Stress in Animals and Humans: A Wholistic Analysis

This comprehensive analysis connects findings from animal studies to human health, emphasizing the importance of managing stress to prevent negative outcomes. For a comprehensive overview of our exploration into the interconnectedness of life and the cosmos, please visit the Holistic Index Page.

5 August 2024

Table of Contents

Introduction

The continued release of the stress hormone cortisol in both animals and humans has potentially major physiological effects. Here we discuss these stress-induced physiological changes in humans and animals, particularly cattle, where stress leads to inferior meat quality. An Earthworm Express article highlights the effects of long-term, low-intensity stress through cortisol and its impact on meat quality, leading to Dark, Firm, Dry (DFD) meat and the impact it can have on human health.

Animal Stress and Meat Quality

Chronic stress in cattle, characterized by long-term exposure to elevated cortisol levels, leads to a depletion of muscle glycogen stores. This depletion results in insufficient lactic acid production post-mortem, which prevents the meat from reaching a low enough pH to maintain proper water-holding capacity. This discussion pertains specifically to the naturally occurring water in meat, excluding any water that might be added during processing.

Animal muscle tissue naturally contains water content ranging between 65% to 80%, depending on the species, muscle type, and other factors such as age and feed. This intrinsic water is divided into three categories: free water, bound water, and immobilized water. Approximately 75% of this water is free water, which is easily lost during processing and cooking. The remaining water comprises bound and immobilized water, which are more closely associated with muscle proteins and cellular structures, making them less prone to loss. This natural water content is crucial for the meat’s juiciness, tenderness, and overall quality, significantly influencing the meat’s appearance and texture post-slaughter (Lawrie & Ledward, 2006; Honikel, 2004).

In the case of Dark, Firm, Dry (DFD) meat, the depletion of glycogen and the insufficient production of lactic acid post-mortem result in a high ultimate pH. This elevated pH impairs the muscle proteins’ ability to bind water effectively. Specifically, the failure to achieve a sufficiently low pH prevents the necessary structural changes in proteins that are needed to retain bound water, which is closely associated with the proteins’ polar groups. Consequently, more free water, loosely held within the muscle fibres, is released, leading to drier meat. The inability to retain adequate water, particularly free water, contributes to the meat’s darker appearance and firmer texture, making it less appealing and reducing its shelf life (Guàrdia et al., 2020; Gispert et al., 2012).

Pale, Soft, Exudative (PSE) meat in pigs arises from acute stress experienced immediately before slaughter. This stress triggers a rapid and significant pH decline while the muscle temperature remains elevated, leading to severe protein denaturation. The rapid decrease in pH, combined with high muscle temperatures, denatures muscle proteins such as myosin and actin. This denaturation is further exacerbated if the carcass is not cooled quickly enough, allowing muscle proteins to remain at high temperatures for extended periods. This results in more extensive denaturation. As these proteins lose their structure, they release more bound water, which becomes free water. The inability to retain bound water, combined with the release of free water, leads to the meat becoming pale, soft, and exudative. This loss of water, coupled with the denatured proteins’ inability to retain it, leads to a less desirable texture and appearance, reducing the meat’s marketability and shelf life (Zhu et al., 2011).

Even with rapid chilling, if the lactic acid levels are too high, the carcass may still develop PSE characteristics due to the overwhelming acidification and protein denaturation. To prevent PSE, the carcass must be cooled rapidly to slow the post-mortem biochemical reactions. The general recommendation is to reduce the carcass temperature to below 10°C (50°F) within 10 hours post-slaughter. Ideally, the carcass should be cooled to 7°C (44.6°F) or lower as quickly as possible to prevent the conditions conducive to PSE development. Maintaining rapid chilling helps reduce glycolysis rates, moderating the pH decline and minimizing protein denaturation (Lawrie & Ledward, 2006; Honikel, 2004).

The occurrence of DFD meat in cattle and PSE meat in pigs are the results of stress and its physiological response similarly, in humans, stress affects our health and in both instances is linked to the gut-brain axis and gut microbiota.

Gut-Brain Axis and Gut Microbiota – The Connection

The gut-brain axis is a complex communication network linking the gastrointestinal system and the central nervous system. Recent discoveries have emphasized the close connection between gut microbiota and brain function. The gut microbiota, composed of trillions of microorganisms, can influence brain function and behaviour through several mechanisms, including the production of neurotransmitters like serotonin and dopamine, modulation of the immune system, and interaction with the vagus nerve (Cryan & Dinan, 2012).

Certain gut bacteria produce neurotransmitters critical for mood regulation. Additionally, metabolites such as short-chain fatty acids (SCFAs) produced by microbial fermentation have neuroactive properties. The gut microbiota also affects the integrity of the intestinal barrier, influencing immune responses and neuroinflammation, which can impact mental health. Dysbiosis, or an imbalance in gut microbiota, has been associated with various mental health conditions, including depression, anxiety, and stress-related disorders (Clarke et al., 2013; Dinan & Cryan, 2017).

In animals, particularly cattle, stress impacts meat quality not only through the release of cortisol but also via other stress-related hormones such as adrenaline and noradrenaline. These catecholamines, released during acute stress, can exacerbate the effects of cortisol by increasing metabolic rate and energy expenditure, leading to further depletion of glycogen reserves in muscles. Chronic low levels of cortisol, beyond affecting meat quality, can have broader negative health effects on animals, including immune suppression, reduced growth rates, and increased susceptibility to diseases. (Lawrie & Ledward, 2006; Honikel, 2004).

-> Gut-Brain Axis and Gut Microbiota in Cows

We have seen that the gut-brain axis (GBA) is a bidirectional communication network that connects the central nervous system (CNS) with the gastrointestinal system, including the gut microbiota. In cows, this axis is important for mediating the effects of stress on health and physiological function. The cow’s digestive system, comprising four stomach compartments (rumen, reticulum, omasum, and abomasum), facilitates complex interactions between ingested feed, microbial populations, and the animal’s physiological state.

We also saw that stress in cows can lead to the release of cortisol and other hormones that alter gut motility, permeability, and blood flow, thereby impacting the gut microbiota. This stress-induced change can cause dysbiosis, an imbalance in the microbial community, where pathogenic bacteria may proliferate at the expense of beneficial microbes. This imbalance can significantly impair the cow’s ability to ferment and break down fibrous plant materials in the rumen, a process critical for producing volatile fatty acids (VFAs) like acetate, propionate, and butyrate, which are vital for the cow’s energy supply.

Under stress, the altered microbial environment in the rumen may produce fewer VFAs and more harmful metabolites, reducing nutrient absorption and the conversion of feed into body protein. Additionally, stress can lower the rumen pH, leading to subacute ruminal acidosis (SARA). SARA inhibits fiber-digesting bacteria and promotes acid-producing bacteria, further disrupting the microbial balance. This condition not only affects the cow’s digestion but can also lead to reduced growth rates and milk production, as well as increased susceptibility to infections and diseases due to compromised immune function.

Human Stress and Gut Health

Chronic stress in humans has the exact same effect as it has in animals. It can disrupt the gut microbiota balance, impacting both mental and physical health through the gut-brain axis. This disruption can lead to dysbiosis, causing gastrointestinal issues such as irritable bowel syndrome (IBS) and chronic diarrhoea. The gut-brain axis, as is the case in animals, facilitates communication between the gut and the brain, and stress can exacerbate these issues by altering gut permeability and microbiota composition (Foster & Neufeld, 2013).

The expression “I feel it in my gut” or “gut feel” is an expression we use which speaks to the physiological reality of the gut-brain connection, where the gut’s health can significantly influence emotional and instinctual responses.

The expression “I feel it in my gut” or “gut feel” relates to the gut-brain axis, a bidirectional communication network between the gastrointestinal tract and the central nervous system. This connection allows the gut to play a significant role in emotional and instinctual responses.

The gut is lined with a vast network of neurons, known as the enteric nervous system (ENS), which can function independently of the brain and spinal cord. The ENS communicates with the central nervous system via the vagus nerve, as well as through endocrine and immune signaling. This extensive neural network in the gut can influence mood and instinctual behaviors by transmitting signals that affect the brain’s emotional and decision-making centers.

For instance, the gut microbiota can produce neurotransmitters such as serotonin, dopamine, and gamma-aminobutyric acid (GABA), which are crucial for mood regulation. Approximately 90% of the body’s serotonin, a key mood-regulating neurotransmitter, is produced in the gut. These neurotransmitters can travel from the gut to the brain, influencing emotional states and contributing to the “gut feeling” or instinctual response.

The gut-brain axis can trigger instinctual responses through its role in detecting and responding to environmental stimuli. The gut is sensitive to stress and danger signals, which can prompt the release of stress hormones like cortisol which we have been discussing in this article. This hormonal response can enhance alertness and readiness for action, a critical component of instinctual behavior.

The gut’s influence on emotional and instinctual responses is mediated through complex interactions between the ENS, gut microbiota, and the central nervous system. This physiological reality is behind the common expression “gut feeling,” indicating how gut health can significantly shape our emotional and instinctual experiences (Mayer, E. A. (2011), Cryan, J. F., & Dinan, T. G. (2012).

The Gut-Brain Axis: The Power of a Good Meal

A good meal can significantly impact mood due to the intricate relationship between the gut and the brain due to the vast network of neurons in the gut. We have seen how this neural network can influence mood by sending signals that affect brain regions responsible for emotion. The gut microbiota plays a role in the enjoyment of a good meal also through their production of neurotransmitters like serotonin, dopamine, and gamma-aminobutyric acid (GABA).

The quality and composition of a meal can influence the gut microbiota and, consequently, neurotransmitter production. Consuming nutritious foods rich in fiber, probiotics, and essential nutrients supports a healthy gut microbiome, promoting the production of mood-enhancing neurotransmitters. This biochemical interaction can lead to improved mood and overall well-being, making a good meal comparable to other pleasurable experiences, such as good sex, in terms of its ability to enhance mood and satisfaction.

The sensory pleasures associated with eating—such as the taste, texture, and aroma of food—can also contribute to a positive emotional state. These sensory experiences, combined with the biochemical effects of the gut-brain axis, are the reasons why a good meal can be so satisfying and impactful on mood.

A good meal therefore elevates the mood through a combination of biochemical processes involving the gut-brain axis, the sensory pleasure of eating, and the nutritional support of a healthy gut microbiome. This holistic experience can indeed be on par with other pleasurable activities, showcasing the connection between food, emotions, and well-being.

Impact of Stress and Emotional Well-being on Gastrointestinal Health: A Case Study

Two individuals under severe stress reported that in one person’s case, severe stress resulted in significant abdominal pain and the other person’s case, prolonged and chronic diarrhoea. Both individuals reported a resolution of the symptoms the moment the stress was removed. The impact of stress and emotional well-being on gastrointestinal health is illustrated in both cases. Stress can activate the hypothalamic-pituitary-adrenal (HPA) axis, leading to the release of cortisol and other stress hormones that alter gut motility, permeability, and microbiota composition (Mayer, 2011).

Mechanisms of Stress Impact

Cortisol and Gut Microbiota: In both animals and humans, stress-induced cortisol release disrupts gut microbiota, leading to dysbiosis. In animals, this can impair nutrient absorption and metabolism, reducing the efficiency of converting feed into muscle and affecting meat quality. In humans, stress-related dysbiosis can exacerbate gastrointestinal symptoms, impacting overall health (Foster et al., 2017).

Gut-Brain Axis: The gut-brain axis mediates the effects of stress on gastrointestinal health. In humans, stress can exacerbate conditions like IBS and chronic diarrhoea, while in animals, chronic stress can lead to gastrointestinal disturbances affecting health and growth (Cryan & O’Mahony, 2011).

Immune Response and Inflammation: Stress increases gut permeability, or “leaky gut,” allowing bacteria and toxins to enter the bloodstream, triggering systemic inflammation. This inflammation can further disrupt metabolic processes and contribute to gastrointestinal distress in humans and reduced feed efficiency in animals. In the cases described, stress-related inflammation likely exacerbated digestive issues.

Implications for Meat Quality and Nutrition

The stress-induced changes in gut microbiota and metabolism in feedlot animals can result in poorer meat quality, such as DFD meat. The nutritional profile of meat can also be affected, often resulting in a less favourable fatty acid composition compared to pasture-raised animals, which experience lower stress levels and maintain a healthier gut microbiome (Scerra et al., 2011).

The expression “happy animals are tasty animals” reflects the reality that stress management benefits both animal welfare and meat quality. Similarly, humans who manage stress maintain normal biological functions and overall well-being.

Conclusion

This comprehensive analysis emphasizes the impact of stress on both animal and human health, particularly through the gut-brain axis and the role of gut microbiota. Chronic stress, characterized by prolonged exposure to elevated cortisol levels, leads to significant physiological changes, including the depletion of muscle glycogen stores in animals. This results in the production of Dark, Firm, Dry (DFD) meat due to insufficient lactic acid post-mortem, and Pale, Soft, Exudative (PSE) meat from acute stress, both of which are indicators of inferior meat quality.

These findings hightites the importance of managing stress to maintain a healthy gut microbiome, which is crucial for optimal digestion, nutrient absorption, and overall well-being. In humans, stress similarly disrupts the gut-brain axis, leading to gastrointestinal issues and affecting emotional and instinctual responses. The production of neurotransmitters by the gut microbiota and the physiological response to stress highlight the interconnectedness of the body’s systems, influencing both mood and physical health.

The expression “happy animals are tasty animals” is rooted in the reality that stress management is essential for both animal welfare and the quality of meat. Similarly, humans who manage stress are more likely to maintain normal biological functions and overall well-being, highlighting the importance of a holistic approach to health. Understanding these interconnected systems and managing stress effectively can lead to improved quality of life and better health outcomes for both animals and humans.

For a comprehensive overview of our exploration into the interconnectedness of life and the cosmos, please visit the Holistic Index Page.

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