Vitamin B12 Deficiency: Why It Is Common and Often Missed

Vitamin B12 deficiency is one of the most rewarding diagnoses in medicine: common, frequently missed, often easy to treat and, if caught early, reversible. It is also one of the most consequential to miss, because prolonged deficiency can cause permanent neurological damage. Its symptoms are nonspecific, its test results can be misleading, and its causes range from autoimmune disease to everyday medicines. For practitioners working with chronic fatigue, cognitive complaints, neuropathy or older patients, it deserves a permanent place on the list of things to consider.

Why the body needs B12

Vitamin B12, or cobalamin, is a cofactor for 2 enzymes. One converts homocysteine to methionine, linking B12 to folate metabolism and to methylation reactions throughout the body. The other converts methylmalonyl CoA to succinyl CoA, a step in the metabolism of certain fatty acids and amino acids. Deficiency therefore impairs DNA synthesis, causing megaloblastic anaemia, and disrupts myelin formation and nerve function, causing neurological damage. The 2 consequences can occur independently: neurological damage may develop without anaemia.

The long journey of absorption

B12 absorption is unusually complex, which explains why so many conditions cause deficiency.

1. B12 in food is bound to protein and released by stomach acid and pepsin.
2. It binds to a carrier protein from saliva, then is released in the small intestine by pancreatic enzymes.
3. It binds to intrinsic factor, a protein secreted by the stomach’s parietal cells.
4. The B12 and intrinsic factor complex is absorbed in the terminal ileum.

A small amount, around one percent of an oral dose, is absorbed passively without intrinsic factor, which is why high dose oral supplements can work even when intrinsic factor is lacking.

The body stores several years’ worth of B12, mostly in the liver, so deficiency usually develops slowly, and symptoms may appear years after absorption or intake falls.

Who becomes deficient

How deficiency presents

Haematological

Macrocytic anaemia with fatigue, breathlessness and pallor. Folate supplementation can correct the anaemia while allowing neurological damage to progress, which is one reason B12 status should be checked before treating macrocytic anaemia with folate alone.

Neurological

Symmetrical tingling and numbness, usually starting in the feet; loss of vibration and position sense; unsteady gait; and, in severe cases, subacute combined degeneration of the spinal cord. Cognitive changes, memory problems, depression and, rarely, psychosis can occur. Neurological symptoms may be the only sign of deficiency.

Other

A sore, smooth tongue, mouth ulcers, and in some cases fertility problems. In infants of deficient mothers, developmental delay and failure to thrive.

Why testing is tricky

Serum B12 is the usual first test, but it has important limitations. It measures total circulating B12, most of which is bound to a carrier protein that does not deliver it to cells. People can have neurological deficiency with a serum level in the low normal range, and some people with low levels are not functionally deficient. Reference ranges and assays vary between laboratories.

When results are borderline, or when symptoms suggest deficiency despite a normal level, further tests help:

• Active B12 (holotranscobalamin) measures the fraction available to cells and may detect deficiency earlier.
• Methylmalonic acid rises when B12 is functionally deficient and is relatively specific, although it also rises in kidney impairment.
• Homocysteine rises in both B12 and folate deficiency and is less specific.
• Intrinsic factor antibodies support a diagnosis of pernicious anaemia when positive; a negative result does not exclude it.

Guidelines from haematology and clinical bodies emphasise that clinical judgement should take precedence when symptoms strongly suggest deficiency, particularly neurological symptoms, and that treatment should not be delayed while waiting for confirmatory results in such cases.

Treatment

Treatment depends on the cause and the presence of neurological features.

• With neurological symptoms or suspected pernicious anaemia, intramuscular B12 is usually given initially and frequently, then continued as maintenance, under medical supervision.
• Dietary deficiency is treated with oral supplements and advice on reliable sources.
• High dose oral B12 has been shown in trials to be as effective as intramuscular treatment in correcting levels for many patients, including some with absorption problems, because of passive absorption. It may not be suitable for everyone, particularly those with severe neurological involvement or uncertain adherence.
• Ongoing monitoring for people with continuing risk factors, such as long term metformin use, is recommended by many diabetes guidelines.

Finding the cause matters as much as treating the deficiency. Pernicious anaemia is associated with a higher risk of gastric cancer and other autoimmune conditions; malabsorption may point to coeliac disease or other gut disease; recreational nitrous oxide use needs specific support.

An illustrative case

A 71 year old man is referred with memory problems and unsteadiness. He has had type 2 diabetes for 12 years, treated with metformin, and has taken a proton pump inhibitor for reflux for 8 years. His blood count is normal. His neuropathy has been attributed to diabetes and his memory to ageing.

His serum B12 is in the low normal range, but his methylmalonic acid is raised. His doctor starts intramuscular B12 given the neurological features, checks for pernicious anaemia and reviews whether the proton pump inhibitor is still needed. Over several months his unsteadiness improves and his family notice he is sharper. Not all of his neuropathy resolves, a reminder that early recognition matters. His B12 is now monitored as part of his diabetes care.

A checklist worth keeping

1. Consider B12 deficiency in anyone with fatigue, anaemia, neuropathy, cognitive change, depression or unsteadiness, especially older adults.
2. Ask about metformin, acid suppressing medicines, diet, gut surgery, gut disease and nitrous oxide use.
3. Do not rely on a normal blood count to exclude deficiency.
4. Use active B12 or methylmalonic acid to clarify borderline results.
5. Do not delay treatment when neurological symptoms strongly suggest deficiency.
6. Look for the underlying cause.
7. Monitor people with ongoing risk factors.