Carbohydrates, abundantly present in foods such as breads, cereals, fruits and vegetables, are the main source of energy in a diet. During digestion, a series of enzymatic reactions break down the carbohydrates in these foods into simple carbohydrates that are easily absorbed in the small intestine. While complex carbohydrates require enzymes such as salivary amylase, pancreatic amylase and maltose for digestion, simple carbohydrates require little or no enzymatic reaction before absorption.
Different forms of carbohydrates are present in foods. Individual units of sugar such as glucose, fructose and galactose are the simplest forms of carbohydrates called monosaccharides, while sucrose, lactose and maltose are disaccharides made up of two monosaccharides linked together. Complex carbohydrates include starch and fiber, which are polysaccharides made up of long chains of glucose units bonded together. Although fiber resists enzyme action and is not broken down during digestion, break down of starch by enzymes starts in the mouth.
Chewing breaks food into small molecules that combine with saliva secreted by the salivary glands in the mouth. Along with mucin and buffers, saliva contains the enzyme salivary amylase, which acts on the starch in food and breaks it down to maltose. Salivary amylase continues for the short duration that the carbohydrates are in the mouth, after which the mixture of the partially digested carbohydrates travels down the esophagus into the stomach. Due to the inhibition of salivary amylase activity by the acidic gastric juices, digestion of carbohydrates does not occur in the stomach.
Pancreatic Amylase and Maltase
As the combination of gastric juices and partially digested food enters the small intestine, the pancreas secretes pancreatic juices, which contain the enzyme pancreatic amylase. This enzyme acts on the remaining polysaccharides and breaks them into disaccharide units of maltose. In the final step of complex carbohydrate digestion, the enzyme maltase present in the lining of the small intestine breaks maltose into two units of glucose. Glucose is then absorbed and enters the bloodstream.
Sucrase and Lactase
Two additional enzymes present in the small intestine digest other disaccharides in foods. The enzyme sucrase digests sucrose or table sugar into its constituent units of glucose and fructose, while lactase breaks lactose or milk sugar into glucose and galactose. These monosaccharides are absorbed in the small intestine and transported to the liver through blood. As the human body can only utilize glucose as a source of energy, the liver converts fructose and galactose into glucose. Glucose either becomes a source of immediate energy or is stored in the liver and muscles in the form of glycogen.
Fiber, present in foods as soluble and insoluble fiber, is the only carbohydrate that is not broken down by digestive enzymes. While soluble fiber becomes a thick gel-like mass in the small intestines due to its ability to dissolve in water, insoluble fiber remains unchanged during digestion. Dietary fiber is an important part of the diet as it helps to prevent constipation, maintain bowel health, reduce blood levels of low-density lipoproteins, control blood glucose levels and may even help you to lose weight.
To get the most out of your carbohydrate intake chose foods that contain complex carbohydrates such as whole grains, whole-grain products, nuts, seeds, dry beans, legumes, peas, fruits and vegetables. Avoid or consume small quantities of sodas, sweet breakfast cereals, fruit drinks and desserts that contain added sugars in the form of high-fructose corn syrup, corn sweeteners, fructose, brown sugar, molasses, raw sugar, dextrose and malt syrup. The ingredient list on food labels can help you in choosing healthy carbohydrates.
Articles For Your Diet
- National Digestive Diseases Information Clearinghouse: Your Digestive System and How it Works
- Clinton Community College: Digestive System
- Oklahoma State University: Carbohydrates in the Diet
- Western Kentucky University: Digestion
- MayoClinic.com: Dietary Fiber: Essential for a Healthy Diet
- Centers for Disease Control and Prevention: Carbohydrates
- Bread with bran image by kotomiti from Fotolia.com