Insulin and ACTH - Double Trouble in the Fall!
By Juliet M. Getty, Ph.D.
Horses are more likely to suffer from laminitis in the fall than any other time of year. Two reasons - Increased insulin secretion due to high levels of simple sugars and starch in pasture grasses, and increased blood ACTH (adrenocorticotropic hormone) secretion from the pituitary gland.
Insulin rise = laminitis
Simple sugars (denoted as ethanol soluble carbohydrates - ESC, on your hay analysis report) along with starch are digested down to glucose. Once glucose enters the bloodstream, it signals the pancreas to produce insulin. Elevated insulin is the most common cause of laminitis. It stimulates the production of "insulin-like growth factors" within the hoof's laminae, resulting in proliferation of the epidermal layer. The laminae have two inter-meshed layers, the epidermal and the dermal layers. When the epidermal layer lengthens and stretches with uncontrolled growth, it can weaken the laminae. This can lead to a structural failure by compromising the connection of the coffin bone to the hoof wall, creating a gap between the wall and the sole. You may see some hemorrhaging under your horse's foot - an indication of laminitis.
Insulin also rises due to the normal hormonal cascade initiated by stress. Stress can take many forms. Intense exercise, mental discomfort, pain, or an empty stomach (there should always be a steady flow of forage through the digestive tract) cause the pituitary gland to release ACTH. ACTH signals the adrenal gland to produce the stress hormones cortisol and epinephrine, both of which are needed to release glucose, for energy, out of glycogen stores in the liver and muscle. Glucose from liver glycogen stimulates the pancreas to secrete insulin. The healthy body has a homeostatic mechanism to maintain these hormones within a normal range.
ACTH naturally rises in the fall
All horses, regardless of health status, experience a rise in ACTH between mid-July and late-November (in the northern hemisphere). In the southern hemisphere, ACTH shows a seasonal rise between January and June. The increase in ACTH secretion helps prepare the horse for winter by encouraging a winter coat, and increasing insulin to promote fat storage.
Winter in a wild setting, when food is scarce, can be life-threatening. Elevated insulin helps protect the wild horse by telling its body to hold on to body fat. It is a survival mechanism. Unfortunately, we duplicate this in a domestic setting when we restrict forage, causing the horse's brain to think that "winter is coming," thereby putting the horse into survival mode and insulin is increased. The result is more body fat and an increased risk of laminitis.
The seasonal rise of ACTH, and hence insulin, can negatively impact the already insulin-resistant horse by further increasing inflammatory insulin, potentially leading to a laminitis attack. Therefore, it is important to be particularly cognizant of factors that affect sugar and starch accumulation in grasses during months when the horse's ACTH is naturally elevated.
Sugar and Starch content of grasses can vary with stress
Stress not only affects your horse; it also influences forages. Simple sugars and starches can vary according to temperature, rainfall, and other stressors. However, not all grasses are the same in the way they accumulate these carbohydrates. This is summarized in the table below:
C3 - Cool Season
(Timothy, fescue, orchardgrass, brome, perennial rye, Kentucky bluegrass)
C4 - Warm Season Grasses and Legumes
(Bermuda, Bahia grass, crabgrass, prairie grass, Teff, Tifton, alfalfa, clover, perennial peanut grass)
Cold temperatures (below 41° F; 5° C)
High ESC + Starch until stem base is no longer green
Low ESC + Starch; Dormant
Lower in ESC + Starch
High ESC + Starch (mainly as starch) in hot weather.
ESC + Starch is lowest in early morning if night was warm enough (above 41° F, 5° C) to allow for the plant to utilize sugars produced the day before. ESC + Starch will be highest in the late afternoon on a sunny day. Cloudy conditions or grass grown in the shade reduces ESC + Starch accumulation.
Lack of water for more than 5 days will increase ESC + Starch. New shoots will be high after it rains following a period of drought.
Excessive grazing or mowing
Accumulate ESC + Starch. Mow only low enough to remove seed heads.
- When the nighttime temperature remains below 41° F (5° C) for 2 to 3 weeks, cool season grasses are high in ESC + Starch, even in the daytime. Wait until the base of the stems are no longer green. If they remain green throughout the winter, consider testing your pasture. Please see these two articles:
- Warm season grasses go dormant and do not accumulate ESC + Starch once cool weather sets in.
- If the nighttime temperature remains above 41° F (5° C), the ESC + Starch level will be lowest in early morning until approximately 10:00 am and then again at night, starting a few hours after the sun sets.
- During times when the horse is not on pasture, allow the horse to graze free-choice on appropriately low sugar/starch hay. Hay needs to be available to your horse at all times, 24/7, so there is some left over in the morning. Slow feeders work very well for these situations.
Insulin resistant (IR) horses should be removed from pasture in climates where the nighttime temperatures start to get cold. Furthermore, ACTH starts to rise during mid-summer, increasing the risk for laminitis especially in IR and cushingoid horses. Test your hay for suitability and feed it free-choice to avoid stress during those times when pasture must be restricted.
[i] Adapted from two sources by Watts, Kathryn A.: (1) 2010, Pasture Management to Minimize the Risk of Equine Laminitis, Veterinary Clinics Equine, 26, 361-369; 2004, Forage and Pasture Management for Laminitis Horses. Clinical Techniques in Equine Practices, 31(1), 88-95.
For Permission to Reprint
For permission to reprint this article, in part or in its entirety, please contact Dr. Juliet Getty directly at Gettyequinenutrition@gmail.com.