Obesity is an epidemic problem with domesticated horses. Although we most easily attribute the problem to overfeeding concentrates combined with too little exercise, the underlying cause is much less apparent. It has to do with the horse’s brain and his response to stress -- a chronic low-grade, inflammatory stress.
Discomfort, from any source, induces a biochemical response in the brain that triggers the horse to do whatever he can to survive. Research with a variety of species has repeatedly shown[i] 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.
Stress can come from many sources – stall confinement, isolation from buddies, sleep deprivation, change in environment, travel to strange locations, excessive training and performing, pain and illness, exposure to toxins, and the most stressful of all – not being allowed to graze on forage at all times. Forage restriction is incredibly stressful.[ii] Putting the horse on a “diet” by limiting the amount of hay he can have will create a chain of chemical reactions that prevent the very outcome the “diet” was meant to ensure. Let’s look at more specifics…
Stress causes the adrenal gland to release the hormone cortisol. Cortisol tells the tissues to ignore insulin’s attempts to get glucose into the cells.[iii] So insulin increases to try to overcome this, but not very successfully. When insulin is elevated, the cells hold on to body fat. And when body fat increases, it releases a hormone called leptin. Normally, leptin is a good thing, but not in this case.
The brain can become resistant to leptin. Under normal circumstances, leptin (secreted from fat tissue), 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.[iv] 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.
The excess body fat of obese horses promotes inflammation through its secretion of substances known as cytokines.[v] Cytokines can damage the areas within the hypothalamus that recognize leptin.[vi] Leptin is high, 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 greater leptin resistance.
Perhaps you’ve had your horse’s cortisol level checked and it is normal. You assume that stress is therefore not an issue. But this can lead to a false assumption. Cortisol can actually be elevated inside the cell and not in the bloodstream, due to the overexpression of an enzyme called 11-beta-hydroxysteroid-dehydrogenase-1, present in fat, liver and brain cells that produces active cortisol. This has been shown in several species, including horses,[vii] and leads to the vicious cycle resulting in hypothalamic damage.
Elevated cortisol can reduce T4 levels leading one to believe that thyroid medication is necessary. But reduced T4 under this circumstance is not an indication that the thyroid gland is underactive, nor is it an indication that more thyroid medication is needed to help the horse lose weight. Furthermore, adding T4 to the diet will not do any good if the horse is stressed, simply because excess cortisol interferes with the conversion of T4 to T3, the active hormonal form.
Some horses have suffered from forage restriction for so many years that their metabolic rate has become severely impaired. For these, modest, short-term weight gain can be a consequence of free-choice feeding. Be patient. The transition can take several months. Allow your horse time to become accustomed—both physically and psychologically—to this new way of eating. Healthy weight loss takes time. When fed following the steps outlined below, the large majority of horses, even those grossly overweight, will adjust, lose weight and in time, arrive at a healthy body condition.
The leptin resistant horse will, first and foremost, have excess body fat. His appetite will seem insatiable and he will rarely lift his head from eating. His metabolic rate is sluggish, causing him to pack on the pounds very easily. He is reluctant to move and his energy level is low.
Reduce inflammation! Three factors to consider:
Important to understand: Once the horse loses body fat, the brain will initially remain leptin resistant, making the horse very hungry so he could gain back all the weight. Therefore, the approach must be to heal the inflammatory signaling in the hypothalamus.
To do this:
Many barn owners are reluctant to feed hay free-choice because of the apparent expense involved in purchasing more hay. But in actuality, horses who are permitted to self-regulate their intake will eat less. It’s only when several hours lapse between feeding that they eat very quickly and consume everything in sight. But when they get the message that hay is always available, that they can walk away from it and it will still be there when they return – then, and only then will they eat just what their bodies need to maintain a healthy weight. They will actually eat less than before.
Absolutely! Living, healthy grass is the best whole food around. Grazing in the open air is the best stress reducer your horse can experience. The amount of grazing depends on your horse’s individual condition. Yes, pasture can be high in sugar and starch but it can vary depending on the month, the time of day, level of rainfall, sunlight, etc. Get to know your pasture grasses.
Turn off the body’s fat-hoarding response by taking measures to reduce stress. Combine this with an anti-inflammatory diet and increased movement, and your horse’s brain will regain its ability to respond properly to leptin. Taking off weight will become much easier.
[i] Block, J.S., He, Y., Zaslavsky, A.M., et. al., 2009. Psychosocial stress and change in weight among U.S. adults. American Journal of Epidemiology, 170, 181-192. Also, Gabriel, J., 2008. The Gabriel Method. Atria Books.
[ii] Getty, J.M. 2014. Restricting forage is incredibly stressful. Choose a different approach to weight loss.
[iii] Tiley, H.A., Geor, R.J., and McCutcheon, L. J., 2007. Effects of dexamethasone on glucose dynamics and insulin sensitivity in healthy horses. American Journal of Veterinary Research, 68(7), 753-759.
[iv] Freidman, J., and Halaas, J., 1998. Leptin and the regulation of body weight in mammals. Nature, 395, 763-770.
[v] Wisse, B., 2004. The inflammatory syndrome: The roles of adipose tissue cytokines in metabolic disorders linked to obesity. Journal of the American Society of Nephrology, 15(11), 2792-2800.
[vi] De Git, K.C., and Adan, R.A., 2015. Leptin resistance in diet-induced obesity: The role of hypothalamic inflammation. World Obesity, (16(3), 207-224.
[vii] Farias, F.H.G., 2007. 11Beta-Hydroxysteroid dehydrogenase activity in feline, equine, and Ossabaw swine adipose tissue. Master of Science Thesis, University of Missouri-Columbia. Also, Morgan, S.A., Sherlock, M., Gathercole, L.L., et al., 2009. 11Beta-Hydroxysteroid Dehydrogenase Type 1 regulates glucocorticoid-induced insulin resistance in skeletal muscle. Diabetes, 58(11), 2506-2515.
[viii] Getty, J.M., 2014. Equine Digestion – It’s Decidedly Different.
[ix] Lestelle, LR., Earl, L.R., Thompson, Jr., et.al., 2011. Insulin-glucose dose response curves in insulin sensitive and insensitive mares and effect of overnight and long-term feeding regimen. Journal of Equine Veterinary Science, 31, 285-286.
[x] Getty, J.M., 2015. "Do you need to analyze your hay?"
[xi] Berkseth, K.E., Guyenet, S.J., Melhorn, S.J., et. al., 2014. Hypothalamic gliosis associated with high-fat diet feeding is reversible in mice: A combined immunohistochemical and magnetic resonance imaging study. Endocrinology, 155(8), 2858-2867.
[xii] Banks, W.A., Coon, A.B., Robinson, S.M., et. al., 2004. Triglycerides induce leptin resistance at the blood-brain barrier. Diabetes, 53(5), 1253-1260.
[xiii] Schell, T., DVM, 2015. Reducing the effects of stress and anxiety of health with ashwaghanda. Nouvelle Research www.nouvelleresearch.com
[xiv] Liburt, N.R., Fugaro, M.N., Wunderlich, E.K., et. al., 2011. The effect of exercise training on insulin sensitivity and fat and muscle tissue cytokine profiles of old and young Standardbred mares. Journal of Equine Veterinary Science, 31(5-6), 237-238.
[xv] Yi, C.X., Al-Massadi, O., Donelan, E., et. al., 2012. Exercise protects against high-fat diet-induced hypothalamic inflammation. Physiology & Behavior, 106(4), 485-490.
[xvi] Getty, J.M., 2014. "The correct way to use slow-feeders."
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