This is an abbreviation of unit 12 from the Advanced Equine Nutrition course. The complete text, a video, and quiz questions are available with The Horse’s Advocate membership. You can also purchase them separately. Note: Passing the Basic Equine Nutrition Course is required before starting the Advanced Course.

Unit 12 – Stress: Its Impact On Horses

Stress is a universal phenomenon affecting every animal, plant, and non-living object. Consider the molten rock that builds pressure under a mountain, culminating in a volcanic eruption. Another example of stress is lightning, which occurs when cloud charges build up. Or the bones in our bodies respond to the weight carried and the twisting and bending pressures of running by building more bone where needed. This Piezoelectric effect created by these stresses is why bones don’t look like 2x4s or I-beams; instead, they have curves and lumps, and astronauts worry about bone loss in weightlessness.

In other words, stress forces the body to adapt to what the environment applies to it. It can be much more than their weight or the twists and turns of running. Stress can be hot or cold, dry or wet, thin or fat, abundance or starvation. It can be a little or a lot, acute or chronic, and physical or perceived. This chapter will focus on cellular stress caused by food, but all stress is additive. The food can be perfect, but if the groom is abusive, the music is too loud, and the window is left open in winter, creating an inescapable draft, these add to inflammation from food.

Horses living in the wild demonstrate remarkable resilience in adapting to stress. They run to escape enemies, migrate to water and food, deliver babies without help, and live in harmony with their families. This adaptability is not just remarkable; it’s inspiring. Most stress they encounter is short-lived or within the boundaries of life, and nature provides mechanisms to counter it. In any form, stress in nature is usually temporary and recoverable. Disease and mortal wounds cause insurmountable stress and eventual death. Chronic inflammation from chronic stress can also lead to disease and death, but these are often slow and insidious, frequently undetected until it becomes hard to recover. A great example of this is obesity and metabolic syndrome in horses and cardiovascular disease in humans.

The Good (Temporary) And Bad (Chronic) Stresses

Stress involves two hormone systems that affect metabolic processes: the immediate response to life-threatening stress and the more sustainable, life-supporting response.

Hormesis And Allostasis

A natural ‘set point’ exists between the daily high and low cortisol levels. This mean average is called hormesis. In simpler terms, hormesis represents the body’s natural state when daily stresses are applied. When the body leaves the set state of hormesis, it triggers reactions to balance the body against the change in stress. An example is when the outside temperature changes and causes the body to shiver or sweat. These changes are usually within a day (foraging versus sleeping) or a season (summer or winter).

When many stressors are applied over time, the set point changes to a higher level, a movement called allostasis.

Oxidative Stress

The previous chapters covered the cell mechanisms for taking in food and converting it into energy. As a measure of potential energy in food, calories consumed must equal those used, stored, and eliminated. Many factors can affect either side of the equation, such as food availability or the need to travel long distances. In the wild, food availability is affected by the seasons, but its availability also changes with movement to faraway lands. However, in the human world of horse care, fences stop migration, and high-carbohydrate food (grains and hay) remain constantly available throughout the seasons. This pattern creates inflammation, called oxidative stress, within the cells and their mitochondria, making energy conversion inefficient. It’s crucial to be aware of man-made environmental factors (fences, food) and their potential impact on your horse’s stress levels, as it underscores your responsibility for your horse’s well-being.

Reducing Reactive Oxygen Species And Oxidative Stress

Most people try to decrease ROS by adding antioxidants to their diets. This action aligns with the common belief to add something to fix a problem. As most of you know by now, my belief is to prevent the issue and respect the evolution that has allowed most things to live in health. So, how can we do this with our horses? 

Studies in humans and other animals show that cells can deal with ROS up to a point. It is helpful to think of cell metabolism as a factory assembly line with thousands of moving parts. Periodically, the line shuts down for maintenance and lubrication. This time off occurs in cell metabolism when no food enters the mouth, normalizing substrate levels in the blood. 

Key Points:

1. The purpose of stress is to help horses adapt to changes in their environment.
2. The immediate response to a life-threatening event releases adrenaline and noradrenaline. The goal is to provide immediate fuel from any source for fighting or running. It is quick to occur and quick to be gone.
3. The less rapid response to non-threatening stress releases cortisol. The goal is to conserve energy.
4. Cortisol causes lipogenesis, blood vessel constriction, water retention, immune sensitivity, increased awareness and arousal, and gluconeogenesis.
5. Adrenaline is for life-saving, and cortisol is for life-preserving.
6. Chronic long-term cortisol causes obesity, hypertension, dehydration, autoimmune or immunosuppression, behavioral problems, and nutritional deficiencies.
7. Hormesis is the average of all environmental stresses, creating a set point of body processes to accommodate the normal stresses of everyday life.
8. Allostasis is the increase in body processes to a higher set point in response to increased stress.
9. Oxidative stress is inflammation within the cell caused by long-lasting metabolic processes that do not allow time for reducing work and cleaning up the byproducts.
10. Reactive Oxygen Species (ROS)are the free radicals formed within cells from the metabolism of fuels. They are molecules with elements that have gained or lost electrons, causing them to have a charge (unpaired electron) and become unstable.
11. ROS at low levels helps cell survival, but it causes cell damage and even death when elevated.
12. Consuming antioxidants is common, but it is less effective than promoting the internal clearing of ROS. These actions include fasting, reducing excess dietary glucose and fructose, reducing triggers of the Polyol Pathway, and exercise.