The food intolerance test entails the measurement of exhaled gas after lactose intake. Food intolerance is mainly lactose intolerance, a condition caused by the lack of the enzyme lactase in the intestines. Symptoms of diarrhea and allergies are manifested after ingestion of foods containing lactose, and the lactose hydrogen blow test is the primary test for food intolerance. This is a relatively simple test in which the person to be tested ingests a portion of lactose and then checks the amount of hydrogen in the exhaled gas. A high hydrogen content in the exhaled breath after ingestion of lactose indicates that the body has a lactase deficiency and is unable to break down the lactose ingested into the body, resulting in symptoms such as diarrhea. People who are lactose intolerant should avoid foods containing lactose to minimize diarrhea.