As a scientist, I rely on research studies to offer insight into the complex mechanisms that affect our horse's health. I also rely on hands-on experience and take a holistic approach in gaining a deeper understanding by looking at the entire horse -- his history, environment, activity, diet, and any physical and emotional stressors he may be enduring.
By Juliet M. Getty, Ph.D.
Research studies typically focus on one factor. But the horse’s body behaves in concert with a vast variety of influences that often cannot be ascertained by the specificity that research studies demand in a controllled setting.
Eat less and exercise more – that’s the way to help your horse lose weight. Right? Well, yes and no. Yes, reducing caloric intake and burning more calories helps your horse’s body use the energy he’s storing. But there’s a component to weight loss that has nothing to do with calories – it has to do with hormones. Hormones, such as cortisol and insulin, dictate to your horse’s body how much fat he will store; these hormones are keenly sensitive to stress. The science is complex but well worth understanding, so let’s take a look.
Research with a variety of species has repeatedly showni that stress tells the body to hold on to fat; the chemical changes that occur are similar to those produced during a famine. This is based on a primitive need to feel safe. Therefore, stress “tricks” the horse’s body into gaining weight just to survive.
If you’re seeking help for your overweight horse, you may be getting advice that is unsuitable for your horse’s long-term health. It may seem to make sense to feed your horse less, but how should this be accomplished? If eating less means taking away hay or pasture, then it’s contradictory to what your horse needs. Yes, do take away fattening cereal grains and sugars, but never, never, never restrict forage. Why? Because restricting forage is the most stressful thing you can do to your horse.
The physiology is indisputable: the horse is a grazing animal designed to chew all day long. His chewing produces saliva, which neutralizes the acid that’s continually flowing in his stomachiii which, therefore, should never be empty. He also needs forage flowing through his digestive tract to exercise those muscles; otherwise the muscles get flabby, which can bring on colic from a weak intestinal tract that torques and intussuscepts. Furthermore, the cecum (hindgut) must be full for digested material to exit, since its exit and entrance are both at the top. Otherwise colic can result from material left at the bottom.
Compensatory behaviors can develop: Deprived horses will chew on whatever they can—fences, trees, even their own manure. It’s pitiful to see. Chewing on non-feedstuffs makes a horse mentally acutely uncomfortable because it goes against his instincts, but physically he is attempting to resolve his discomfort and follow his innate drive to eat.
Now, about those hormones…
Food deprivation can be stressful and has been shown to result in weight gain in humans. How stress impacts increased body fat accumulation has been studied extensively.v The research using horses is minimal, but it’s there.
Cortisol is elevated during acute forms of stress such as pain,vi intense exercise,vii or transport,viii but appears to decline during feed deprivation. The mechanism that prolonged stress has on reduced cortisol is likely due to the alteration of the hypothalamus-pituitary-adrenal (HPA) axis. The HPA axis involves the hypothalamus portion of the brain and secretes several hormones within a fraction of a second of a stressful event. The key hormone, corticotropin-releasing factor (CRF), signals to the pituitary gland to release adrenocorticotropic hormone (ACTH). ACTH reaches the adrenal glands (near the kidneys) to initiate the secretion of cortisol, which circulates throughout the body to stimulate physiological stress responses.xi In the case of prolonged stress, these stress-activated biological reactions start to shut down, suppressing the HPA axis and negatively impacting the activation of CRF.
The studies that determined reduced cortisol when horses were fed forage on a restricted basis cannot necessarily be taken as absolute truth because of potentially confounding variables. For example, in many cases, you’re not told if the horses came from a setting in which their feed was already given in measured amounts rather than offered free choice – in other words, they may have already been living in a condition of prolonged stress. In that case, it would make perfect sense for the cortisol level to be low because the horse has endured the chronic stress adaptation of the HPA axis. I have found no research that took such feeding method change into account, except for one study where the ponies were initially kept on pasture and were of good health, only to have all but one of them gain weight after the experiment.
The key here is that cortisol levels decline over a period of months or even years of the chronic, unrelenting stress that forage restriction creates. So, in actuality, forage restriction over time is even more stressful than pain! Pain initiates an immediate cortisol response. But chronic stress taxes the HPA axis to such a degree that the horse can no longer regulate hormonal responses designed to protect him and keep him healthy.
Normally, cortisol secretion follows a circadian rhythm. Under normal circumstances, cortisol should be highest in the morning and gradually decrease over the course of the day.xvi However, in a recent study, researchers found the effect of feed deprivation to be just the opposite: cortisol concentrations were lowest at 8:00 am, and peaked in the middle of the afternoon. We do know that in this particular study some of the horses were kept on pasture beforehand, and others (stallions) were initially housed in individual dry lots with hay, though it is not stated whether the hay was provided ad libitum, or by scheduled feedings. Nonetheless, the researchers admitted that the abnormal cortisol levels may have been caused by differing feeding times or changes in feeding locations, both remarkably stressful to horses.
Regardless of cortisol levels, forage restriction can lead to pain within the stomach or anywhere along the gastrointestinal tract, and when that happens, pain can cause cortisol to increase. Severe ulcerations can develop rapidly when hay is withheld,i even within one or two hours. The stomach releases acid continuously, making the horse’s digestive system vulnerable to painful ulcerations when there isn’t a steady flow of forage at all times. When feed restriction causes pain, the stress of that pain can lead to an increase in cortisol through the HPA axis. Elevated cortisol stimulates the liver to derive glucose from glycogen stores, resulting in increased insulin secretion from the pancreas.
Stress elevates epinephrine. Another hormone, known as epinephrine, comes into the picture during stress episodes. You may be familiar with its other name – adrenaline. It is released during periods of acute stress or fear. It’s often referred to as the “flight or fight” hormone. Horses don’t stick around to fight when threatened; they run instead. However, if confined, they cannot run and they may feel as though their very lives are threatened. This heightened state of stress affects body weight in the same way that cortisol does by increasing glucose released from glycogen stores or glucose production from amino acids, leading to elevated insulin. When prolonged, insulin sensitivity (the body’s ability to use insulin effectively) declines, promoting fat storage.
Deprivation of hay overnight makes things worse. Researchers from Louisiana State University, found that mares having enough hay during the day but deprived of hay overnight showed the greatest degree of insulin resistance. Are you doing this to your horse? You may not think so, but if there is no hay left over in the morning, how do you know if your horse just ran out 10 minutes ago, or if he’s been without hay since 2 o’clock in the morning? If he’s running out, he is experiencing physical pain and discomfort from acid bathing the unprotected lining in the upper part of his stomach. Compound that with what your horse’s brain is thinking. He perceives that as “winter’s coming,” telling his body to hold on to fat. Both stressors ultimately raise insulin, and when insulin is high, there is no way your horse is going to burn fat. If he loses weight, what he’s losing is mostly muscle. So the vital thing to learn from this is that we are basically telling our horses to remain overfat to prepare for an “impending famine.”
Insulin resistance is impacted by hay vs pasture grazing. We know another interesting thing from those same researchers at Louisiana State University. They took a look at horses who were fed hay in a dry lot versus those that were able to graze on pasture and found that insulin resistance decreased when the horses were allowed to graze on pasture and increased when they were put in a dry lot. The researchers didn’t give a reason, but I think we can surmise that it had to do with the fact that the horses were able to exercise by moving around more and that they were less stressed.
Anticipation of the next meal impacts stress response. Researchers at Ohio State University found that boredom and anticipation of their next meal also affects cortisol levels. In other words, a horse that’s allowed to have only periodic feeding, and is therefore left with an empty stomach for hours, will have an increased level of cortisol flowing through his veins. You know what this looks like on the outside—how the horse will grab at the hay and eat it very quickly, because he terribly uncomfortable.
Leptin may play a significant role. Fat tissue secretes a hormone known as leptin. Under normal circumstances, leptin goes to the satiety center in the hypothalamus portion of the brain to tell it that the horse has had enough to eat and is satisfied.xxiv This is the body’s way of maintaining normal weight: fat increases, leptin rises, the brain says the body has had enough to eat, and weight comes down. But in the case of obesity, the horse may become leptin resistant due to inflammatory cytokines released by body fat that potentially damage the areas within the hypothalamus that recognize leptin.xxv Leptin is high in this circumstance, but the brain is not responding to it. The result? The appetite does not decrease; instead the horse keeps on eating, getting more obese, producing more cytokines, increasing inflammatory damage to the hypothalamus, resulting in even greater leptin resistance.
Restricting forage results in loss of muscle mass. In a recent study, forage restriction was proposed as an effective way to lose weight. However, even though horses lost weight when forage was limited, there was no change in body condition score, neck or girth circumference, cresty neck or rump fat. Instead, the longissimus dorsi muscle thickness was reduced. So, the weight loss had to have come largely from muscle loss! This further damages the metabolic rate. Furthermore, leptin declined when horses were limit-fed for 28 days. What does that tell you? It says that the horse is hungry! And there’s no food. Is that not stressful?
Sluggish metabolism is exacerbated by forage restriction. A recent study revealed that overweight horses do not always consume more than horses of normal weight. Therefore, not all obese horses are leptin resistant. Instead, a sluggish metabolic rate is likely keeping the horse in an obese condition on the same amount of calories. Adipose (fat) tissue is metabolically slower than muscle. Removing forage from the diet will force the horse’s body to breakdown muscle tissue for glucose, since elevated insulin inhibits the release of glucose from glycogen stores in the body. Consequently, the metabolic rate becomes even slower as more muscle is lost. This contradicts the effort to help a horse remain healthy while losing weight.
Leaky gut syndrome significantly increases inflammation. Stress can affect the intestinal barrier that protects normal gastrointestinal function. This is known as “leaky gut” syndrome and can create severe health problems in all species, including horses. It is a condition where the intestines become permeable to dangerous substances that can then enter the blood stream and create a variety of illnesses. Researchers from North Carolina State University recently studied leaky gut syndrome in horses, determining that leaky gut can lead to increased concentration of endotoxins promoting significant oxidative damage to the mucosal lining. The influx of endotoxins, primarily lipopolysaccharides, into the bloodstream stimulates the production of proinflammatory cytokines, potentially leading to hypothalamic inflammation that can lead to elevated leptin.
So what should you do to help you horse lose weight?
Feed appropriate forage free choice. Yes, really. Steadily grazing on forage matches what a horse would naturally do and makes the cells more responsive (more sensitive) to insulin. A healthy, insulin sensitive horse is a horse that will not easily gain weight when fed forage free-choice.
Make exercise an important part of your plan. Activity has been shown to increase insulin sensitivity in horses.
Reduce concentrates. Calorie reduction, though important, should only be accomplished by reducing or even eliminating commercial feeds and cereal grains.
Make sure your hay is appropriate to feed free choice. It is best to have it tested, so you know that it is low enough in sugar, starch, and calories. To evaluate the testing report, look at the column labeled “Dry Matter.” Add the ESC percentage (simple sugars) to the starch. This amount should not exceed 11%. The Digestible Energy (DE) is an indicator of calories and should not be more than 0.94 Mcals/lb (2.06 Mcals/kg) on a dry matter basis.
Test your pasture. Pasture grazing is the best way to keep your horse healthy. Grasses are not only highly nutritious, but grazing supports both physical and mental health. Get to know your pasture and periodically have it analyzed to offer your horses grazing opportunities at the most opportune times and conditions.
Allow your horse to self-regulate by always having forage available, with no gaps. He will soon get the message that he can walk away, and the hay will be there when he returns. He will eat less and eat more calmly.
Grazing muzzles may be used, but only with caution. They can defeat the purpose if they cause stress. Horses will make attempts to remove the muzzle by drinking more water. They will also be less active and spend more time standing still. If you try a grazing muzzle, be sure you limit its use to no more than 3 hours per day because the digestive tract needs more forage than such muzzles allow. Be absolutely certain that the mask allows for proper water drainage. And never put a horse out in pasture with a sealed muzzle! Not only is that unbelievably cruel—and stressful—it is also incredibly dangerous.
Consider slow feeders. Not all horses require them, but they are helpful initially to allow for slowing down intake.
Pay attention to inflammation. Once the horse loses body fat, the brain may remain leptin resistant, making the horse very hungry and he could gain back all the lost weight. Therefore, the approach must be to heal the inflammatory signaling in the hypothalamus. Ways to accomplish this can be found in a recent article on obesity,xxxvi as well as on the benefits of colostrum.
Reducing calories through depriving forage ironically keeps your horse overweight. Instead, pay close attention to meeting the horse’s instinctive need to consistently graze. The research validates this. These instincts are based on compelling physiological and mental needs. Make no mistake about this: when we ignore or deny those needs, we seriously imperil our horses. The closer you get to a feeding environment that simulates a natural setting, the healthier your horse will be. Give your horse a chance to be a horse, and let him tell you how much forage he needs.