Insulin resistance in the laminitic horses: Part 1: Digestion and how food passes through a horse

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.

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