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Galactose

Fructose Figure 18.1

The liver converts the monosaccharides fructose and galactose into glucose.

Glucose

Galactose

Fructose Figure 18.1

The liver converts the monosaccharides fructose and galactose into glucose.

Carbohydrate Sources

Carbohydrates are ingested in a variety of forms. Complex carbohydrates include the polysaccharides, starch from grains and vegetables and glycogen from meats. Foods containing starch and glycogen usually have many other nutrients, including valuable vitamins and minerals. The simple carbohydrates include disaccharides from milk sugar, cane sugar, beet sugar, and molasses and monosaccharides from honey and fruits. Digestion breaks complex carbohydrates down to monosaccha-rides, which are small enough to be absorbed.

Cellulose is a complex carbohydrate that is abundant in our food—it provides the crunch to celery and the crispness to lettuce. We cannot digest cellulose, and most of it passes through the alimentary canal largely unchanged. However, cellulose provides bulk (also called fiber or roughage) against which the muscular wall of the digestive system can push, facilitating the movement of food. Hemicellulose, pectin, and ¡ignin are other plant carbohydrates that provide fiber.

Carbohydrate Utilization

The monosaccharides that are absorbed from the digestive tract include fructose, galactose, and glucose. Liver enzymes convert fructose and galactose into glucose (fig. 18.1). Recall that glucose is the form of carbohydrate that is most commonly oxidized in glycolysis for cellular fuel.

Sugar substitutes provide concentrated sweetness, so fewer calories are needed to sweeten a food. Aspartame, which is a dipeptide (two joined amino acids), is two hundred times as sweet as table sugar (sucrose). This means that just a pinch of it equals the sweetness of a teaspoonful or more of sugar. Saccharin, another sugar substitute, is three hundred times as sweet as sugar.

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