A B12 deficiency can go unnoticed for years. [Photo: Getty]

Ahead of next month’s “Veganuary”, experts are warning those ditching animal products not to forget a B12 supplement.

Vitamin B12 helps keep our blood healthy and immune system in check, according to the NHS.

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It is only found naturally in meat, fish, eggs and dairy. Vegans are therefore forced to rely on a supplement or fortified foods, like cereals and soya drinks.

Professor Tom Sanders - emeritus professor of nutrition & dietetics at King’s College London - warns around 20% of vegans are seriously deficient in the essential nutrient.

Veganuary alone won’t do much damage, but Professor Tim Key - deputy director of the cancer epidemiology unit at the University of Oxford - worries many use the month as a springboard to give up meat and dairy all together.

Our bodies “silently cope” with B12 deficiency for up to four years, before irreversible neuropathy - nerve damage - strikes, Professor Sanders warns.

According to the Mayo Clinic, “your body is capable of storing several years’ worth of vitamin B12”.

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Around 150,000 people worldwide took part in Veganuary last year, according to Vegan Food & Living.

Initially adopted for animal welfare reasons, people are increasingly giving up meat for the environment.

Livestock farming is responsible for 18% of human produced greenhouse gas emissions worldwide, The Conversation reported.

Others believe going vegan is good for their health, cutting the saturated fat from meat and dairy, while upping fruit and vegetable consumption.

While this may be true, many underplay the importance of vitamin B12.

“Many vegans think B12 deficiency is a myth thanks to self-appointed experts on the internet,” Professor Sanders said.

Others may be reluctant to take supplements full stop, thinking they are “unnatural”, Professor Key added.

The slow onset of severe symptoms may also cause many to downplay B12’s importance.

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“It takes a few years to develop,” Professor Sanders said. “Many ignore the signs of pins and needles.”

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A lack of the nutrient can trigger vitamin B12 deficiency anaemia. This occurs when the body produces abnormally large red blood cells that cannot carry out their normal function of carrying oxygen around the body.

As well as pins and needles, symptoms include extreme fatigue, a sore tongue, mouth ulcers, muscle weakness, blurred vision, poor memory and even psychological problems, like depression.

“You’re not going to get B12 deficient in one month but you could then become vegan permanently so you need to read up,” Professor Key said.

Professor Sanders adds a deficiency can be “so easily avoided by supplements or fortified food”.

The Mayo Clinic recommends a daily dose of 2.4 micrograms.

If deficiency develops, B12 injections or tablets may be required for the rest of your life, according to the NHS.