Review: Methods and biomarkers to investigate intestinal function and health in pigs.

Society is becoming increasingly critical of animal husbandry due to its environmental impact and issues involving animal health and welfare including scientific experiments conducted on farm animals. This opens up two new fields of scientific research, the development of non- or minimally invasive (1) methods and techniques using faeces, urine, breath or saliva sampling to replace existing invasive models, and (2) biomarkers reflecting a disease or malfunction of an organ that may predict the future outcome of a pig's health, performance or sustainability. To date, there is a paucity of non- or minimally invasive methods and biomarkers investigating gastrointestinal function and health in pigs. This review describes recent literature pertaining to parameters that assess gastrointestinal functionality and health, tools currently used to investigate them, and the development or the potential to develop new non- and minimally invasive methods and/or biomarkers in pigs. Methods described within this review are those that characterise gastrointestinal mass such as the citrulline generation test, intestinal protein synthesis rate, first pass splanchnic nutrient uptake and techniques describing intestinal proliferation, barrier function and transit rate, and microbial composition and metabolism. An important consideration is gut health, and several molecules with the potential to act as biomarkers of compromised gut health in pigs are reported. Many of these methods to investigate gut functionality and health are considered 'gold standards' but are invasive. Thus, in pigs, there is a need to develop and validate non-invasive methods and biomarkers that meet the principles of the 3 R guidelines, which aim to reduce and refine animal experimentation and replace animals where possible.