Carbohydrates

A. E. HARPER.

FROM carbohydrates we get most of the energy which, we need to act and move, perform work, live. Among the carbohydrates are sugars, starches, and celluloses. All green plants form carbohydrates.

Carbohydrates make up about half of the usual American diet and an even greater proportion of the diets of the peoples of most other countries, for the seeds of cereals, high in carbohydrates, form a staple food almost everywhere. They grow almost everywhere. They give the highest yields of energy per unit of land cultivated. They are easy to store and transport. They are inexpensive.

Carbohydrates are important in nutrition for many reasons other than as a source of energy. Some of them make our food sweet. Some of them cling to our teeth and serve as food for bacteria that cause tooth decay. Some determine what types of bacteria will grow in our intestines. The bulk in our food, which helps to prevent constipation, consists mostly of carbohydrates. The body needs carbohydrates in order to use fat efficiently. Some diseases, such as diabetes, develop because the body is unable to use carbohydrates properly.

To understand how carbohydrates perform these functions and how they are used in our bodies, we need to know something about their chemistry. This requires a little discussion of a technical nature, which, if you will bear with me, will help to make some of the more interesting aspects clearer.

The carbohydrates contain carbon, hydrogen, and oxygen.

The hydrogen and oxygen usually occur in the same proportion as in water. When they are burned, water is formed and carbon is left. Early chemists thought therefore that carbohydrates were hydrates of carbon and gave them the name carbohydrate. That turned out to be incorrect, but the name remains.

Some carbohydrates are relatively small molecules. Others, larger and more complex, consist of a few or many of the smaller molecules linked in chains. The chains of the largest molecules may contain more than one thousand units. They may be straight or branched. Some contain only one kind of unit. Some contain several different kinds.

The members of the simplest class, the monosaccharides, have a single unit.

Those in the next class are oligosaccharides. They contain only a few units. These two groups are commonly known as sugars and include such familiar foodstuffs as cane sugar and milk sugar.

Carbohydrates made up of long chains of monosaccharides are known as pol),saccharides. Among them are starch, cellulose, plant gums and mucilages, and other structural and storage carbohydrates.

The individual members of each of these classes differ in the types of small units they contain and in the way the units are linked together.

Glucose is the commonest of the monosaccharides. It was named after the Greek word meaning "sweet," long before its chemical structure was determined. It is now known to be a polyhydroxyaldehyde (a long chemical name which I shall explain later) and may be taken as representative of the class of simple carbohydrates.

Glucose contains 6 atoms of carbon, 12 of hydrogen, and 6 of oxygen. The molecular formula is C₆H₁₂O₆. It may exist in more than one form. but the structure can be visualized by picturing the 6 carbon atoms as joined in a line.

The carbon atom at one end has attached to it 1 hydrogen atom and 1 oxygen atom to form the aldehyde group.

The carbon at the other end has 2 hydrogens and 1 hydroxyl group. The hydroxyl group, which is present in all alcohols, consists of a hydrogen atom linked to an oxygen and is represented chemically as OH.

Each of the remaining four carbons of the glucose molecule carries 1 hydrogen and 1 hydroxyl group. Thus we have polyhydroxyaldehyde. Glucose is called a pentahydroxyaldehyde because it contains 5 hydroxyl groups. The other simple carbohydrates, or Sugars, contain essentially the same groups. They differ in the way the groups occur in relation to each other. Some carbohydrates contain one or More groups that are different from those in glucose.

The properties of the individual monosaccharides depend partly on the positions of the hydroxyl groups within the molecule and partly on whether the molecule contains groups that differ from those in the glucose structure.

They may differ also in the number of carbon atoms in the chain. Most common monosaccharides pentoses and hexoses, respectively contain 5 or 6 carbon atoms.

The more complex carbohydrates, as stated, consist of chains of monosaccharide units. In the formation of each link between units, a hydrogen atom (H) is split away from one unit and a hydroxyl group (OH) from the other, to yield a molecule of water (H₂O).

The properties of the complex carbohydrates depend on the number of units they contain, the types of units of which the chains are composed, and the position of the links that join the individual units together. In the oligoand polysaccharides, the monosaccharide units are in one of several possible ring forms rather than in the straight chain form described earlier.

MOST OF THE different kinds of carbohydrates are plant products. Plants make them by photosynthesis, a complex chemical process that consists of a series of reactions, at least one of which can occur only with the aid of sunlight and the green plant pigment, chlorophyll.

Energy from sunlight is trapped by the pigment and is used to transfer hydrogen from water to a substance that binds it. The bound hydrogen can then be passed on to other compounds. Carbon dioxide from the air enters the plant and is also combined with substances in the plant tissue, probably without the aid of sunlight. The bound hydrogen is transferred to the substance containing the carbon and oxygen from the carbon dioxide to give a product containing carbon, hydrogen, and oxygen.

Repetition of this series of reactions provides a continuous supply of small building blocks from which the plant can make the most complex carbohydrates and other chemical compounds.

The final result of photosynthesis is that light energy from the sun is converted into chemical energy, largely in the form of carbohydrate.

A large part of this chemical energy is eventually consumed by people and animals when they use plants and plant products as food. As the dry matter of most plants consists of 60 to 90 percent of carbohydrate, man consumes a great deal of the energy trapped by plants directly in this form. A part of it he consumes indirectly, after it has been converted into the flesh of animals.

Although the point has little to do with nutrition, it is interesting that many of the other sources of energy used extensively by man wood, coal, oil, and peat also arise directly or indirectly from carbohydrates of plant origin.

ALTHOUGH CARBOHYDRATES, as said at the outset, function in nutrition primarily as a source of energy, there is no definite nutritional requirement for carbohydrates.

People and animals very likely can survive quite well on diets containing no carbohydrate, because the body can also use fats and proteins directly as sources of energy and because the body can make enough carbohydrate for its special needs from other corn-pounds, such as amino acids, the building blocks of proteins. If the animal body were unable to use carbohydrates as a source of energy, however, the available food materials and, therefore, the population of the world would be very limited.

Many different kinds of carbohydrates occur in foods. Not all are of equal importance in nutrition. Their chemical structures determine which of the different carbohydrates can be digested, absorbed, and used by the body and, therefore, their value as foodstuffs. Actually, relatively few of the many carbohydrates that occur in nature are used efficiently by man, but these few make up by far the greatest proportion of the carbohydrates found in most common foods.

Starch, which consists of glucose units, is the only polysaccharide that man can use efficiently. Nutritionally it is far and away the most important carbohydrate.

Cereal grains, our most important source of carbohydrates, are rich in starch. Rice, wheat, sorghum, corn (maize), millet, and rye contain about 70 percent of starch. Potatoes and other tubers and roots are also rich in starch. Beans and the seeds of many other legumes are high in protein, but 40 percent or more of their dry matter is starch.

Only two of the disaccharides (these contain two monosaccharide units) are of much importance nutritionally.

One is sucrose cane sugar or beet sugar, which is available as a highly refined and relatively pure carbohydrate. It is also one of the carbohydrates in many fruits and vegetables and represents almost one-fourth of the carbohydrates eaten in the United States.

The other important disaccharide is lactose, or milk sugar, which makes up almost 40 percent of the solids in fresh whole milk. It is the only disaccharide synthesized by animals. It is the only carbohydrate of animal origin that is of significance in nutrition. It is made up of one glucose unit and one galactose unit. Galactose is a hexose and differs only slightly in chemical structure from glucose.

The monosaccharides are important in nutrition mainly because they are the units of the more complex carbohydrates. A few of them do occur and are eaten in the free form. Glucose and fructose, a hexose quite closely related structurally to glucose, are in honey and fruits. Fructose derives its name from the Latin word for fruit and sometimes is known as fruit sugar.