Carbohydrates An Ideal Source of Energy
Carbohydrates (“carbs”) are needed in the diet primarily to supply energy to body cells. Some cells, such as those in the brain and other parts of the nervous system and in the blood, use only the carbohydrate glucose for fuel. During high-intensity exercise, muscles use primarily energy sources from carbohydrates for fuel.
Simple and Complex Carbohydrates Carbohydrates are classified into two groups: simple and complex. Simple carbohydrates are the single sugar molecules (monosaccharides) and the double sugars (disaccharides). Three monosaccharides are glucose, fructose, and galactose. Glucose, the most common of the sugars, is used by both animals and plants for energy. Fructose is a very sweet sugar that is found in fruits, and galactose is the sugar in milk.
The disaccharides, pairs of single sugars, include sucrose (table sugar: fructose + glucose), maltose (malt sugar: glucose + glucose), and lactose (milk sugar: galactose + glucose). Simple carbohydrates add sweetness to foods. They are found naturally in fruits and milk and are added to soft drinks, fruit drinks, candy, and desserts. There is no evidence that any type of simple carbohydrate is more nutritious than others.
Complex carbohydrates include starches and most types of dietary fiber. Starches are found in a variety of plants, especially grains (wheat, rye, rice, oats, barley, and millet), legumes (dried beans, peas, and lentils), and tubers (potatoes and yams). Most other vegetables contain a mix of complex and simple carbohydrates. Fiber, which is discussed later in this chapter, is found in fruits, vegetables, and grains.
During digestion, your body breaks down carbohydrates into simple sugar molecules, such as glucose, for absorption. Once glucose is in the bloodstream, the pancreas releases the hormone insulin, which allows cells to take up glucose and use it for energy. The liver and muscles also take up glucose and store it in the form of a starch called glycogen. The muscles use glucose from glycogen as fuel during endurance events or long workouts.
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Refined Carbohydrates versus Whole Grains Complex carbohydrates can be further divided into refined, figure 8.2 The parts of a whole grain kernel. or processed, carbohydrates and unrefined carbohydrates, or whole grains. Before they are processed, all grains are whole grains, consisting of an inner layer of germ, a middle layer called the endosperm, and an outer layer of bran (Figure 8.2). During processing, the germ and bran are often removed, leaving just the starchy endosperm. The refinement of whole grains transforms whole-wheat flour into white flour, brown rice into white rice, and so on.
Refined carbohydrates usually retain all the calories of their unrefined counterparts, but they tend to be much lower in fiber, vitamins, minerals, and other beneficial compounds. Refined grain products are often enriched or fortified with vitamins and minerals, but many of the nutrients lost in processing are not replaced.
Unrefined carbohydrates tend to take longer to chew and digest than refined ones; they also generally enter the bloodstream more slowly. This slower digestive pace tends to make people feel full sooner and for a longer period. Also, a slower rise in blood glucose levels following consumption of complex carbohydrates may help in the management of diabetes. Whole grains are also high in dietary fiber (discussed later).
Consumption of whole grains has been linked to a reduced risk of heart disease, diabetes, and cancer, and plays an important role in gastrointestinal health and body weight management. For all these reasons, whole grains are recommended over those that have been refined. This does not mean you should never eat refined carbohydrates such as white bread or white rice; it simply means that whole-wheat Recent evidence suggests that people who eat more whole grains, especially those high in dietary fiber, tend to have a lower body weight than people who eat fewer whole grains. bread, brown rice, and other whole grains are healthier choices. See the box “Choosing More Whole-Grain Foods” for tips on increasing your intake of whole grains. and glucose levels rise following a meal or snack containing any type of carbohydrate. Some foods cause a quick and dramatic rise in glucose and insulin levels, while others have a slower, more moderate effect. A food that has a rapid effect on blood glucose levels is said to have a high glycemic index. The glycemic index of a food indicates the type of carbohydrate in that food. Unrefined complex carbohydrates, high-fiber foods, and high-fat foods tend to have a lower glycemic index, but other factors such as what other foods are consumed at the same time can affect the body’s response to carbohydrate.
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