"There are some tragic cases of children whose brains failed to develop because of their parents being ill-informed vegans," says David Benton, who studies the link between our diets and brain chemistry at Swansea University. In one example, the child was unable to sit or smile. In another, they slipped into a coma.

Later in life, the amount of B12 in a person's blood has been directly correlated with their IQ. In the elderly, one study found that the brains of those with lower B12 were six times more likely to be shrinking.

Even so, low B12 is widespread in vegans. One British study found that half of the vegans in their sample were deficient. In some parts of India, the problem is endemic — possibly as a consequence of the popularity of meat-free diets.