Thiamine was the first of the water-soluble vitamins to be identified and lead to the discovery of more such trace compounds essential for survival. All living organisms use thiamine, but it is synthesized only in bacteria, fungi, and plants. Animals must obtain it from their diet, and thus, for them, it is an essential nutrient.
In 1884, Kanehiro Takaki (1849–1920), a surgeon general in the Japanese navy, rejected the previous germ theory for beriberi and hypothesized that the disease was due to insufficiencies in the diet instead. Switching diet on a navy ship, he discovered that substituting a diet of white rice only, with one also containing barley, meat, milk, bread, and vegetables nearly eliminated beriberi on a 9-month sea voyage.
Thiamine is an essential substance involved in many complex biochemical reactions that help generate energy for the cell. But it is above all indispensable to the functioning of the nervous system, where its role is in the synthesis of the neurotransmitter acetylcholine, which transmits sensory impulses from one neuron to the other.
Clinical deficiency is rarely observed in dogs and cats because most commercial pet foods have adequate supplementation. Clinical signs of deficiency include anorexia, failure to grow, muscle weakness and neurologic dysfunction such as ventriflexion of the head in cats with paresis, and cardiac hypertrophy and ataxia in dogs.
Thiamine is found in a wide variety of foods at low concentrations. Yeast, yeast extract, and pork are the most highly concentrated sources of thiamine. Cereal grains, particularly whole grain cereals are the most important dietary sources of thiamine, by virtue of their ubiquity. Some other foods rich in thiamine are oatmeal, flax, and sunflower seeds, brown rice, asparagus, kale, cauliflower, potatoes, oranges, liver (beef, pork, and chicken), and eggs.