# 8.1: Stage I of Carbohydrate Catabolism


Learning Objectives

• Describe the metabolism of carbohydrates.
• Know the source and function of common carbohydrates in the diet.

Dietary carbohydrates are sugars and sugar derivatives whose formulas can be written in the general form: Cx(H2O)y. (The subscripts x and y are whole numbers.). Some typical carbohydrates are sucrose (ordinary cane sugar), C12H22O11; glucose (dextrose), C6H12O6; fructose (fruit sugar), C6H12O6; and ribose, C5H10O5. Glucose is also the monomer from which the polymers cellulose and starch are built up. In food science and in many informal contexts, the term "carbohydrate" often means any food that is particularly rich in the complex carbohydrate starch (such as cereals, bread and pasta) or simple carbohydrates, such as sugar (found in candy, jams, and desserts). In the strict sense, "sugar" is applied for sweet, soluble carbohydrates, many of which are used in food.

Carbohydrates may be classified according to their degree of polymerization, and may be divided initially into three principal groups, namely sugars, oligosaccharides and polysaccharides[14]as shown in Table $$\PageIndex{1}$$.

Table $$\PageIndex{1}$$ The Major Dietary Carbohydrates. Source: Wikipedia

Class (DP*) Subgroup Components
Sugars (1–2) Monosaccharides Glucose, galactose, fructose, xylose
Disaccharides Sucrose, lactose, maltose, trehalose
Oligosaccharides (3–9) Oligosaccharides Maltodextrins, raffinose, stachyose, fructo-oligosaccharides
Polysaccharides (>9) Starch, Glycogen Amylose, amylopectin, modified starches
Cellulose Cellulose, Hemicellulose, Pectins, Hydrocolloids

DP * = Degree of polymerization

## Digestion of Carbohydrates

The human body breaks down complex carbohydrates into glucose and other monosaccharides. Glucose in the blood (often referred to as “blood sugar”) is the primary energy source for the body. Sugars provide calories, or “energy,” for the body. Each gram of sugar provides 4 calories. Glucose can be used immediately or stored in the liver and muscles for later use.

Carbohydrate digestion begins in the mouth (Figure $$\PageIndex{1}$$) where salivary α-amylase attacks the α-glycosidic linkages in starch, the main polycarbohydrate ingested by humans. Cleavage of the glycosidic linkages produces a mixture of dextrins, maltose, and glucose. The α-amylase mixed into the food remains active as the food passes through the esophagus, but it is rapidly inactivated in the acidic environment of the stomach.

Figure $$\PageIndex{1}$$ The Principal events and sites of carbohydrate digestion.

The primary site of carbohydrate digestion is the small intestine. The secretion of α-amylase in the small intestine converts any remaining starch molecules. Starch is then cleaved into glucose molecules. Disaccharides such as sucrose and lactose are not digested until they reach the small intestine, where they are acted on by sucrase and lactase, respectively. The major products of the complete hydrolysis of disaccharide and polysaccharides are three monosaccharide units: glucose, fructose, and galactose. These are absorbed through the wall of the small intestine into the bloodstream.

## Fuel for the Brain

The brain is a marvelous organ. And it's a hungry one, too. The major fuel for the brain is the carbohydrate glucose. The average adult brain represents about $$2\%$$ of our body's weight, but uses $$25\%$$ of the glucose in the body. Moreover, specific areas of the brain use glucose at different rates. If you are concentrating hard (taking a test, for example), certain parts of the brain need a lot of extra glucose while other parts of the brain only use their normal amount. Something to think about.

Some foods that are high in carbohydrates include bread, pasta, and potatoes (Figure $$\PageIndex{2}$$) . Because carbohydrates are easily digested, athletes often rely on carbohydrate rich foods to enable a high level of performance.

Figure $$\PageIndex{2}$$ Foods that serve as carbohydrate sources.

# Blood sugar level

The blood sugar level, blood sugar concentration, or blood glucose level is the concentration of glucose present in the blood of humans and other animals. Normal blood glucose level (tested while fasting) for non-diabetics is between 70 to 110 mg/dL. Glucose levels are usually lowest in the morning, before the first meal of the day, and rise after meals for an hour or two by a few millimoles. Blood sugar levels outside the normal range may be an indicator of a medical condition. A persistently high level is referred to as hyperglycemia.  Long-term hyperglycemia causes many health problems including heart disease, cancer, eye, kidney, and nerve damage

A persistently low levels of blood glucose are referred to as hypoglycemia. . Symptoms may include lethargy, impaired mental functioning; irritability; shaking, twitching, weakness in arm and leg muscles; pale complexion; sweating; loss of consciousness. Diabetes mellitus is characterized by persistent hyperglycemia from any of several causes, and is the most prominent disease related to failure of blood sugar regulation.

• ## Summary

• Carbohydrate digestion begins in the mouth (in the presence of salivary α-amylase) and continues in the small intestine. The major products of the complete hydrolysis of disaccharides and polysaccharides are three monosaccharide units: glucose, fructose, and galactose. These are absorbed through the wall of the small intestine into the bloodstream.
• The blood sugar level, blood sugar concentration, or blood glucose level is the concentration of glucose present in the blood of humans and other animals.

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