As a non-ruminant herbivore, horses are adapted to survive on a continuous diet of fibre rich feed, with a significant portion of daily energy needs met by the absorption of Volatile Fatty Acids (VFA) from the fermentation of fibre by the hindgut microbiome. However, horses are also able to digest and absorb other energy substrates, particularly glucose (from sugar and starch) and triglycerides (from fat). This means that horses have a complex digestive system for breaking down a range of nutrients into energy, and therefore a complex metabolic system which is governed and regulated by a network of organs, glands and hormones which together are known as the endocrine system.
The endocrine system has several functions within your horse’s metabolism – including appetite regulation and energy balance, energy metabolism and energy storage – and hormones are fundamental to these functions. But what exactly are hormones, how do they work and how do they influence daily metabolism and nutrition?
In the first of a two part blog, Lisa Elliott MSc – Equine Nutrition Solutions explores what are hormones, their functions and how they help our horses optimise nutrient and energy metabolism.
What are Hormones?
The word ‘Hormone’ is derived from the Greek work ‘Hormon’ which means to ‘activate’ and that is exactly what hormones do. Hormones are chemical messengers that have many different functions, the effects of which within your horse’s body are both wide-ranging and varied. A hormone is basically a message which leads to a physiological action, whatever that may be. Some familiar examples of hormones include insulin, which is important within glucose metabolism, alongside oestrogen and progesterone, which are involved in the female reproductive cycle.
The Endocrine System
The endocrine system consists of a group of tissues that release hormones into the bloodstream to travel to other parts of the body. Most endocrine tissues are glands that release hormones directly into small blood vessels within and around the tissue. Several important hormones are released from tissues other than glands, such as the heart, kidney, and liver. Some hormones act only on a single tissue, while others have effects on virtually every cell in the body. Hormones are present in the blood in very small quantities, so laboratory tests that measure hormone levels need to be very sensitive in order to accurately detect them.
Major Endocrine Glands
Nestled near the base of your horse’s brain, the pituitary gland is connected to the hypothalamus area of the brain and assumes a pivotal role, orchestrating the production of vital hormones that govern various bodily functions, including the activities of multiple other endocrine glands. Often hailed as the “master gland,” its extensive hormone repertoire renders it susceptible to a spectrum of disorders, each exhibiting distinct symptoms, depending upon the affected region within the gland.
Adjacent to the kidneys lie the adrenal glands, containing two distinct regions—the cortex and the medulla. The adrenal cortex has three tiers, each dedicated to synthesising specific steroid hormones. The outer layer surrenders mineralocorticoids, pivotal in regulating sodium-potassium balance, the middle tier manufactures glucocorticoids, instrumental in nutrient metabolism and inflammation modulation, while the inner tier produces sex hormones like oestrogen and progesterone. Meanwhile, the adrenal medulla plays a pivotal role in stress response and glucose regulation, secreting epinephrine, and norepinephrine to enhance heart function, elevate blood pressure, boost blood glucose levels, and impede digestion.
The pancreas, a complex organ, functions both as a digestive enzyme factory and hormone hub and is responsible for producing insulin. Within the pancreas are three distinct cell types, each tasked with synthesising a unique hormone. Predominantly, beta cells in the pancreas secrete insulin – which has a widespread influence over organ function, particularly in the liver, adipose tissue, and muscles.
Positioned in the neck as a bilateral structure, the thyroid gland governs numerous bodily processes through the secretion of iodine-containing hormones, T3 and T4. These thyroid hormones chiefly regulate the body’s metabolic rate, influencing various cellular processes. Working in concert with growth hormone and insulin, thyroid hormones facilitate tissue construction, but their excessive secretion can precipitate protein and tissue degradation.
Extending along the sides of the neck, the parathyroid glands intricately modulate the body’s calcium and phosphorus levels in the bloodstream. This regulatory dance involves the interchange of calcium and phosphate metabolism, vitamin D function, and bone formation, orchestrated by parathyroid hormone and calcitonin secretion from the parathyroid glands.
Key Hormones involved in Energy Balance and Metabolism
Your horse’s endocrine system produces several key hormones which have important roles to play within nutrition – through energy balance and metabolism:
- Insulin is an extremely important hormone within your horse’s body, whose main role is to remove glucose from the bloodstream by allowing it to enter the cells and is, therefore, pivotal in regulating blood sugar levels. After a meal as the level of blood glucose increases (following digestion), the pancreas releases insulin which stimulates the uptake of glucose from the blood stream into tissues and eventually cells. Glucose can then then be used to produce energy within cells or stored as glycogen for later use, predominantly in muscles or the liver.
- Glucagon is produced by alpha cells in the pancreas and works in the opposite way to insulin. Whereas insulin takes glucose out of the blood and moves it into tissue, glucagon sends a message to the liver to convert stored glycogen to glucose which is put back into the blood. Glucagon works in a homeostatic manner with insulin to help keep blood glucose levels normalised and therefore, like Insulin is also vital in helping to maintain blood sugar levels. Additionally, whereas insulin stimulates the storage of fat, glucagon stimulates the mobilization of fat stores into the bloodstream.
- Ghrelin is secreted by the hypothalamus, and by specialised cells of the stomach. Plasma ghrelin concentrations generally rise before meals and correlate with hunger scores. Ghrelin induces hunger and food intake and for this reason, ghrelin is often referred to as the ‘hunger’ hormone. Ghrelin stimulates appetite and may, therefore, have an important role in helping to regulate your horse’s appetite through the seasons, alongside its counterpart – Leptin.
- Leptin is a hormone produced in fat cells (adipocytes) that provides information to the brain (via the hypothalamus) regarding the availability of body fat stores, promoting satiety or in other words, a ‘feeling of fullness’ and a resulting reduction in food intake. Leptin is, therefore, often referred to as the ‘satiety’ hormone. Put simply, whilst ghrelin stimulates appetite, leptin suppresses it, and these two hormones work together to help maintain normal body condition and regulate feed intake accordingly.
- Melanocortins like Alpha-Melanocyte Stimulating Hormone (a-MSH) and Adrenocorticotropic hormone (ACTH) help mediate the appetite suppressant effect of leptin and horses demonstrate a season rhythm in circulating concentrations of a-MSH and ACTH, with higher concentrations in autumn. Horses naturally have a decrease in appetite and metabolic rate as winter approaches and it is possible that increases in both a-MSH and ACTH contribute to this decrease in autumn, helping to promote natural weight loss and maintain a healthy body condition throughout the year.
Adrenal Hormones
Catecholamines are released by the adrenal glands and play a major role in the regulation of cardiovascular function and carbohydrate/fat metabolism especially during ‘flight and fright’ responses (stressful stimuli), as well as during exercise. Namely norepinephrine and epinephrine (or ‘adrenaline’), both hormones stimulate the break down and mobilization of lipid and glucose stores, to be used as energy and fuel muscles.
Cortisol is a glucocorticoid hormone, which is secreted by the adrenal glands in response to stress and stimulates the break down and mobilisation of fats and amino acids as substrates to produce glucose through a process known as Gluconeogenesis. By stimulating the breakdown of fats and protein into glucose, cortisol helps ensure there is a direct source of energy for wherever it is needed, whether that is in the brain or powering the contraction of muscles to help the horse deal with a stressful situation
Both the catecholamines and cortisol, together with glucagon are known as glucose counter-regulatory hormones because they oppose the action of insulin on glucose metabolism.
Whether working together or opposing the actions of others, all these hormones work within a finely tuned system, controlled ultimately by the pituitary gland in the brain to ensure optimal energy balance and metabolism and help your horse to function, second by second, day by day and season by season.
But what happens when this system goes wrong and what are the effects on your horse? In part two of our hormone-focused blog, we will investigate the impacts on your horse’s health and well-being when the endocrine system doesn’t function as it should.
If you have any questions about creating the best diet for your horse and keeping your horse healthy through the right nutrition, please contact Coligone – 0333 0503785/07986 183616 or sales@hbradshaws.co.uk
