Oatmeal, whole-grain bread and pasta and brown rice are all tasty examples of complex carbohydrates. These types of carbohydrates digest more slowly than the simple carbs found in cake, cookies and sugary beverages, as well as fruit and milk, and more quickly than the other two macronutrients, protein and fat. A type of complex carbohydrate called dietary fiber doesn't dissolve at all. Although there's no way to exactly quantify the rate of digestion of complex carbs, understanding the digestive process can give you a better idea.
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Digestive Process of Carbs
Digestion of both simple and complex carbs begins in the mouth. Your teeth and tongue begin to break down the food into smaller pieces, while enzymes in saliva start to chemically break down starches, a type of complex carb, into smaller components. Digestion of carbs continues in the small intestine, where pancreatic amylase further breaks down starches in sugars. Cellulose, or indigestible plant fiber, in complex carbs is not affected by amylase. Here, simple sugars have been fully broken down and are ready for absorption; sugar units called disaccharides and oligosaccharides from complex carbs have one more step to undergo. In this step, enzymes attached to small hairlike projections on the intestinal wall called villi break these sugars down into smaller sugar units that can be absorbed into the bloodstream.
Complex Carbs vs. Simple Carbs
The rate of digestion of different kinds of carbs is fairly simple to understand. The more complex the structure, the longer it takes for the carbohydrate to break down. Simple sugars are composed of single- or double-sugar units, which are quickly broken apart. Complex carbs in the form of starches are composed of long chains of sugar units; the bonds take longer to break apart, thus the time to absorption is longer. Fiber isn't broken down at all; it travels through the digestive system mostly unchanged and is excreted in waste.
Carbs vs. Protein and Fat
Although complex carbs take a long time to digest, protein and fats take even longer. Protein digestion doesn't begin until food reaches the stomach, where gastric juices begin to break down the bonds of proteins, reducing them into their individual components, called amino acids. Proteins then travel to the small intestine, where digestion is completed by pancreatic enzymes. Amino acids are then absorbed into the bloodstream. Fat is the last to be digested. Pancreatic juice and finally bile acids from the liver work on fats to break them into free fatty acids and monoglycerides before they can be absorbed into the bloodstream.
Benefits of Complex Carbs
Carbohydrates are the body's primary source of energy, so it's important to get enough in your daily diet. The publication "Dietary Guidelines for Americans, 2010" recommends that all adults get 45 percent to 65 percent of their calories from carbohydrates. Getting the bulk of your calories from complex carbohydrates is better for your health. Besides fruit and milk, simple carbs are often found in foods without a lot of nutritional value, and because they are quickly broken down and absorbed, they provide only short-term energy and can often spike your blood sugar, leading to an energy slump. Complex carbs in vegetables and whole grains provide long-lasting energy. They also provide dietary fiber to keep your digestive system in top form.
- Wiley: Digestion and Absorption of Carbohydrates
- Chemistry for Biologists: Carbohydrates
- Mansoura University: Protein Metabolism >> Protein Digestion and Absorption >> Lecture 1
- National Institute of Diabetes and Digestive and Kidney Diseases: Your Digestive System and How It Works
- University of Washington: Digestion and Absorption of Fats
- U.S. Department of Agriculture and U.S. Department of Health and Human Services: Dietary Guidelines for Americans, 2010