A method of assessing the state of the lining of the small intestine in a mammal or human injesting a suitable labelled test substrate which requires conversion by a brush border enzyme for transport across the small intestinal lining. Suitable substrates include sucrose and maltose. The transport of these can very conveniently be measured by assessing the amount of label present in carbon dioxide expelled in the breath after a period of time. It has been shown that this method has application to assessing damage cause by chemotherapeutic induced mucositis as well as gastroenteritis with an infectious cause.