The Chemist’s View of Carbohydrates Carbohydrates: Compounds composed of carbon, oxygen, and hydrogen arranges as monosaccharides or multiples of monosaccharides. Simple Carbohydrates: Monosaccharides and disaccharides. Complex Carbohydrates: Polysaccharides composed of straight or branched chains of monosaccharides. The carbohydrates are made of carbon (C), oxygen (O), and hydrogen (H). Each of these atoms can form a specified number of chemical bonds: carbon forms four, oxygen forms two, and hydrogen forms one.
The Simple Carbohydrates Six simple carbohydrates, or sugars, are important in nutrition. The three monosaccharides (glucose, fructose, and galactose) all have the same chemical formula, but their structures differ. The three disaccharides (maltose, sucrose, and lactose) are parts of monosaccharides, each containing a glucose paired with one of the three monosaccharides. The sugars derive primarily from plants, except for lactose and its component galactose, which came from milk and milk products. Two monosaccharides can be linked together by a condensation reaction to form a disaccharide and water. A disaccharide, in turn, can be broken down into its two monosaccharides by a hydrolysis reaction using water.
The Circulatory System The complex carbohydrates are the polysaccharides (chains of monosaccharides): glycogen, starches, and fibers. Polysaccharides: Compounds composed of many monosaccharides linked together. Both glycogen and starch are storage forms of glucose– glycogen in the body, and starch in plants– and both yield energy for human use. Glycogen: An animal polysaccharide composed of glucose. Starches: Plant polysaccharides composed of glucose. The fibers also contain glucose (and other monosaccharides), but their bonds cannot be broken by human digestive enzymes, so they yield little, if any, energy. Fibers: The non-starch polysaccharides that are not digested by human digestive enzymes.
Digestion and Absorption of Carbohydrates In the digestion and absorption of carbohydrates, the body breaks down starches into disaccharides and disaccharides into monosaccharides; it then coverts monosaccharides mostly to glucose to provide energy for the cells’ work. The fibers help to regulate the passage of food through the GI system and slow the absorption of glucose, but contribute little, if any, energy.
Lactose Intolerance Lactose intolerance is a common condition that occurs when there is insufficient lactase to digest the disaccharide lactose found in milk and milk products. Lactose Intolerance: A condition that results from inability to digest the milk sugar lactose. Symptoms include GI distress. Because treatment requires limiting milk intake, other sources of riboflavin, vitamin D, and calcium must be included in the diet.
Glucose in the Body Dietary carbohydrates provide glucose that can be used by the cells fro energy, stored by the liver and muscles as glycogen, or converted into fat is intakes exceed needs. All of the body’s cells depend on glucose; those of the central nervous system are especially dependent on it. Without glucose, the body is forced to break down its protein tissues to make glucose and to alter energy metabolism to make ketone bodies from fats. Ketone Bodies: The product of the incomplete breakdown of fat when glucose is not available in the cells. Blood glucose regulations depends primarily on two pancreatic hormones: insulin to remove glucose from the blood into the cells when levels are high and glucagon to free glucose from glycogen stores and release it into the blood when levels are low. Insulin: A hormone secreted by special cells in the pancreas in response to increased blood glucose concentration. The glycemic index measures how blood glucose responds to foods. Glycemic Index: A method of classifying foods according to their potential for raising blood glucose.
Health Effects and Recommended Intakes of Sugars Sugars pose no major health threat except for an increased risk of dental caries. Dental Caries: Decay of teeth. Excessive intake may displace needed nutrients and fiber and may contribute to obesity when energy intake exceeds needs. A person deciding to limit daily sugar intake should recognize that not all sugars need to be restricted, just concentrated sweets, which are relatively empty of other nutrients and high in kcalories. Sugars that occur naturally in fruits, vegetables, and milk are acceptable.
Health Effects and Recommended Intakes of Starch and Fiber An adequate intake of fiber: Fosters weight management. Lowers blood cholesterol. May help prevent colon cancer. Helps prevent and control diabetes. Diabetes: A disorder of carbohydrate metabolism resulting from inadequate or ineffective insulin. Helps prevent and alleviate hemorrhoids. Helps prevent appendicitis. Helps prevent diverticulosis. An excessive intake of fiber: Displaces energy- and nutrient- dense foods. Causes intestinal discomfort and distension. May interfere with mineral absorption.
Recommended Intakes of Starch and Fiber Clearly, a diet rich in complex carbohydrates– starches and fibers– supports efforts to control body weight and prevent heart disease, cancer, diabetes, and GI disorders. For these reasons, recommendations urge people to eat plenty of whole grains, vegetables, legumes, and fruits– enough to provide 45-65% of the daily energy intake from carbohydrate.