Millions of Britons may be writing off the early symptoms of B12 deficiency as inevitable signs of getting older
Adults only need around two micrograms of vitamin B12 daily - less than a sliver of a grain of salt - to survive. But getting too little can have severe health consequences.
Despite a century of research into vitamin B12, deficiencies remain rife - with most cases going unrecognised due to their slow progression.
What are the symptoms?
Because no symptom is specific to B12, most people put them down to getting older. Also part of the problem is that no one ever thinks to question them because they develop so gradually.
Fatigue, weakness and breathlessness are the earliest signs, as are tingling or numbness in the hands and feet. Patients also report poor balance, memory difficulties and brain fog.
The irony of B12 deficiency is that the people it affects the most - older adults - are also those most likely to have their symptoms dismissed.
Persistent fatigue is the most common example - and it has a well-understood biological cause. Doctors have long associated fatigue with anaemia; without sufficient B12, the bone marrow is unable to produce healthy red blood cells.
Instead, it releases oversized cells that carry oxygen less efficiently, causing fatigue that feels indistinguishable from simple exhaustion.
In a 2026 study, low B12 levels were found to interfere with DNA inside mitochondria and reduce energy production in muscle cells, offering a possible explanation for why people report fatigue and brain fog well before anaemia shows up in their blood work.
Who is most at risk?
B12 is found in its natural form almost exclusively in animal-derived foods like meat, fish, eggs and dairy, putting vegans and vegetarians at a significantly elevated risk.
But the deficiency is also extremely common in older adults - for reasons that have nothing to do with diet.
Over time, the body becomes less efficient at extracting B12 from food. Some adults may even have sub-optimal levels of stomach acid, required to release the vitamin during digestion.
In some cases, autoimmune gastritis is to blame. This is where the stomach cells responsible for producing the acid and protein, also known as intrinsic factor, come under attack and prevent the absorption of B12.
How to diagnose and treat a deficiency?
B12 injections are an effective treatment for diagnosed deficiency, particularly in cases where impaired absorption is the cause.
Anyone suffering from brain fog or persistent tiredness can ask their GP for a blood test.
The condition is straightforward to detect and treat - the main difficulty is often recognising that what feels like ageing is something else entirely.