Is there a universal definition of gut health in pigs?

What a healthy gut is, depends on whom you ask. A pig producer, for instance, will define ‘gut health’ in terms of ‘good pig performance’, where a veterinarian may be defining this as ‘lack of disease’. A nutritionist may think mostly of solutions, whereas a scientist may say that a clear definition of gut health still needs to be found. Often, gut health is described in terms of what it is not, i.e. disease or absence thereof. Think of visible signs of animals not being healthy, e.g. the necessity of veterinary medication or low performance.

Gut health in humans

In the science of the human body and gastro-intestinal tract, a clear definition is given of gut health. In 2011, Stephan C. Bischoff, researcher at the University of Hohenheim, Stuttgart, Germany, summed up in BMC Medicine that gut health covers multiple positive aspects of the gastro-intestional tract, being:

• Effective digestion and absorption of food;
• Normal and stable intestinal microbiota;
• Effective immune status;
• State of well-being.

For 2 main reasons, the agricultural community worldwide has recently also taken a much stronger interest in gut health in production animals. Since animal health is often measured in terms of high performance and production, the gut becomes of major interest due to its function. In addition, gastro-intestinal maladies rank among the highest causes of neonatal morbidity and mortality, including domestic livestock species.

What is the gut?

In order to be able to uncover what ‘gut health’ means, it would be best to characterise the gut as well as to identify its main functions. The gut is an important immune organ as it contains the largest lymphoid mass of the pig’s body. In addition, the gut is the home of millions of resident microbes, together known as the ‘gut microbiota’. Thirdly, the microbiota genome has been estimated to contain 150-fold more genes than the host genome.

Using these characteristics, three different, important functions can be identified for the gut.

Digesting and absorbing nutrients effectively

Digestion is the result of enzymatic breakdown of macromolecules and absorption of nutrients to the blood or lymph circulatory systems. This starts in the pig’s mouth and continues throughout the gastro-intestinal tract.

It is vital to keep in mind that the pig gut is not something static – it changes throughout a pig’s life. With the development of the pig come quick and marked alterations in digestive and absorptive functions.

Providing a home for commensal microbiota

The gut forms a home for a wide range of bacteria, the density of which is growing from the stomach (108 per g digesta at a pH of 3-4.5) to even 1011 at a pH of 6-6.5 in the colon. A similar growth appears to be true for lactobacilli, which are more prevalent towards the end of the intestinal tract. Coliforms also grow in numbers, but in the hindgut their numbers decrease slightly.

Again, essential in all this is to realise that the gut is constantly changing, so this should not be considered with a static view. After all, a change of microbiota mass and composition comes with the development of the pig and the time. Furthermore, a healthy microbiota is characterised by a good diversity, a good barrier function and a microbial community composition that is strongly influenced by factors as nutrient availability and peristalsis.

Effective barrier function through the mucosa

The mucosa forms a barrier between the body and the luminal environment. Its role is to allow efficient absorption of nutrients while excluding the passage of pathogens, antigens and toxins.

A normal gut function

A normal gut function can thus be assessed, by for instance analysing the intestinal microbiome, and by using markers of intact immunity, intestinal integrity and functionality. Nevertheless, the question remains whether all these markers together indicate ‘gut health’.

One key concept to get closer to gut health is to introduce the term ‘resilience’. Interesting in this respect is a publication by Thomas F. Döring of Humboldt University, Berlin, Germany. In an article in the Journal of the Science of Food and Agriculture in 2013, he and his team defined ‘resilience’: “A measure of the persistence of systems and their ability to adsorb change and disturbance and still maintain the same relationships or state variables.” ‘Disturbance’ in this context can be seen as an external principle, i.e. the source of the disturbance being outside of the system that is assessed for its resilience. This can be characterised by both frequency and severity.

So in short, resilience is always defined as a capacity or ability of a system or individual to react (respond) to an external force (disturbance) while fulfilling some further conditions at the end of the response (outcome).

Find out more about Gut Health

Gut infections can be a major source of efficiency loss, downtime and additional costs for veterinary treatment. Check out the Gut Health special section, which delves deeper into several feeding and managements practices regarding gut health and how they can positively influence animal health, performance and profitability of the farm.

Resilience and gut health

Why apply the term ‘resilience’ to gut health? Resilience matches perfectly with a more dynamic formulation of health. In this approach, health is not a potential end-goal or target, but an ability to respond and interact with environment. Connecting the dots between definition and the pig gut, ‘responses’ then include parameters of normal or abnormal gut functioning; ‘disturbances’ include for instance antibiotic therapy, pathogens or a different feed; the ‘outcome’ is the continuation of the gut function.

In short: ‘resilience’ is an important criterion for ‘health’, when studying factors which can influence the highly dynamic gut ecosystem for pigs.

This text is an approved summary of the presentation ‘What is a healthy gut?’, given by Dr Charlotte Lauridsen, head of Immunology and Microbiology at the Department of Animal Science, Aarhus University. She spoke at Pig Progress’ ‘Healthy Gut, Healthy Piglet’ seminar during EuroTier 2016, in November 2016, in Hanover, Germany.