The carbohydrates are a vast and diverse group of nutrients found in most foods. This group includes simple sugars (like the sugar you add to your morning coffee) and complex forms such as starches (contained in pasta, bread, cereal, and in some fruits and vegetables), which are broken down during digestion to produce simple sugars. The main function of the simple sugars and starches in the foods we eat is to deliver calories for energy. The simple sugar glucose is required to satisfy the energy needs of the brain, whereas our muscles use glucose for short-term bouts of activity.
The liver and muscles also convert small amounts of the sugar and starch that we eat into a storage form called glycogen. After a long workout, muscle glycogen stores must be replenished. Both simple sugars and starches provide about 4 calories per gram (a gram is about the weight of a paper clip). Because carbohydrates serveprimarily as sources of calories (and we can get calories from there macronutrients), no specific requirement has been set for them .
But health experts agree that we should obtain most of our calories (about 60 percent) from carbohydrates. Our individual requirements depend on age, sex, size, and activity level. In contrast to the other carbohydrates, fiber (a substance contained in bran, fruits, vegetables, and legumes) is a type of complex carbohydrate that cannot be readily digested by our bodies. Even though it isn’t digested, fiber is essential to our health. Nutrition professionals recommend 25 to 30 grams of fiber daily.
Simple sugars make foods sweet. They are small molecules found in many foods and in many forms. Some simple sugars occur naturally in foods. For example, fructose is the sugar that naturally gives some fruits their sweet flavor. Table sugar, the sugar that we spoon onto our cereal and add to the cookies we bake, also called sucrose, is the most familiar simple sugar. A ring-shaped molecule of sucrose actually consists of a molecule of fructose chemically linked to a molecule of another simple sugar called glucose. Sugars such as fructose and glucose are known as monosaccharide, because of their single (mono) ring structure, whereas two-ringed sugars such as sucrose are known as disaccharides. Another disaccharide, lactose, the sugar that gives milk its slightly sweet taste, consists of glucose linked to yet another simple sugar called galactose. The inability to digest lactose to its constituent sugars is the cause of lactose intolerance, a condition common to adults of Asian, Mediterranean, and African ancestry.
The table sugar that we purchase is processed from sugar cane or sugar beets. As an additive to many differenttypes of prepared or processed foods, sucrose adds nutritive value (in the form of calories only), flavor, texture, and structure, while helping to retain moisture. Today, sucrose is most often used to sweeten (nondietetic) carbonated beverages and fruit drinks (other than juice), candy, pastries, cakes, cookies, and frozen desserts. One of the most commonly consumed forms of sugar is called high-fructose corn syrup. High-fructose corn syrup is also commonly used to sweeten sodas, fruit drinks (not juices), some ice creams, and some manufactured pastries and cookies. Other forms of sucrose include brown sugar, maple syrup, molasses, and turbinado (raw) sugar.
Foods that are high in added sugar are often low in essential nutrients such as vitamins and minerals. Unfortunately, these foods are often eaten in place of more nutrient-rich foods such as fruits, vegetables, and low-fat whole-grain products, and they may prevent us from obtaining essential nutrients and lead to weight gain. Nutritionists are concerned by the enormous increase in sugar consumption by Americans during the past 30 years, particularly because much of this sugar is in the form of soft drinks. On average, teens today drink twice as much soda as milk, and young adults drink three times as much soda as milk. As a result, their intake of calcium-rich foods is low, a factor that is thought to contribute to lower bonemass. This can lead to an increased risk of bone problems as we grow older. The increase in sugar consumption also has been attributed to the increasing availability of low-fat versions of such dessert and snack foods as cookies, cakes, and frozen desserts. Often, the sugar content of these foods is high because sugar is used to replace the flavor lost when the fat is decreased. Sugar promotes tooth decay, when consumed in forms that allow it to remain in contact with the teeth for extended periods
Thus, foods that are high in sugar, or sugar and fat, and have few other nutrients to offer appear at the top of the Food Guide Pyramid because they should be eaten sparingly. In contrast, choosing fresh fruits, which are naturally sweetened with their own fructose, or low-fat yogurt, which contains lactose (natural milk sugar), allows us to get the vitamins and minerals contained in those foods as well as other food components that contribute to health but may not have yet been identified. On the positive side, there is no credible evidence to demonstrate that sugar causes diabetes, attention deficit hyperactivity disorder, depression, or hypoglycemia. No evidence has been found that sugar-containing foods are “addictive” in the true sense of the word, although many people report craving sweet foods, particularly those that are also high in fat.
Found almost exclusively in foods of plant origin, complex carbohydrates are long chains of molecules of the simple sugar glucose. The complex carbohydrates in plant foods can be divided into two groups: starch and fiber. Starch is the form of carbohydrate that is found in grains, some fruits and vegetables, legumes, nuts, and seeds. It provides energy for newly sprouting plants. Fiber is the tougher material that forms the coat of a seed and other structural components of the plant. Starches are digested by our bodies into their constituent glucose molecules and used for energy, whereas fiber is not. Starch, like simple sugars, provides 4 calories per gram, whereas fiber (sometimes called nonnutritive fiber) provides no calories. Like simple sugars, the role of starches in our diets is mainly to provide energy. Fiber is actually a family of substances found in fruits, vegetables, legumes, and the outer layers of grains. Scientists divide fiber into two categories: those that do not dissolve in water (insoluble fiber) and those that do (soluble fiber).
Insoluble fiber, also called roughage, includes cellulose, hemicelluloses, and lignin, found in vegetables, nuts, and some cereal grains. Soluble fibers include pectin, found in fruits, and gums, found in some grains and legumes. Fiber-rich diets, which include ample amounts of whole-grain foods, legumes, and fresh vegetables and fruits, have been linked with a lower risk of several diseases. Nutrition scientists are just beginning to understand the role of dietary fiber in maintaining health. Fiber appears to sweep the digestive system free of unwanted substances that could promote cancer and to maintain regularity and prevent disorders of the digestive tract. Fiber also provides a sense of fullness that may help reduce overeating and unwanted weight gain. Diets that are rich in fiber and complex carbohydrates have been associated with lower serum cholesterol and a lower risk for high blood pressure, coronary artery disease, and some types of cancer. But does this mean that it’s okay just to take a fiber pill? No! Rather, the studies that have shown the beneficial effects of a high fiber diet (containing 25 to 30 grams of fiber per day) have been those in which the dietary fiber is in the form of fruits, vegetables, whole grains, and cereals. These and other studies suggest that not only the fiber in these foods but also
For the same reason that people have recently sought substitutes for fat, noncaloric sugar substitutes became popular in the 1960s as people began to try to control their weight. Sugar substitutes are of two basic types: intense sweeteners and sugar alcohols. Intense sweeteners are also called non-nutritive sweeteners, because they are so much sweeter than sugar that the small amounts needed to sweeten foods contribute virtually no calories to the foods. These sweeteners also do not promote tooth decay. Currently, four such intense sweeteners are available, both for use in processed foods and for home consumption. The U.S. Food and Drug Administration (FDA) has set “acceptable daily intakes” (ADI) for these sweeteners. The ADI is the amount that can be consumed daily over a lifetime without risk.