What is British Marmite or Australian Vegemite made of?

Marmite and Vegemite are traditionally eaten as a savory spread on bread or toast. The product is made of brewer's used yeast, which would be thrown away otherwise. The product sales boosted after the discovery that it is a rich source of the vitamin B complex. During the First World War, British troops were issued with Marmite as part of their rations.