed only by passive diffusion at roughly 1% efficiency. Supplements are dosed high to make sure enough gets through that second pathway, especially in women whose absorption is impaired by age, medication or low stomach acid.^[1]

The science on women's health

Energy and fatigue

B12 deficiency causes fatigue through two routes. It reduces red blood cell production which lowers oxygen delivery to tissue. It also slows the methylation cycle that produces neurotransmitters like serotonin and dopamine.^[1] Women with low ferritin from heavy periods often also run low on B12. The two deficiencies stack.

The evidence on supplementation is more nuanced than marketing suggests. A 2021 systematic review of 16 randomised trials in over 6,000 participants found that B12 did not improve cognitive function or depressive symptoms in people who were not already deficient. B12 helps when there is a measurable deficiency. It does not act as a stimulant in replete adults.^[3] The case for supplementation is strongest in vegetarians, vegans, women on metformin or PPIs and women over 50.

Pregnancy and cognition

B12 status before conception matters. Maternal B12 deficiency is linked to neural tube defects independent of folate status. It also affects infant neurodevelopment in exclusively breastfed babies of deficient mothers.^[1] Vegan and vegetarian women planning pregnancy should confirm B12 status with a blood test that includes serum B12 plus methylmalonic acid or holotranscobalamin. Serum B12 alone misses early deficiency.^[6]

In older women, low B12 is associated with cognitive decline through high homocysteine. Treatment reverses the metabolic markers. Whether it reverses cognitive symptoms depends on how long the deficiency has lasted before correction.^[3]

EU authorised health claims

Under EU law, vitamin B12 is officially authorised to carry the following health claims:

• Vitamin B12 contributes to the reduction of tiredness and fatigue
• Vitamin B12 contributes to normal energy-yielding metabolism
• Vitamin B12 contributes to normal functioning of the nervous system
• Vitamin B12 contributes to normal psychological function
• Vitamin B12 contributes to the normal function of the immune system
• Vitamin B12 contributes to normal homocysteine metabolism
• Vitamin B12 contributes to normal red blood cell formation
• Vitamin B12 has a role in the process of cell division^[2]

References

[1] National Institutes of Health, Office of Dietary Supplements. Vitamin B12: Fact Sheet for Health Professionals. ods.od.nih.gov/factsheets/VitaminB12-HealthProfessional

[2] Commission Regulation (EU) No 432/2012 establishing a list of permitted health claims made on foods. Official Journal of the European Union, 25 May 2012. eur-lex.europa.eu

[3] Markun S, Gravestock I, Jäger L, Rosemann T, Pichierri G, Burgstaller JM. Effects of vitamin B12 supplementation on cognitive function, depressive symptoms, and fatigue: a systematic review, meta-analysis, and meta-regression. Nutrients 2021;13(3):923. doi.org/10.3390/nu13030923

[4] Temova Rakuša Ž, Roškar R, Hickey N, Geremia S. Vitamin B12 in foods, food supplements, and medicines: a review of its role and properties with a focus on its stability. Molecules 2022;28(1):240. doi.org/10.3390/molecules28010240

[5] Jung C, Park S, Kim H. Concomitant use of metformin and proton pump inhibitors increases vitamin B12 deficiency risk in type 2 diabetes. Journal of Diabetes Investigation 2025;16(6):1020-1027. doi.org/10.1111/jdi.70037

[6] Weikert C, Trefflich I, Menzel J, et al. Vitamin and mineral status in a vegan diet. Deutsches Ärzteblatt International 2020;117(35-36):575-582. doi.org/10.3238/arztebl.2020.0575