Biomarkers of gastrointestinal functionality in animal nutrition and health

Effective gastrointestinal functionality is crucial in determining animal health, welfare and performance. A new definition of gastrointestinal functionality has been recently presented and it has identified the key components that contribute to effective gastrointestinal functionality and health. These components are: diet, effective structure and function of the gastrointestinal barrier, host interaction with the gastrointestinal microbiota, effective digestion and absorption of feed, effective immune status, and neuroendocrine function of the gut. Each of these components are linked to each other by several complex mechanisms and pathways, however, having identified some key components of gastrointestinal functionality offers the opportunity to evaluate potential biomarkers that can allow us to measure the functionality of the gastrointestinal system in farm animals. Numerous and rapidly evolving methodologies are producing an ever-increasing number and types of biomarkers, each with their own distinct advantages and disadvantages. Moreover, differences in models and methodologies make it difficult to extrapolate finding across species and to make meaningful comparisons, even when studies seem quite similar. This review will highlight the intrinsic challenges in choosing what biomarker to measure, where and when to measure it. Because of the complexity of the interactions between the key components of gastrointestinal functionality, we propose that the use of a single biomarker might not be feasible, rather we propose the development of a panel of biomarkers of gastrointestinal functionality that needs to be indicative not only of effective functionality and health of the gastrointestinal tract, but also of animal performance, health and welfare.