How to substantiate health claims for gut health and immunity? Health and functional claims made on foods, frequently focus on the digestive tract and already, there is a large market for gut-functional food products worldwide. However, we need to take into account the guidelines as drawn up by the European Food Safety Authority (EFSA). These guidelines make the substantiation of a health claim in the area of gut health and immunity quite a challenge. In this white paper we will discuss the definition of a health claim, the most important EFSA guidelines and our solution to substantiating health claims: challenge studies.