Here at My Pet Nutritionist, we take a holistic view of your pet’s health. So, whilst we focus primarily on nutritional adjustments we can make, we look at everything else that is going on for your pet too. Not surprisingly we find that stress is a key player in many of the concerns brought to us. Most of us are familiar with the concept of stress, but do we truly understand the mechanism and the far-reaching impact it has on the body?
Stress is huge, so we’ll try to condense it as much as we can. Stay with us.
Despite the biological stress response being around for millennia, we first started paying attention to the actual response in the early part of the 1900s. Walter Cannon was the first Professor of Physiology at Harvard, and he was particularly interested in how the body managed to maintain its balance even when faced with things that could threaten it’s being.
Building on this, Hans Selye established that no matter the stressor, each body (and brain) experienced the same cascade of responses. He also proposed that during the response to one stressor, our ability to withstand another is diminished. We often reference his theory when we discuss trigger stacking – and we’ve all been there – that day, when you miss your alarm, you spill the milk as you’re putting it in your coffee, then you lose your keys – and your ability to manage challenges that day just seems reduced. But on a biological level, we also see that when we are going through particularly stressful times, our immune system isn’t quite as nifty as dealing with threats and so we start picking up bugs and that old faithful cold sore rears its ugly head.
The same concept applies to dogs, and if you sadly share your life with a reactive dog, you’ll see trigger stacking in them too. Equally, if they live with chronic stress, their immune function may go rogue too. This is why we always consider stress levels in those dogs who suffer with inflammatory skin issues.
The official response starts in the brain. The amygdala, being the part of the brain that deals with emotions, sends a message to the hypothalamus which is like the command centre of the body. It communicates with the rest of the body through the autonomic nervous system which controls involuntary body functions like breathing, blood pressure and heartbeat.
The autonomic nervous system is then split further into two branches, the sympathetic nervous system, and the parasympathetic nervous system. These are the systems that we are particularly interested in in the stress response.
The Sympathetic Nervous System
This system is known as the fight or flight system. We can think of it like the gas pedal of the stress response.
- Dilates pupils – to see oncoming danger,
- Inhibits saliva production,
- Dilates bronchia,
- Increases heart rate,
- Inhibits the activity of the digestive system,
- Relaxes urinary bladder,
This system is like a redistribution centre. When the body is faced with a stressor that can challenge body homeostasis, it redirects resources to re-balance. Blood is sent to the limbs to mobilise and why waste energy on food digestion?
But this is where we see many issues. As we know, to absorb and utilise nutrients the digestive system needs to do its job. If the stress response is inhibiting the action of the digestive system, then it can’t do what it needs to.
As part of sympathetic response, corticotropin-releasing-factor (CRF) is released. The name isn’t particularly important, but it seemingly plays a considerable role in altering gastrointestinal functions.
It has been found that CRF:
- Inhibits gastric acid secretion,
- Inhibits small intestine transit,
- Increases intestinal permeability,
- Increases visceral sensitivity.
Gastric acid is a digestive fluid formed in the stomach, produced by parietal cells. The highly acidic environment in the stomach causes proteins from foods to lose their folded structure (denature) which exposes the peptide bonds. It is therefore a key player in protein digestion. Gastric acid secretion is regulated by the parasympathetic nervous system via the vagus nerve and by the hormone gastrin which stimulates the parietal cells.
There is also an increased risk of infection with reduced gastric acid secretion. Low or no gastric acid can reduce the disinfectant properties of the gastric lumen.
Gastric acid suppression is also associated with the development of food intolerances and allergies.
The small intestine is chiefly involved in the digestion and absorption of nutrients, it receives pancreatic secretions and bile through the hepatopancreatic duct to aid its functions. In the stress response, it is thought that this mechanism encourages vomiting to expel potential pathogens, as in the large intestine, transit time is increased, again to aid expulsion. Poor motility can lead to small intestine bacterial overgrowth (SIBO) which is frequently implicated in nutritional deficiencies, malabsorption, weight loss and bowel changes like diarrhoea.
The intestinal barrier is made up of tight junctions which allows the absorption of nutrients whilst limiting the transport of potential harmful antigens. Early stress and chronic stress have been seen to increase intestinal permeability both through TJ function and mucous degradation.
Animal studies have demonstrated that early life stress impairs development of mucosal barrier function, becoming a predisposing factor to intestinal disorders in adult life. However, these stress induced changes do seem to respond to probiotic intervention. Probiotics have been seen to restore colonic tight junction integrity and enhance the mucus barrier.
This is largely because stress impacts the bacterial community found in the gut. Studies have shown that early life maternal separation, for example, results in a significant decrease in fecal lactobacillus numbers, three days post separation. Mothers who report feeling stressed have a significantly higher number of proteobacteria and lower lactic acid bacteria.
Evidence suggests that long term stress facilitates pain perception and sensitises pain pathways. There is a clear gene-environment interaction which results from maladaptive changes in neuronal circuitry. Stress-induced visceral pain alters DNA methylation and histone acetylation patterns within the brain, leading to an increase in pro-nociceptive neurotransmitters. The effect of chronic stress on visceral pain is best illustrated by the high co-morbidity of anxiety and depression with other functional pain disorders like irritable bowel syndrome.
As long as the brain perceives the threat hasn’t yet passed, CRF will continue to be released, triggering the release of further hormones including the one we all know and love, cortisol.
Cortisol gets a bad rap, because it is known as the central indicator of the stress response. But the two drivers of the fight or flight response are in fact the sympathetic nervous system and adrenaline, cortisol just comes along to regulate it.
Cortisol functions to maintain homeostasis; it is required for normal cellular and metabolic function. Animals deprived of adrenal function are incredibly hard to keep alive (again, we really need to rethink whether our tests are ethical). And the fun fact for today, cortisol serves to inhibit inflammation.
So we need cortisol, and it is the release of cortisol that provides the negative feedback to get us out of the fight and flight response and into rest and digest, or more technically known as the parasympathetic nervous system.
The Parasympathetic Nervous System
- Constricts pupils,
- Increases saliva production,
- Reduces heart rate,
- Stimulates the activity of the digestive organs,
- Constricts urinary bladder.
Both the parasympathetic and sympathetic nervous system are able to stimulate the enteric nervous system.
This is the system that is described as the second brain. It is embedded in the lining of the gastrointestinal system and can operate independently of the brain and spinal cord. The enteric nervous system has a number of roles which include responding to bulk in the digestive tract which influences peristalsis, it mixes and churns the contents of the tract and also plays a role in the secretion of various hormones.
But this brain in the gut can suffer a sort of mental breakdown. Exogenous stimulation from food-based antigens, toxins and invading microbes sensitise the gut which promotes mast cell degranulation and the release of pro-inflammatory mediators, altering enteric nervous system function. From a protective standpoint, this in born defence mechanism flushes out offending organisms and ingested antigens, but chronic stress can also activate mast cells, producing a range of uncomfortable symptoms including diarrhoea, cramping and visceral hypersensitivity (and more!).
If you are still with us at this point, you’ll have gathered that stress significantly influences digestive function. Not only does this potentially lead to nutrient malabsorption but altered motility can also contribute to a range of issues throughout the digestive tract.
The important thing is to mitigate stress wherever possible.
A stressor is a stimulus that challenges the integrity or health of the body. The stress response is a compensatory reaction made to that challenge.
- Psychological – emotional stressors; depression, anxiety, fear, trauma,
- Physical – over-exertion, accidents, trauma, pain,
- Environmental – pollutants, extreme temperatures, allergens, radiation,
- Biological – bacteria, mould, virus, parasites,
- Chemical – toxins, pesticides, herbicides, heavy metals, fumes, dust,
- Consumable – highly processed foods, GM foods,
There is such a thing as good stress though and that can include exercise, challenges, learning new skills and more! But the body can’t differentiate stress, the response is the same – so be mindful of how much your dog has on his plate.
There are some beautiful herbs and diet modifications that be used to support those stressed dogs, along with other lifestyle changes, so if you would like some support then please check out our services.
Thanks for reading,