Several essential nutrients are not available from plants or fungi. Creatine, carnosine, taurine, EPA and DHA omega-3, haem iron and vitamins B12 and D3 generally only occur naturally in foods from animal products.

The nutrients found in vegan foods are minimal. To get the minimum amount of vitamin B6 each day from the richest plant sources, you'd have to eat about five cups worth of potatoes.

Vitamin B12 is only found in animal products like eggs and meat.

Low B12 is widespread in vegans. There are some cases of children whose brains failed to develop because of their vegan parents.

Later in life, the IQ is directly correlated with the amount of B12 in a person's blood. the brains of those with lower B12 is six times more likely to be shrinking.

Iron plays a prominent role in brain development and is essential throughout our lives. One study found that giving women iron supplements led to intellectual gains. In those whose blood iron levels improved, their cognitive performance increased between five- and seven-fold.

Those adopting a vegan diet are particularly prone to iron deficiency. The form most readily absorbed by the body is only found in animal proteins.

Some of the essential nutrients can be acquired from supplements. However, nutrients such as taurine are easily overlooked. Others are so obscure that vegans are unlikely to have even heard of them.

• There are small amounts of taurine in some dairy products, but the primary dietary sources are meat and seafood. Vegetables do not contain taurine.
• Choline is found in eggs, beef and seafood. People who consume eggs tend to have almost double the choline levels and enjoy a lasting cognitive edge.
• Creatine supplies our cells with energy. Vegans and vegetarians have deficient levels in their bodies.