PARASYMPATHETIC NERVOUS SYSTEM AND GUT HEALTH

The nervous system is comprised of many parts, but for this article the sympathetic nervous system, and parasympathetic nervous system are the focus. These two systems connect the same areas of the body, but with opposing actions.

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The Sympathetic Nervous System

The sympathetic nervous system responds to danger and stress, readying the body for physically strenuous lifesaving situations by redirecting resources from the visceral organs to the muscles. Heart rate is elevated, digestion inhibited and the body readied for action. This is commonly known as a “fight or flight” response, which can be seen as alert behaviours in dogs.

The Parasympathetic Nervous System

The parasympathetic nervous system in contrast, is known for “rest and digest” actions. Conversely to the sympathetic nervous system, the parasympathetic nervous system stimulates digestion and reduces heart rate, enabling a more relaxed body state for digestion, rest and recovery.

Constant State of Alert

There are situations where the body, rather than moving between activation of the sympathetic nervous system and parasympathetic nervous system dependent on the environment and the presence of dangers, remains in this “fight or flight” state. In a dog this could be seen as struggling to settle alone or without external soothing. Other characteristics include always being alert and looking for danger, following you around the house or pacing, explosively switching between calm and reactive behaviours, and struggling to recover to a calm state after an event. Other signs to note are disturbed sleep and upset digestion. Frequent upset digestion or loose stools, even loss of appetite can relate to this prolonged stress state. Resources are consistently directed away from the digestive system as the body remains ready for physical exertion in a dangerous situation. Regarding sleep, dogs require between 12 and 14 hours of sleep daily. Inadeqate sleep, in duration or quality, feeds the dysregulation of the nervous system further. The prolonged “fight or flight” state inhibits quality rest and sleep, as the body is readied for movement, rather than engaging this “rest and digest” state.

The Gut-Brain Axis

We have known for some time that there is direct two-way communication between the brain and the gut. We ourselves know how stressful situations can make us feel sick or upset our digestion. We also experience how changes to our diet, or upset digestion, affect our moods and mental state. We see the same connection in dogs.

Albright et al state “Psychological stress and physical disease can create dysbiosis, inflammation, and disruption of the microbiota–gut–brain axis”. Meaning that stress can cause imbalance in the gut microbiome environment, resulting in digestive upset. A perfect example of the gut-brain axes effects.

Impact of Gut Health on Behaviour

Since the understanding of the gut-brain axis, the gut microbiome has appeared as a study focus in regulation of behaviour and mental health. Diet, as a key method to affect the gut microbiome, is now studied to understand more of how these interactions work. The idea of “psychobiotics” is being studied; to understand how diet and functional nutrition can deliver beneficial impact upon mental health and behaviour. In humans we understand that healthy diets positively affect gut-brain communications, to potentially avoid or improve mental health disorders. Both short-term and long-term dietary habits and nutrients have shown to effect microbial diversity and populations in humans. Development of microbiome-targeting nutritional approaches is anticipated as future treatments of mental health disorders.

Cooccurrence of behavioural and digestive issues

In dogs, it is commonplace for behavioural issues, such as aggression and phobias, to occur alongside gastrointestinal problems. Dual focus on both the gut microbiome and behavioural solutions as a treatment therefore are most effective. Studies have identified differences in the gut microbiome populations between normal, aggressive and phobic dogs. This shows the correlation between the gut microbiome and mental state and behaviour. Dogs exhibiting aggressive behaviour were characterized by “a peculiar gut microbiome structure, a high biodiversity and an enrichment in generally subdominant bacterial genera”. Interestingly, Faecal Microbiome Transplants (Microbiome Restorative Therapy) has shown the impact of the microbiome population also. The receiver of the stool transplant exhibiting changes to more closely resemble the characteristics of the donor animal. Ranging from reduced aggression in aggressive dogs receiving a faecal transplant from an amicable dog, to weight loss and improved mental state respectively in mice where recipient mice were obese and depressed, and their donor mice showed opposite characteristics.

The results of the 2020 study by Mondo, et al suggest that scenarios of gut microbiome dysbiosis and subsequent long-term stress could affect behaviour, through the release of potentially neuroactive microbial by-products to the local gut environment.

Gut bacteria and stress response

Gut bacteria characterised as beneficial has previously shown to reduce levels of cortisol and adrenaline, supporting a reduction in stress and anxiety. This finding is supported by studies on germ-free mice. These are mice which are missing their usual gut microbiome, therefore allowing particular bacterial populations to be studied in isolation. Germ-free mice react dramatically to stress, which was reversed when Bifidobacterium infantilis was introduced. Different bacteria show different effects, from calming, to anxiety. Lactobacillus & bifidobacteria populations resulted in signals being sent to areas of the brain which regulate emotions, therefore impacting mood and behaviour.

The link between gut health and behaviour is being studied, and our insights are ever expanding. What is clear, however, is that gut health and behaviour are closely related. Modulating gut health through healthy and appropriate diet supports the health and diversity of the microbiome, allowing for mental health and behaviour to be supported and remove any behavioural problems arising through imbalances within the microbiome. Functional nutrition, including additions to the diet such as pre, pro and post-biotics utilise this knowledge that gut health has wide-ranging effects across numerous areas of the body. These “biotic” functional ingredients have shown to support the gut microbiome, through promoting diversity, and supporting the population of beneficial gut bacteria at the expense of pathogenic bacterial populations, improving the gut health generally. This positive impact upon the gut health and microbiome population then starts a domino effect through the direct links to other areas of the body, to benefit other areas of health – particularly, in the case of this article, the brain and behavioural aspects.

Summary

For dogs who show characteristics of stress and anxiety, and a prolonged “fight or flight” state from activation of the sympathetic nervous system, there are several approaches of support. Prioritising calm environments and reduced stresses, excitement and overstimulation can help dogs to move into the parasympathetic “rest and digest” state. Supporting their gut microbiome is also recommended, to promote the improved mental state and behavioural improvements associated with a diverse and healthy gut microbiome population. Psychobiotics and the use of diet and nutrients to manage mental health, mood and behaviour through functional nutrition, and alongside behavioural training, can equip dogs with the tools required to modulate behaviour and stress states.

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References

1. Eckert, R. Randall, D. Augustine, G. (1988) Animal Physiology: Mechanisms and Adaptations. 3rd edn. New York: W. H. Freeman and Company. Pages 226 to 228.
2. Habib, R. Shaw-Becker, K. (2021) The Forever Dog: A new science blueprint for raising healthy and happy canine companions. London: Thorsons (An imprint of Harper Collins Publishers). Pages 163 to 167.
3. Albright, J. Haug, L. Bidirectional Communication: The Gut–Brain Axis in Companion Animal Health. Veterinary Clinics of North America: Small Animal Practice, Volume 56, Issue 3, 2026, Pages 649-663. https://doi.org/10.1016/j.cvsm.2026.01.008.
4. Mondo E, Barone M, Soverini M, D’Amico F, Cocchi M, Petrulli C, Mattioli M, Marliani G, Candela M, Accorsi PA. Gut microbiome structure and adrenocortical activity in dogs with aggressive and phobic behavioral disorders. Heliyon. 2020 Jan 29;6(1):e03311. doi: 10.1016/j.heliyon.2020.e03311. PMID: 32021942; PMCID: PMC6994854.
5. Berding K, Vlckova K, Marx W, Schellekens H, Stanton C, Clarke G, Jacka F, Dinan TG, Cryan JF. Diet and the Microbiota-Gut-Brain Axis: Sowing the Seeds of Good Mental Health. Adv Nutr. 2021 Jul 30;12(4):1239-1285. doi: 10.1093/advances/nmaa181. PMID: 33693453; PMCID: PMC8321864.