What is insulin?
What are the primary sources of carbohydrate in a horse’s diet?
What is insulin resistance?
What causes or contributes to insulin resistance?
How do I tell if my horse is insulin resistant?
What are the best ways to help my insulin resistant horse?
What are the warning signs of laminitis?
What is insulin and insulin resistance?
Insulin is a hormone that is produced by the Islets of Langerhans which are clusters of cells present in the pancreas. Its actions are widespread and profound on the body and they mostly target metabolism of sugar and starches (carbohydrates), fats, and proteins. It is required by most cells in the transport of glucose across the cell membrane and the pancreas releases it in response to elevations in blood glucose. More is needed when foods high in carbohydrates are digested and less is needed during exercise and physical activity.
||Fluorescent microscopic image of a pancreatic islet.
Our body depends on these pancreatic cells releasing insulin to maintain normal levels of blood glucose.
||Light microscopic image of a cross-section of an islet in the pancreas..
Inset: a small islet is shown at higher magnification.
|Photos courtesy of Dr. LuGuang Luo,
Roger Williams Medical Center, Providence, RI.
Sugar (glucose) in the blood comes primarily from the foods ingested. The body also has the ability to make glucose in the liver, a process called gluconeogenesis. Grains like oats and corn as well as commercial grain mixtures and sweet feeds contain a high amount of sugar and starch and when ingested will increase blood glucose levels substantially more than a feed like plain beet pulp. According to a database by Dairy One, oats and corn have an average NSC (Non-Structural Carbohydrate = total sugar and starch) level of 44% and 73%, respectively. See the Table of NSC Values of Various Equine Feeds.
Grass and hay also contain sugar and starch, sometimes at very high levels. An average amount of sugar in grass hay, according to Dairy One's database, is 11%, but this can vary widely. Katy Watts, of Rocky Mountain Research, produced Timothy hay that contained 31% sugar. This means that if a horse eats 15 pounds of such hay a day, then he would ingest more than four and a half pounds of sugar every day. For more information about sugar in grass and hay, please see www.safergrass.org.
Insulin resistance is a condition in which the cells of the body become less sensitive to the effects of insulin, i.e. cellular receptors for insulin do not properly respond to insulin and the pancreas produces more insulin in order to keep blood glucose (sugar) levels controlled. This can happen if there are abnormalities of the binding of insulin to the receptor or of the receptor's response to insulin.
Many factors can cause or contribute to insulin resistance, including:
- genetic predisposition
- a diet that is high in sugar and starch (non-structural carbohydrates)
- inadequate exercise and activity
- dietary/mineral imbalances
- being overweight or obese
Certain breeds of horses such as Morgans, Arabians, Peruvian Pasos, Paso Finos, as well as ponies, may tend towards insulin resistance as a genetic adaptation to sparse vegetation. The problem is exacerbated by domestic lifestyles with insufficient turnout and exercise and by feeding practices that are not appropriate for them, i.e. too high in carbohydrates. Horses have evolved to be active and moving the majority of the day and did not evolve to eat the levels of grain that are commonly fed.
Insulin resistance can be diagnosed by a qualified veterinarian by testing for blood insulin and glucose levels. External signs of insulin resistance may include:
- abnormal fatty deposits, especially along the crest, rump, and above the eyes
- excessive urinating (polyuria) and drinking (polydypsia)
- voracious appetite
Obesity is associated with insulin resistance, but it does not necessarily imply it; a horse can be obese without being insulin resistant, and vice versa. Horses diagnosed with Equine Cushing's Disease may or may not also be insulin resistant, i.e. a horse can be insulin resistant, Cushing's, both insulin resistant and Cushing's, or neither.
It is possible for a horse to be fat in some areas, e.g. the crest, while having ribs that show and loss of muscle along the top line. A diet high in soluble carbohydrates leads to high blood glucose levels which in turn leads to an increase in insulin. Increased insulin in the presence of high glucose allows glucose to flood the cells. Since cells have no mechanism to excrete glucose and must utilize it once it is within the cell, cells become less sensitive to insulin which decreases the cells' permeability to glucose. Eventually, cells may lose their sensitivity to insulin to such an extent that the body cannot produce enough insulin to get glucose into the cells; then the cells begin to starve and muscle wasting results, but the abnormal fatty deposits remain.
Managing the Insulin Resistant Horse
Successful management of the insulin resistant horse is a multi-pronged approach that involves:
- Minimizing the sugar and starch level in the horse's total diet (including that in hay) and maintaining a healthy and appropriate body weight.
If the NSC content of the hay is too high (the number depends on the individual horse and degree of insulin resistance, but generally above 10%), it can be reduced by an average of 30% by soaking in water. For details about soaking hay to reduce its soluble carbohydrate content, see Katy Watts' Hay Soaking Study. For a list of feeds and hay alternatives, and their NSC values, see NSC Values of Various Equine Feeds.
Keep an eye on your horse's weight. This can be done by monitoring his body condition score. Body condition scoring objectively evaluates a horse on the amount of fat deposited in specific body locations (e.g. neck/crest, rump, behind the shoulders, ribs) and assigns a number, 1 (extremely thin) to 9 (very obese); he should be close to a 5. Note that a horse with a body condition score of less than 5 can still be insulin resistant, e.g. a horse can have ribs showing and still be insulin resistant. Learn more about body condition scoring from the University of Maine Cooperative Extension and Susan Evans Garlinghouse.
- Optimizing nutrient levels and their balance
In addition to the level of carbohydrates, the dietary levels and balance of minerals and essential fatty acid balance both affect insulin resistance and are affected by it. Magnesium, zinc, chromium, and biotin are some of the nutrients that can positively affect insulin resistance.
Magnesium is one mineral that affects both insulin secretion and action and is therefore important in glucose homeostasis; low intracellular magnesium levels result in impaired insulin function. Dietary magnesium levels in hay may be low either in absolute terms or in relative terms. Calcium is a competitor to magnesium. Hence, there can be an effective magnesium deficiency if calcium is too high relative to magnesium. In other words, the ratio of calcium to magnesium is just as important as the absolute levels of each mineral. Similarly, imbalances amongst other minerals can and often do exist in the typical equine diet and may be a source of problems. See the Mineral Ratios article for more on mineral interactions.
- Increasing activity and exercise
Exercise and activity is also a critical aspect in managing insulin resistance; exercise decreases blood glucose levels, and hence insulin levels.
A proper diet is no substitute for exercise and exercise is not a substitute for a proper diet.
- Maintaining the hooves appropriately
Proper hoof care is another essential part of managing the insulin resistant and/or Cushing's horse. Feet should be maintained in a balanced trim with low heels and a short breakover (breakover shortened when needed by backing up the toes and/or by a toe rocker, not by removing the sole callous). For help in what to look for in a balanced hoof, see Gene Ovnicek's Natural Balance Tutorial.
The Warning Signs of Laminitis
Laminitis can accompany insulin resistance in horses and often occurs before it can be diagnosed by radiographic changes.
Probable warning signs that may indicate laminitis are:
If your horse exhibits any of these signs, contact your veterinarian and enlist the help of an experienced hoof care provider. It is vital to identify and remove the cause of the laminitis.
- divergent hoof wall growth rings, i.e. faster growth at the heel than at the toe
- distinctive rings (founder rings)
- pounding or abnormal pulse from the digital arteries and/or unusual heat in the hooves.
The digital artery can be felt as it passes over the proximal sesamoid bone or just above the lateral cartilage (on either side of the limb).
- rocking back on the hind feet in order to turn
- constant shifting from one foot to the other
- cramped muscles in the hind end and abnormally weighting the hind end
If your horse shows signs of active laminitis, e.g. pounding digital pulse or rocking back on the hind feet in order to turn, it is important to mechanically support the hooves by duct taping blue styrofoam to the bottom of the hooves or by placing the horse in an area with deep sand. Blue styrofoam can be purchased from a local home-building store or preshaped pads may be obtained from EDSS. Application instructions can be downloaded from here.
More subtle signs that may indicate laminitis are:
- difficulty turning or pivoting on one hoof
- stiff or choppy gaits
- "arthritis" type of pain
- ouchiness or short-striding on rocks or hard surfaces when that was not previously a problem
- soreness in the back and/or shoulder muscles, in the absence of saddle fit or work-related issues
- bruising or discoloration of the white line (the white line should normally be clear)
- stretching/widening of the white line (the white line should be tight; approximately 2 to 5 mm, depending on the size of the horse and its hooves)
Be vigilant and aware of these signs. An ounce of prevention (and early intervention) are worth much more than a pound of cure.