Category: Digestion

Let’s look at insulin resistance in horses. Research released in June 2011 suggests that any elevation in insulin levels in a horse is dangerous.

A horse with elevated insulin and would be similar to a prediabetic person.

There don’t seem to be hard numbers on how many laminitis horses exist in the United States, but there were at least 57 million U.S. adults ages 20 or older who had prediabetes in 2007, according to the Department of Health and Human Services. The United States has about 313 million residents in 2011, which would indicate about one in five people is prediabetic.

Another sobering statistic: Diabetes was the seventh leading cause of death in humans in 2006, according to the National Diabetes Information Clearinghouse.

In people, type 1 diabetes results in the pancreas producing little or no insulin. Type 2 diabetes results in the body not using that insulin effectively and blood glucose reaching dangerous levels.

Horses normally don’t reach the point of being so insulin resistant that they develop diabetes. The problem is what elevated insulin does to their feet.

So what is insulin and blood glucose and why is it killing humans and horses alike?

Insulin is a hormone, a chemical substance produced by the body that is carried to target cells waiting for that hormone to give them instructions. Hormones control and regulate the activity inside those cells and are essential for digestion, metabolism, growth, reproduction and mood control.

The pancreas produces insulin to regulate glucose (the pancreas also produces enzymes to digest food).

Glucose is a carbohydrate. Carbohydrates are found in plants and can be simple or complex.

Glucose is the most simple carbohydrate, the smallest possible sugar molecule. Glucose is a major source of energy for cells.

Starches are complex carbohydrates and can be made up of hundreds or thousands of glucose molecules.

In the small intestine, starch is broken down into individual glucose molecules. The glucose then enters the bloodstream, and the blood glucose level rises. Blood glucose levels rise considerably after a meal.

Blood glucose is carried throughout your body. As elevated blood glucose hits the pancreas, beta cells in the pancreas release insulin. The insulin transports glucose into the cells for use as energy, at which point blood glucose levels drop back to normal. Cells cannot access glucose without the help of insulin.

Glucose that is not needed for energy is turned into glycogen and stored in the liver and muscles. Glycogen is used for brain functioning, maintenance of the central nervous system and performing high-intensity physical activity.

If blood glucose levels get too low, alpha cells in the pancreas release glucagon. Glucagon is a hormone that tells the liver to release glycogen for energy. Insulin and glucagon maintain consistent levels of blood glucose using this pattern.

If blood glucose levels rise too rapidly due to high sugar and starch intake, and the pancreas churns out more insulin, the cells can become less receptive to the onslaught of insulin; over time, it may take more insulin to transport glucose into the cells. This is one form of insulin resistance and leads to excess insulin and glucose in the bloodstream.

Horses have no insulin receptors in their feet. According to research released in 2011 by researchers in Australia, excess insulin in the bloodstream can bind with receptors in the equine foot that were designed to receive insulin-like growth factor 1. Scientists theorize the insulin-like growth factor 1 may have limited use in making the hoof grow outward as a foal turns into a horse. In adult horses, these receptors are fooled by the insulin and bind to it, leading to abnormal hoof growth and laminitis. Insulin-like growth factor 1 also has been shown to be responsible for metastitis of malignant tumors in humans, according to the Animal Health Foundation, which funded the equine insulin research in Australia (http://www.ahf-laminitis.org/2011/06/major-breakthrough-in-understanding.html).

Meanwhile, the excess glucose in the bloodstream due to insulin resistance is converted into fat and stored in adipose tissue, leading to weight gain.

According to the website of Dr. Frank Reilly of Equine Medical and Surgical Associates, after a grain meal, the insulin level goes up three to four times the regular level due to insulin responding to glucose pouring into the bloodstream.  This is normal and lasts a few hours.

In insulin resistance, the base level of insulin can be constantly elevated three times higher, and a grain meal can cause the insulin level to go up 30, 40, 50 or more than 100 times the usual level and remain there for longer periods.

Reilly says people with insulin resistance and diabetes have their lifespans shortened by seven to 12 years; for horses, this translates to 2.5 to 4.5 years, he says.

Horses that develop laminitis may have a considerably shorter lifespan.

Insulin resistance also can be caused by excess fat. Fat cells can release a toxin that interferes with insulin’s action on the cells. The higher the insulin and blood sugar levels, the more excess glucose is converted to fat in adipose tissue, and the fatter the horse gets.

Insulin resistance creates yet another vicious cycle by lowering thyroid levels. The thyroid gland produces hormones that regulate the body’s metabolism. When the thyroid gland produces less of these hormones – thyroxine (T4) and tri-iodothyronine (T3) – metabolism slows, leading to weight gain.

Other causes of insulin resistance are Cushing’s disease and stress (stress leads to increased cortisol, which increases blood glucose and interferes with insulin).

Let’s look at how a piece of food passes through the laminitic or foundered horse. This will serve as the framework for the second part of the series on how high insulin levels and insulin resistance can cause laminitis. This part does not segue into the next part but rather serves as a short tutorial on where the food goes and what happens to it. Learning this process may come in handy if you have an emergency and a vet tries to describe to you a problem in the digestive track. It’s hard to make informed decisions when you can’t picture what’s going on.

Intake

When a horse sees food it wants to eat, it starts salivating and producing stomach acids. Once the horse puts the food in its mouth, digestive juices from the salivary glands start breaking down the food chemically while the horse’s teeth break the food down mechanically. The tongue combines this finer material with the saliva to make a moist bolus that can be swallowed. Horses have three pairs of salivary glands that produce up to 10 gallons of saliva per day. Saliva is made up of bicarbonate, which buffers the stomach from amino acids, and the enzyme amylase, which assists with carbohydrate digestion.

Esophagus

The food enters the esophagus, a tube 50 to 60 inches long. The esophageal muscles push the food down toward the stomach by relaxing in front of the food and tightening behind it, known as peristalsis, a one-way process in the horse. The force of the food tells the lower esophageal sphincter to open, allowing the food into the stomach.

Stomach

The stomach in a horse is small. It holds 8 to 16 quarts but functions most efficiently when it’s only about three-quarters full.

The stomach muscles churn the food, while gastric juices continue to break down the food into a fluid called chyme. Hydrochloric acid breaks down solid particles, and pepsin, an enzyme, digests protein. The chyme then goes through the pyloric valve into the small intestine. Food can pass through the stomach in as little as 15 minutes.

Small intestine

The small intestine is 70 feet long and 3 to 4 inches in diameter and holds up to 48 quarts, or 12 gallons. It has three parts: the duodenum, jejunum and ileum. This is the organ where most digestion takes place. Partially digested food is digested more, and nutrients are absorbed through the walls of the small intestine, entering the bloodstream. Food passes through the small intestine in 30 to 90 minutes. The faster the food passes through, the less it is digested.

The chyme is met by digestive juices from the pancreas and liver as well as from the wall of the small intestine.

The pancreas contributes enzymes that help break down proteins, carbohydrates and fats.

The liver produces bile, which aids in the digestion of fat. Horses do not have gallbladders, so bile from the liver flows directly into the small intestine.

The small intestine handles 30 to 60 percent of carbohydrate digestion and absorption and nearly all amino acid absorption. The small intestine also absorbs vitamins A, D, E, and K and minerals such as calcium and some phosphorus.

Carbohydrates are broken down into simple sugars, and sugar entering the bloodstream causes the pancreas to release insulin. It is this process that can lead to laminitis (see Part 2 on Nov. 2, 2011).

Nutrients are absorbed through the walls of the small intestine. Food can pass through in less than an hour.

It then enters the large intestine, or hindgut, where fermentation takes place.

Note that this fermentation differs from the function of the small intestine, where digestion and absorption of sugar, starch, protein and fats takes place.

Large intestine: Five parts

The large intestine is made up of five parts: the cecum, 4 feet long and 1 foot in diameter; the large colon, 12 feet long and 10 inches in diameter; small colon, 10 feet and 4 inches in diameter; rectum; and anus.

Cecum

The cecum, which holds 8 to 10 gallons, breaks down the complex carbohydrates and fiber found in hay and grass. The cecum and other parts of the hindgut break down food through fermentation by bacteria. The cecum absorbs fatty acids and vitamins. Food stays here for up to seven hours.

But the microflora, or the bacterial population, in the hindgut is sensitive to changes in diet. An overload of high-sugar food or high levels of fructan can upset the normal population. Lack of proper digestion and fermentation can result in laminitis and colic. The microbial population does adapt to changes in the diet, but this process takes about three weeks.

Food stays in the cecum for about seven hours.

Large colon

The large colon, or large intestine, holds 80 quarts. The large colon has a right and left ventral colon and dorsal colon. Nutrients such as B-vitamins, minerals and phosphorus are absorbed.

Food stays here for up to 2 1/2 days.

Small colon, rectum and anus

The food then passes to the small colon, but by this point, most of the nutrients are gone.

Moisture is absorbed, leaving behind fecal balls, which are passed out the rectum and anus.

Horses normally produce 33 to 50 pounds of feces per day.

Digestion takes 36 to 72 hours.