Some of the most recognisable warning signs can show up on your tongue.
According to National, Heart, Lung, and Blood Institute (NHLB), a smooth, thick, red tongue is a sign of vitamin B12 deficiency.
Other symptoms of vitamin B12 deficiency involve the digestive tract.
As the NHLB explains, these symptoms include nausea (feeling sick to your stomach) and vomiting, heartburn, abdominal bloating and gas, constipation or diarrhoea, loss of appetite, and weight loss.
READ MORE: Eight telltale signs you have low vitamin B12 levels – what to look for