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Stress, particularly chronic stress, is one of the most destructive influences on the body, leading to suppressed activity of anti-oxidant systems and immune function, decreased glucose uptake and utilization, increased lipid peroxidation, and increased inflammation. Subjecting animals to a period of stress produces characteristic changes in several hormones and parameters associated with the central nervous system and the hypothalamic-pituitary-adrenal axis (HPA).
First, it is important to understand how the HPA axis works. It starts with the hypothalamus, a specialized area of the brain that is a part of the neuroendocrine system. The hypothalamus has many functions, such as controlling the horse’s body temperature, water balance, thirst, and hunger. This control area of the brain also affects such vital functions as heart rate, blood pressure, and digestion, and acts as a controller of the pituitary gland that sits just below the hypothalamus. During times of stress, the hypothalamus releases corticotrophin-releasing factor, which in turn signals the pituitary gland to release adrenocorticotropic hormone, or ACTH. This hormone then travels through the bloodstream to the adrenals, two triangle-shaped glands located on the top of the kidneys. When ACTH reaches the adrenals, it causes them to release the hormone known as cortisol.
Overburdening this HPA axis can lead to an unhealthy increase in cortisol, a reduced sensitivity of the HPA to feedback down-regulation, and a disruption in the circadian rhythm of cortisol secretion. It can also lead to central nervous system changes, including the stress-induced depletion of catecholamine neurotransmitters such as norepinephrine and dopamine. An acute increase in beta-endorphin levels is also observed under stressful conditions.
During a challenge or stress, more sugar (glucose) is released into the blood from the horse’s storehouses. Initially, this glucose is quickly taken up by the tissues to carry-out their work. But as stress becomes long-term or chronic, several things happen. Cortisol, the hormone most associated with long-term stress, acts to decrease the movement of glucose from the bloodstream into muscle cells (and several other types of cells). This is meant to be a protective response, helping to conserve blood glucose for essential functions, such as brain activity. But in the exercising horse, it can mean that athletic performance is lower than expected because of the decrease of glucose available to the working muscles. Additionally, build-up of waste products from the increased cellular metabolism can impair the cell’s ability to convert glucose efficiently to ATP, the cell’s fuel. This negative cycle can eventually result in as much as a 40% decrease in cellular energy.
For an animal to successfully combat stress, adaptation is required. Adaptation is best thought of as the ability to be exposed to a stressor, while responding with little or no characteristic hormonal disturbances. Adaptation also implies being capable of rapidly reassuming homeostasis (or equilibrium) after the stressor is withdrawn. As an example, a well-trained equine athlete can participate in an event that would induce a large HPA fluctuation (stress response) in a sedentary animal, and yet the conditioned horse will be relatively unaffected. This is a result of adaptation that has occurred during the horse’s training process. Additionally, if the equine athlete is exposed to stressors it was not trained for, hormonal disruptions characteristic of a stress response would be expected; however, this response might not be as great as that found in a less fit individual. Furthermore, after the stress ends, the trained athlete would be expected to re-establish homeostasis more rapidly. This is an example of non-specific resistance to stress gained by a training-induced higher level of fitness.
The benefits provided by feeding plant adaptogens can be compared to the training an athlete undergoes in order to prepare for competition. Plant adaptogens cause the animal’s physiology to begin the adaptation process to stress.
When a stressful situation occurs, feeding adaptogens generates a degree of generalized adaptation (or non-specific resistance) that allows your horse’s physiology to handle the stressful situation in a more resourceful manner. Adaptogens help the adrenal glands to mount an immediate hormonal response, by manufacturing and releasing more stress hormones. But when stress stops, the adaptogens help the adrenal glands return to a resting state more quickly. If stress is prolonged and severe, the adrenals reserve their resources by reducing the amount of hormones released due to adaptogenic maintenance of hypothalamic receptor sensitivity. This conserved energy is available to continue the body’s response to stress, thereby delaying adrenal exhaustion.
Adaptogens help glucose to cross cellular membranes more easily, and can lower or stabilize the levels of sugar more quickly to normal. They are also valuable in facilitating the conversion of glucose to ATP and in helping to rid the cell of toxic waste by-products.
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