Even those with the normal range could be missing out on the essential vitamin

B12 deficiency symptoms are notoriously easy to miss as they’re both wide-ranging and typically slow to develop. And while the risks increase with age, younger people can be affected equally.

Research shows that the problem may be more widespread than previously assumed after a study of 231 healthy adults aged 71, on average, found that those with lower B12 levels performed worse on time-sensitive neurological and cognitive tests than participants with higher levels.

The vitamin is essential for a functioning nervous system, healthy red blood cell formation and DNA synthesis.

Crucially, it also helps convert protein and fat into energy, as well as supports the formation of neurotransmitters and hormones that regulate brain function.

What are the symptoms?

Fatigue is "the most common" early symptom of B12 deficiency, according to registered dietitian and nutritionist Shelley Balls.

But because tiredness has so many possible causes, it is worth keeping an eye out for a wider pattern of symptoms.

This could encompass weakness, low mood, pale skin, heart palpitations, tingling in the hands or feet, appetite and weight loss and occasionally fertility problems.

The real damage occurs when these symptoms are ignored, Shelley warned. This is why treatment is so important.

Because the body stores B12, symptoms can sometimes take years to emerge, but megaloblastic anaemia can be a sign of a long-standing deficiency.

Certain medications raise the risk too, including metformin, commonly used to manage diabetes - which can hinder B12 absorption.

How to keep a deficiency at bay?

Eating a varied diet is essential for maintaining optimal B12 and folate levels. Animal products like meat, fish and dairy are the main sources of B12, while vegans can turn to fortified nutritional yeast.

Folate is found in leafy greens, broccoli, peas, citrus fruits, chickpeas and kidney beans.

Marmite is another source of B12, but it's high in salt, with a single serving providing 76 per cent of the recommended daily intake.