Dietary Fiber Is Not Digestible in the Stomach

Dietary fiber is found in nuts, grains, legumes, fruits and vegetables.
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Fiber. The word is everywhere you turn. TV commercials, food labels and even your doctor stress the importance of getting enough fiber in your diet. What exactly is fiber and why is it so important? Dietary fiber specifically describes the parts of plant cells resistant to digestion by the enzymes in your digestive tract. The stomach plays a vital role in digestion, but it cannot digest dietary fiber.


    Digestion, the process of breaking down food particles, begins in the mouth and continues in the digestive tract, which includes the esophagus, the stomach, the small intestine and the large intestine. Digestion involves physical processes, such as chewing and swallowing, which require the mouth and esophagus, and the churning and mixing action of the stomach. Enzymes produced by the salivary glands, the stomach, the pancreas and the liver speed up the chemical reactions involved in digestion. Dietary fiber remains intact as it moves through the digestive tract.

Stomach Function

    The stomach plays a role in both physical and chemical digestion. Although it is a single organ with one cavity, doctors divide the stomach into four sections based on their functions. The uppermost portion, called the cardia, grinds the food particles and begins mixing it with the acids and enzymes forming a partially digested mass named chyme. The fundus, the main upper section of the stomach, retains the chyme to allow the enzymes to take further action. After 40 to 60 minutes in the fundus, the chyme moves into the body of the stomach where the acid and enzymes produced by the cells lining the stomach break the mixture into smaller particles. The antrum, the last section of the stomach, functions as a gate to gradually allow the chyme to move into the upper portion of the small intestine, known as the duodenum. Through all the intricate workings of the stomach, the physical processes and enzyme actions fail to affect dietary fiber.

Fiber Fermentation

    Although fiber survives virtually unchanged through the digestion processes in the stomach and small intestine, the bacteria in the large intestine can break down some types of fiber in the process of fermentation. Fermentation is the anaerobic, meaning without oxygen, process that involves the breakdown of dietary components to usable energy. In this case, the bacteria in the colon break down certain fiber molecules, such as pectin, guar and gum, into short-chain fatty acids and gases. Your body can absorb the fatty acids and use them for energy. The gases produced by fermentation explain why high-fiber foods can cause bloating and embarrassing gas. Cellulose and lignin, which are other types of fiber, are not fermented.

Role of Fiber

    Dietary fiber is an important part of your diet. Fiber affects the rate of digestion, the absorption of nutrients and the movement of waste. Insoluble fiber helps prevent constipation by adding bulk and moisture to your stool. Soluble fiber can help lower cholesterol levels by binding to bile acids and cholesterol in the intestines and carrying them out of the body through excretion. Soluble fiber also slows down the digestion of carbohydrates, which helps stabilize blood sugar levels to prevent type 2 diabetes.

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