Carbohydrates

Carbohydrates, fats, and proteins all meet the body’s basic energy needs. Carbohydrates are the major source of energy because they break down rapidly and are readily available for use. This chapter defines basic terminology related to carbohydrates and discusses the body’s use of carbohydrates and the way carbohydrates relate to the other energy nutrients. Green plants manufacture carbohydrates during a complex process called photosynthesis. In this process, carbon dioxide from the air and water from the soil are transformed into sugars and starches. Sunlight and the green pigment chlorophyll are necessary for this conversion. All the food we eat is a product of photosynthesis. If this process did not occur, the whole food chain would collapse, and life would cease. On the basis of their chemical structure, carbohydrates are divided into two major groups: sugars and starches. Sugars have a simple structure; starches are more complex. Therefore, sugars are often called simple carbohydrates, and starches are called complex carbohydrates.

Composition of Carbohydrates
Understanding the composition of carbohydrates involves understanding three structures: molecule, element, and atom:

1. A molecule is the smallest quantity into which a substance may be divided without loss of its characteristics. For example, the formula for water is H2O. If the hydrogen atoms are pulled from the oxygen atom, the resulting products are hydrogen and oxygen, which bear no resemblance to water. Molecules are made of elements. In the case of water, H2O, the elements are hydrogen and oxygen.
2. An element is a substance that cannot be separated into simpler parts by ordinary means.
3. An atom is the smallest particle of an element that retains its physical characteristics.

Basic Terminology
Carbohydrates are composed of the elements carbon, hydrogen, and oxygen. The ratio of hydrogen to oxygen is the same as that for water: two parts of hydrogen to one part of oxygen. The simplest carbohydrates have the formula C6H12O6. Carbohydrates are frequently abbreviated CHO.
Simple carbohydrates (sugars) include monosaccharides and disaccharides (mono- means one, di- means two, and -saccharide means sweet). Starches are called polysaccharides.

Simple Carbohydrates
Simple carbohydrates are of two types: monosaccharides and disaccharides.

1. A monosaccharide contains one molecule of C6H12O6.
2. A disaccharide is composed of two molecules of C6H12O6 joined together (minus one unit of H2O).

When the body joins two monosaccharide molecules, a molecule of water is released in the process.

Monosaccharides
Monosaccharides are the building blocks of all other carbohydrates. The three monosaccharides of importance in human nutrition are glucose, fructose, and galactose. Note the -ose ending in the name of each of these sugars. All monosaccharides and disaccharides end with the letters -ose.

Glucose
The monosaccharide glucose in the body is commonly called blood sugar. It is the major form of sugar in the blood. Normal fasting blood sugar (FBS) is 70 to 100 milligrams per 100 milliliters of serum or plasma. Impaired fasting glucose (IFG) is 100 to 125 milligrams per 100 milliliters of serum or plasma (www.care.diabetesjournal.org). Regardless of the form of sugar consumed, the body readily converts it to glucose.
Another name for glucose is dextrose (abbreviated D). Clients in health-care facilities often receive intravenous feedings. Intravenous simply means within or into a vein. The most common intravenous feeding is D5W (5% dextrose in water), used primarily to deliver fluids to the client.

Fructose
Fructose is found in fruits and honey. It is the sweetest of all the monosaccharides. Fructose is used extensively in soft drinks, canned foods, and various other processed foods. High-fructose corn syrup (HFCS) is very sweet because the cornstarch has been treated with an enzyme that converts some of the naturally present glucose to the sweeter fructose. The human body readily converts fructose to glucose. HFCS has become increasingly controversial in its use by the food industry. A possible link has been identified with HFCS consumption and diabetes. Countries using HFCS in their foods have a 20% higher prevalence of diabetes than those that do not use it. It is estimated that in the United States, 55 lb per person of HFCS are ingested annually. Many countries, including Australia, China, Denmark, France, Italy, Sweden, and the United Kingdom, have a per capita consumption of approximately 1 lb annually.

Galactose
The monosaccharide galactose comes mainly from the breakdown of the milk sugar lactose. Yogurt and unaged cheese may contain free galactose. It is the least sweet of all the monosaccharides. The body converts galactose into glucose after ingestion.

Disaccharides
When two monosaccharides are linked, a disaccharide is formed. The three important disaccharides are:

1. Sucrose
2. Lactose
3. Maltose

Sucrose
The most prevalent disaccharide, sucrose, is ordinary white table sugar made commercially from sugar beets and sugar cane. Brown, granulated, and powdered sugars are all forms of sucrose. Sucrose is also found in molasses, maple syrup, fruits, and vegetables. The two monosaccharides joined to form sucrose are glucose and fructose.

Lactose
Because lactose occurs naturally only in milk, it is commonly referred to as milk sugar. Lactose is the least sweet of the disaccharides. The two monosaccharides that make up lactose are glucose and galactose.

Maltose
Maltose is a double sugar that occurs primarily during starch digestion. The disaccharide maltose is produced when the body breaks starches into simpler units. Smaller amounts of this disaccharide are present in malt, malt products, beer, some infant formulas, and sprouting seeds. Maltose consists of two units of glucose.