Carbohydrates are made up of groups of molecules, known as saccharides. These saccharides contain carbon, hydrogen and oxygen atoms in different combinations. There are two main classes of carbohydrates: simple and complex. Simple carbohydrates have one or two saccharides (mono- and di-saccharides, respectively) linked together. They are primarily the sugars, found in fruit, honey, milk (as lactose) and commercial sweeteners. Complex carbohydrates contain many saccharides (polysaccharides) and are known as the starches and fibers found in vegetables, grains and legumes.
The primary role of carbohydrates is to provide energy for the body. The breakdown of glucose within the cells produces molecules of energy that can be used. Glucose is the preferred source of energy within the body, although protein and fat can be used if necessary. Once energy needs are met, glucose is stored in the liver as glycogen, then can be diverted to making other compounds, such as keratin (found in fingernails), ribose (found in DNA and RNA) and hyaluronic acid (used to lubricate the joints). Excess glucose is converted to triglycerides and stored in fatty tissue.
The process of digestion begins in the stomach as carbohydrates are broken down into their monosaccharide components. Most digestion occurs in the small intestine as the mass of food is exposed to special enzymes. Starch is digested in a much slower fashion then simple carbohydrates. The human body lacks the necessary enzymes to break down fibers. Instead, fiber is broken down into water, gas and other components by bacteria in the intestinal track, slowing the movement of food leading to a feeling of being full.
Once the carbohydrates are broken down into their monosaccharide components, they are absorbed through the intestinal wall into the bloodstream where they travel to the liver and are converted to glucose. The liver then controls the secretion of glucose into the bloodstream. If the blood concentration is too high, the pancreas secretes insulin to move glucose into the cells and out of the bloodstream. If the blood glucose level begins to fall, glucagon is secreted to increase the amount of glucose that the liver secretes back into the blood.
Carbohydrates are found in every major food group, though in differing concentrations, from grains to fruits to vegetables and even protein. Grains contain the highest amount of carbohydrates per serving, with varying amounts of fiber. Whole grain and bran products have higher fiber then white and refined grains. Vegetables contain mostly starches, especially in beans, peas, corn and potatoes. Watery vegetables such as lettuce, cucumber and sprouts contain lesser concentrations of starch. Fruits contain mostly sugars, though the skins do contain a good deal of fiber. While no specific recommendations have been set, it is estimated that 50% to 60% of calories should come from carbohydrates, specifically complex carbohydrates. The average adult should consume 20g to 35g of fiber each day, and limit simple sugars to 10% of calories. That means that in an average diet of 1,800 calories, 900 to 1,080 calories should come from carbohydrates, and sugar should be limited to 45g per day.
A healthy diet should contain 6 one-ounce servings of whole grain carbohydrates, 3 to 5 servings of vegetables, and 2 to 4 servings of fruit each day. Select green-leafy vegetables whenever possible, and try to limit starchy vegetables like peas, corn and potatoes. Fruit makes a great desert or snack instead of processed sugars like cookies or cake. Try to avoid white and refined grains, and choose instead whole grain breads, pastas and cereals. By selecting healthy carbohydrates and monitoring calories, nothing has to be off limits. But be aware of the calories that sweets and carbohydrates are contributing to your daily intake.Source:
Dudek, Susan G. Nutrition Essentials for Nursing Practice 4th Edition. Lippincott, Williams and Wilkins. Philadelphia: 2001. p19-45.