Cancer of the stomach and oesophagus is among the world's top five cancers. Survival rates are very poor as the disease presents late and early symptoms are non-specific. The study team has developed a non-invasive test for cancers of the stomach and oesophagus based on the detection of volatile organic compounds in exhaled breath. These compounds are known to be produced by both cancers as well as cancer associated bacteria within the gut. The proposed innovation is to improve the accuracy of this test by investigating whether simple metabolic substrates can increase the production of these volatile organic compounds by both the tumour and its associated bacteria.