Fruits and Vegetables

Carbohydrates, Our Main Source of Energy

Carbohydrates are probably the most important of the three main classes of foods since they are our main source of energy and should constitute at least 50–60 percent of the diet.

There has been a shift in this century away from the healthful consumption of fresh fruits and vegetables and complex carbohydrates—the starches and fiber foods—toward a diet of more refined carbohydrates and simple sugars that are implicated in a variety of diseases, among them obesity, diabetes, cardiovascular problems, and tooth decay.

Carbohydrates are organic molecules; that is, they contain carbon and come from living sources. They are composed of carbon (C), hydrogen (H), and oxygen (O) (thus, the abbreviation CHO) in a 1:2:1 ratio. The basic relationship is that of carbon coupled with water molecules. Carbohydrates are a quick source of energy for the body, being easily converted to glucose, the fuel for the body’s cells. Each gram of carbohydrate releases four calories, units of heat or energy, for the body.

Carbohydrates are produced by photosynthesis in plants. The carbohydrates are the primary source of energy in nature’s plant foods—fruits, vegetables, grains, legumes, and tubers. These foods play a very important role in the functioning of the internal organs, the nervous system, and the muscles. They are the best source of energy for endurance athletics because they provide both an immediate and a time-released energy source as they are digested easily and then consistently metabolized in the bloodstream.

Carbohydrates are also needed to regulate protein and fat metabolism. With the proteins and fats, the carbohydrates help to fight infections, promote growth of body tissues such as bones and skin, and lubricate the joints. Many carbohydrate foods are also high in fiber, and the fiber content of foods is important in the bulking of the stool, which aids in regular elimination of waste materials through the colon. Indeed, fiber is thought to be helpful in preventing colon diseases such as colon cancer and diverticulosis and is being prescribed by more doctors as a dietary necessity.

Three principal carbohydrates are present in foods. Carbohydrates are classified according to their structure.

  • First are the sugars, both monosaccharides (simple sugars), such as those found in honey and fruits, and oligosaccharides (multiple sugars), such as table sugar and malt sugar, which both happen to be disaccharides (two-sugar molecules).
  • Then there are the starches, or complex carbohydrates, found primarily in vegetables such as carrots and potatoes and in whole grains such as rice and corn.
  • Finally, there is fiber, mainly cellulose and hemicellulose, the indigestible roughage found in most unprocessed, carbohydrate-containing foods.


The basic unit of the simple sugars (monosaccharides) is one hexose (containing six carbon atoms) or pentose (five carbon atoms) molecule. These simple sugars are easily and quickly digested and utilized by the body. They have the same chemical makeup but vary in structure. The disaccharides, such as table sugar or milk sugar, require some enzymatic breakdown but are easily converted into monosaccharides for digestion. The following represent the common basic sugars.


Glucose is the metabolized form of “sugar” in the body. It is found in some fruits, such as grapes. It can also be hydrolyzed from starch, cane sugar (sucrose), milk sugar (lactose), and malt syrup (maltose). Glucose is carried in the blood and is the principal sugar used by the tissues and cells for energy. Glucose can be measured by the “blood sugar” test, which reads the current concentration of glucose in the serum or plasma. A high glucose level can signal a diabetic condition; low blood sugar is called hypoglycemia. Both of these abnormalities can become chronic and even life-threatening. The adrenal and pancreatic hormones, adrenaline and insulin, are very important in sugar metabolism. High-sugar and refined-carbohydrate diets, stress, and lack of exercise can generate elevated glucose levels in both blood and tissue. Pregnancy is also a stress on carbohydrate metabolism because of its high metabolic demands.

Fructose is found in most fruits and fruit juices, as well as in honey and some vegetables. Fructose is sweeter than cane sugar and is absorbed directly into the blood. Cane sugar (sucrose) is metabolized into fructose and glucose. Fructose can be changed to glucose in the liver or in the small intestine for a quick source of energy.

Galactose comes from the metabolism of the milk sugar lactose, which breaks down into galactose and glucose. Galactose is converted to glucose in the liver and is synthesized in the mammary glands to make the lactose of mother’s milk.


These sugars can be hydrolyzed into two monosaccharides with the addition of a water molecule. They are all water soluble and will crystallize when dehydrated.

Lactose (milk sugar) is the only sugar of animal origin, the sugar of mother’s milk. It is composed of one molecule each of glucose and galactose. Lactose is broken down by the enzyme lactase, which may be deficient or absent in some races of people, leading to problems with milk digestion.

Sucrose (“white sugar”) is found in sugar cane and sugar beets, maple syrup, molasses, sorghum, and pineapple. Sucrose is composed of one molecule each of fructose and glucose. Sucrose is very sweet and can be metabolized in the body. Its crystalline form, table sugar, is used excessively in our society, not only as a sweetener on food and in beverages but also in cooking and “hidden” in preparation of many other common foods and condiments, such as catsup, mayonnaise, salad dressings, and baby foods.

Addiction to sucrose begins early and is supported by millions of dollars of advertising. The average American consumes well more than 100 pounds of sucrose a year, and this particular disaccharide is responsible for a wide variety of problems. It has been implicated in obesity, tooth decay, diabetes, and many psychological and emotional problems, including premenstrual syndrome and stress/burnout syndromes. Using less sugar would be a boost to anyone’s health.

Maltose (malt sugar) is a short chain of two glucose molecules. It is produced during the breakdown of starches in many cereal grains. Maltose is present in beers, malted snacks, and some breakfast cereals and is the sweetener of many crackers (read labels!). It is easily broken down into glucose molecules for quick utilization by the body.


The second category of carbohydrates is the starches, or polysaccharides. These are also termed the complex carbohydrates, as they are composed of long chains of glucose molecules. Starches require amylase enzymes (other biochemical catalysts) to be broken down into simple sugars for digestion, absorption, and utilization.

Starch provides a more consistent blood sugar level than the simple sugars, which cause the glucose level in the blood to rise and fall rapidly. In the traditional diet, a high percentage of foods consumed included the complex carbohydrates of potatoes, vegetable roots, and whole grains such as wheat, rice, and corn. This was much healthier than the present-day preference for high-sugar and refined-flour diets, which are associated with degenerative tissue disease and aging.

There are several types of starches. If the polysaccharide chains are shorter and branched, the starch is called amylopectin—the most common one found in foods. Amylose has long chains of glucose molecules, which are easily separated by the enzyme amylase. Glycogen is the animal-source starch contained in muscle and liver. It is similar in structure to amylopectin and can be broken down to release glucose for energy needs or be formed from extra glucose and stored in the liver. Glycogen is really the form in which glucose is stored in our body. Dextrins are partially digested starches that are formed in the breakdown of starch.


The third component of carbohydrates is fiber—mainly the indigestible cellulose commonly found in the skins of fruits and vegetables and in the coverings of cereal grains, such as wheat bran. This fiber in foods provides little energy or caloric value. As mentioned earlier, it fosters good intestinal function and elimination. Low-fiber diets are associated with constipation, gastrointestinal disorders, diverticulosis, and colon cancer, while a high-fiber diet may prevent these problems. Fiber in the diet may also reduce the risk of appendicitis.

Cellulose is the most common fiber contained in basic foods. Other fibers include the hemicelluloses, found in the cell walls of plants, which have a high ability to bind water. This helps in digestion and elimination. Psyllium seed husks are a good example of a hemicellulose. They are a popular fiber supplement used to provide bulk and to speed transit time through the bowels. Pectin is another hemicellulose, which, besides absorbing water, can lower the amount of fat absorption. This is the pectin found in the rind of citrus fruits and in the pulp of apples, which is also used in making jams.

Other fibers used in the diet include both agar and alginate (derived from seaweed) and carrageen, which comes from the Irish moss plant. All indigestible polysaccharides, they are used in food preparation and in cosmetics for their smooth gelatinous consistency. Carrageen is used commonly with dairy products such as yogurt to create a smooth consistency. Agar is used to bring a gelatinous quality to foods and desserts. Alginate can bind up minerals and metals, such as cadmium, mercury, lead, and arsenic, in the intestines and has been found useful in detoxification programs.

Several other high-fiber substances that have some use in the diet have been shown in preliminary research to help reduce cholesterol levels because of their ability to hinder fat absorption from the intestines. Guar-gum may also be used to slow glucose uptake in the intestines and may be helpful in mild diabetes. Konjar root flour from Japan has also been shown in tests to have some influence in moderating diabetes, in lowering cholesterol levels, and in weight control.

Chitosan, derived from oyster shells, has also been used to lower cholesterol levels.


Although the carbohydrate-containing foods often constitute the majority of our diet, there are no specific requirements published for our carbohydrate needs. They are one of the best sources of energy and are simple for the body to use; however, since the body can make its own glucose from stored glycogen and the amino acid L-alanine, the government lists no minimum requirement. Yet, carbohydrate intake is important to health. Many of the carbohydrate foods contain essential vitamins and minerals as well as the dietary fiber necessary for colon health and proper elimination.

On the other hand, people can live without carbohydrate intake; in fact, in many weight loss programs carbohydrate consumption is severely limited. It is wise in these cases to consume supplemental fiber. Also, some people have a tendency to overeat carbohydrate foods, even to become “carb addicts.” With this, weight may increase; obesity is associated most frequently with carbohydrate overindulgence. Allergies and emotional shifts, including “carbohydrate depression,” have also been associated with sensitivity to overconsumption of this macronutrient.

Peoples of different cultures consume varying amounts of carbohydrates. Native or traditional diets may be very high in carbohydrates, while in cold climates, as with the Eskimo culture, people may consume very few carbohydrates.

The average American diet includes about 40–50 percent carbohydrates; sadly enough, about half of that is from the refined and processed flours and sugars in breads, candies, cookies, and cakes. These foods deplete the body of many B vitamins and of minerals such as chromium. In addition to the already-mentioned diseases of obesity and tooth decay, it is possible that this type of diet (high in simple and refined sugars, high in fats, and low in complex carbohydrates) may be influencing the incidence of diabetes, high blood pressure, heart disease, anemia, skin problems, kidney disease, and cancer.

I feel that a diet of about 60–70 percent carbohydrate foods is ideal, especially when caloric consumption will support our best weight range. Intake of the refined carbohydrate foods should be minimal; primary intake should be the complex carbohydrates (many vegetables, whole grains, and legumes) and some simple “naturally occurring” sugars from the fruits and vegetables. For the adult, this higher carbohydrate/fiber diet, along with about 15–25 percent fat and 15–20 percent protein, is likely to be the best long-range healthy diet.

Carbohydrate Digestion and Metabolism

Carbohydrates—sugars and starches—are broken down in the gastrointestinal tract by various enzymes for absorption into the blood. The disaccharides (lactose, sucrose, and maltose) are converted into their monosaccharides (glucose, fructose, and galactose). The polysaccharides (starches) are converted by salivary amylase in the mouth into dextrin, a shorter-chain starch; then the dextrins are reduced to maltose by pancreatic amylase released into the small intestine.

The maltose is further broken down into glucose by maltase enzymes at the intestinal lining. Also in the small intestine, sucrose is changed into glucose and fructose by the enzyme sucrase, while lactase converts lactose, the milk sugar, into glucose and galactose. The monosaccharides, or simple sugars, such as glucose, galactose, and fructose, are the end products of carbohydrate digestion and are all absorbed into the bloodstream through the intestinal lining. The blood circulates to the liver, where fructose and galactose are easily converted into glucose, the fuel the body uses for energy.

The healthy liver regulates the use of glucose as it allows certain levels to circulate in the blood for use by the cells of the body. If the carbohydrate intake is higher than immediately needed, the liver will normally convert extra glucose into glycogen, a highly branched polysaccharide, and this glycogen can be stored in the liver or in the muscles. At a later time, if energy is needed when there are no dietary carbohydrates available, the liver will convert glycogen back to glucose and return it to the bloodstream, while the muscles may use the muscle glycogen directly for energy.

If we consume higher levels of carbohydrates than are immediately needed or can be converted to glycogen (that is, if there are already sufficient storage sources), then the liver will convert the excess glucose into fatty acids and then triglycerides that can be stored as body fat, a process termed lipogenesis.

If carbohydrates are consumed in high quantities on a regular basis by a person with a sedentary lifestyle, weight gain occurs. Fat is a reserve source of energy. With decreased carbohydrate intake and increased activity levels, fat reserves are converted back to fatty acids for body fuel, a process called lipolysis. This generally produces weight loss.

Even when there is little or no intake of carbohydrates, the body attempts to maintain a steady blood sugar level through many mechanisms. Glucose is used by the liver as a source of energy to help synthesize a variety of essential substances. Insulin, a pancreatic hormone, regulates blood sugar levels by stimulating glucose uptake by the cells.

Activity and exercise also can reduce blood sugar by increasing tissue glucose needs. A number of hormones influence the production of glucose when the body, and especially the brain, needs more energy. Epinephrine (adrenaline) stimulates glycogen breakdown and raises blood sugar. Steroids enhance conversion of fats and proteins into glucose, and adrenocorticotrophic hormone (ACTH) can interfere with insulin activity. Glucagon is produced in the pancreas and can raise blood sugar, while thyroid hormone may increase intestinal absorption of glucose as it attempts to stimulate metabolism.

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Written by Elson M. Haas MD

Explore Wellness in 2021