Vitamin B12 Deficiency and Fatigue
Vitamin B12 deficiency is a slow-developing and often dramatically underdiagnosed cause of fatigue, brain fog, and neurological symptoms. It can take years to develop and years more to be identified because the symptoms overlap with so many other conditions.
Why B12 is critical for energy
B12 is essential for red blood cell formation, myelin production (the insulating sheath around nerve fibres), and DNA synthesis. Deficiency causes megaloblastic anaemia (large, poorly functioning red cells), nerve damage, and impaired methylation — a key process in energy metabolism and mood regulation.
The neurological symptoms of B12 deficiency are particularly striking: cognitive slowing, memory problems, depression, tingling or numbness in hands and feet, and poor balance. These can develop before anaemia appears on a standard blood count.
Who is at risk
Vegans and vegetarians are the highest-risk group, as B12 is found almost exclusively in animal products. Without supplementation, most plant-based eaters will become deficient within 2–5 years of eliminating animal foods. B12 fortified foods and supplements are essential, not optional.
People over 50 absorb B12 significantly less efficiently due to reduced stomach acid production (B12 requires intrinsic factor, secreted in the stomach, for absorption). Metformin (a common diabetes medication) also impairs B12 absorption. Pernicious anaemia — an autoimmune condition — blocks intrinsic factor production entirely.
Testing and treatment
Standard serum B12 tests underestimate deficiency — a level in the 'low normal' range (200–300 pmol/L) can still cause neurological symptoms. Methylmalonic acid (MMA) and homocysteine are better markers of functional B12 status if deficiency is suspected despite normal serum levels.
Treatment for functional deficiency is daily oral B12 supplementation (1,000 mcg of methylcobalamin or cyanocobalamin) or, for pernicious anaemia, regular injections (which bypass the absorption defect). B12 is water-soluble with no known toxicity, so dosing is not a concern.
Our assessment screens for B12 deficiency risk factors and includes it in your personalised fatigue analysis.
Frequently Asked Questions
What are the symptoms of vitamin B12 deficiency?
Symptoms of vitamin B12 deficiency can include fatigue, cognitive slowing, memory problems, depression, and tingling or numbness in the hands and feet.
Who is most at risk for vitamin B12 deficiency?
Vegans, vegetarians, and individuals over 50 are at the highest risk for vitamin B12 deficiency, as the vitamin is primarily found in animal products.
How is vitamin B12 deficiency diagnosed?
Vitamin B12 deficiency is often diagnosed through serum B12 tests, but methylmalonic acid (MMA) and homocysteine levels can provide better insights if deficiency is suspected.
What is the treatment for vitamin B12 deficiency?
Treatment typically involves daily oral B12 supplementation or regular injections for those with pernicious anemia, as B12 is water-soluble and has no known toxicity.