Carbohydrates have six major functions within the body:
Glucose is the only sugar used by the body to provide energy for its tissues. Therefore, all digestible polysaccharides, disaccharides, and monosaccharides must eventually be converted into glucose or a metabolite of glucose by various liver enzymes. Because of its significant importance to proper cellular function, blood glucose levels must be kept relatively constant.
Among the enormous metabolic activities the liver performs, it also includes regulating the level of blood glucose. During periods of food consumption, pancreatic beta cells sense the rise in blood glucose and begin to secrete the hormone insulin. Insulin binds to many cells in the body having appropriate receptors for the peptide hormone and causes a general uptake in cellular glucose. In the liver, insulin causes the uptake of glucose as well as the synthesis of glycogen, a glucose storage polymer. In this way, the liver is able to remove excessive levels of blood glucose through the action of insulin.
In contrast, the hormone glucagons is secreted into the bloodstream by pancreatic alpha cells upon sensing falling levels of blood glucose. Upon binding to targeted cells such as skeletal muscle and brain cells, glucagon acts to decrease the amount of glucose in the bloodstream. This hormone inhibits the uptake of glucose by muscle and other cells and promotes the breakdown of glycogen in the liver in order to release glucose into the blood. Glucagon also promotes gluconeogenesis, a process involving the synthesis of glucose from amino acid precursors. Through the effects of both glucagon and insulin, blood glucose can usually be regulated in concentrations between 70 and 115mg/100 ml of blood.
Other hormones of importance in glucose regulation are epinephrine and cortisol. Both hormones are secreted from the adrenal glands, however, epinephrine mimics the effects of glucagon while cortisol mobilizes glucose during periods of emotional stress or exercise.
Despite the liver's unique ability to maintain homeostatic levels of blood glucose, it only stores enough for a twenty-four hour period of fasting. After twenty four hours, the tissues in the body that preferentially rely on glucose, particularly the brain and skeletal muscle, must seek an alternative energy source. During fasting periods, when the insulin to glucagons ratio is low, adipose tissue begins to release fatty acids into the bloodstream. Fatty acids are long hydrocarbon chains consisting of single carboxylic acid group and are not very soluble in water. Skeletal muscle begins to use fatty acids for energy during resting conditions; however, the brain cannot afford the same luxury. Fatty acids are too long and bulky to cross the blood-brain barrier. Therefore, proteins from various body tissues are broken down into amino acids and used by the liver to produce glucose for the brain and muscle. This process is known as gluconeogenesis or "the production of new glucose." If fasting is prolonged for more than a day, the body enters a state called ketosis. Ketosis comes from the root word ketones and indicates a carbon atom with two side groups bonded to an oxygen atom. Ketones are produced when there is no longer enough oxaloacetate in the mitochondria of cells to condense with acetyl CoA formed from fatty acids. Oxaloacetate is a four-carbon compound that begins the first reaction of the Krebs Cycle, a cycle containing a series of reactions that produces high-energy species to eventually be used to produce energy for the cell. Since oxaloacetate is formed from pyruvate (a metabolite of glucose), a certain level of carbohydrate is required in order to burn fats. Otherwise, fatty acids cannot be completely broken down and ketones will be produced.
So why are carbohydrates important if the body can use other carbon compounds such as fatty acids and ketones as energy? First of all, maintaining a regular intake of carbohydrates will prevent protein from being used as an energy source. Gluconeogenesis will slow down and amino acids will be freed for the biosyntheses of enzymes, antibodies, receptors and other important proteins. Furthermore, an adequate amount of carbohydrates will prevent the degradation of skeletal muscle and other tissues such as the heart, liver, and kidneys. Most importantly, ketosis will be prevented. Although the brain will adapt to using ketones as a fuel, it preferentially uses carbohydrates and requires a minimum level of glucose circulating in the blood in order to function properly. Before the adaptation process occurs, lower blood glucose levels may cause headaches in some individuals. To prevent these ketotic symptoms, it is recommended that the average person consume at least 50 to 100g of carbohydrates per day.
Although the processes of protein degradation and ketosis can create problems of their own during prolonged fasting, they are adaptive mechanisms during glucose shortages. In summary, the first priority of metabolism during a prolonged fast is to provide enough glucose for the brain and other organs that dependent upon it for energy in order to spare proteins for other cellular functions. The next priority of the body is to shift the use of fuel from glucose to fatty acids and ketone bodies. From then on, ketones become more and more important as a source of fuel while fatty acids and glucose become less important.
A less important function of carbohydrates is to provide sweetness to foods. Receptors located at the tip of the tongue bind to tiny bits of carbohydrates and send what humans perceive as a "sweet" signal to the brain. However, different sugars vary in sweetness. For example, fructose is almost twice as sweet as sucrose and sucrose is approximately 30% sweeter than glucose.
Sweeteners can be classified as either nutritive or alternative. Nutritive sweeteners have all been mentioned before and include sucrose, glucose, fructose, high fructose corn syrup, and lactose. These types of sweeteners not only impart flavor to the food, but can also be metabolized for energy. In contrast, alternative sweeteners provide no food energy and include saccharin, cyclamate, aspartame, and acesulfame. Controversy over saccharin and cyclamate as artificial sweeteners still exists but aspartame and acesulfame are used extensively in many foods in the United States. Aspartame and acesulfame are both hundreds of times sweeter than sucrose but only acesulfame is able to be used in baked goods since it is much more stable than aspartame when heated.
Dietary fibers such as cellulose, hemicellulose, pectin, gum and mucilage are important carbohydrates for several reasons. Soluble dietary fibers like pectin, gum and mucilage pass undigested through the small intestine and are degraded into fatty acids and gases by the large intestine. The fatty acids produced in this way can either be used as a fuel for the large intestine or be absorbed into the bloodstream. Therefore, dietary fiber is essential for proper intestinal health.
In general, the consumption of soluble and insoluble fiber makes the elimination of waste much easier. Since dietary fiber is both indigestible and an attractant of water, stools become large and soft. As a result, feces can be expelled with less pressure. However, not enough fiber consumption will change the constitution of the stool and increase the amount of force required during defecation. Excessive pressure during the elimination of waste can force places in the large intestine wall out from between bands of smooth muscle to produce small pouches called diverticula. Hemorrhoids may also result from unnecessary strain during defecation.
The disease of having many diverticula in the large intestine is known as diverticulosis. Although diverticula is often asymptomatic, food particles become trapped in their folds and bacteria begin to metabolize the particles into acids and gases. Eventually, the diverticula may become inflamed, a condition known as diverticulitis. To combat the disease, antibiotics are administered to the patient to destroy the bacteria while the intake of fiber in the diet is decreased until the inflammation has subsided. Once the inflammation has been reduced, a high fiber diet is begun to prevent a relapse.
Besides the prevention of intestinal disease, diets high in fiber have other health benefits. High fiber intake reduces the risk of developing obesity by increasing the bulk of a meal without yielding much energy. An expanded stomach leads to satisfaction despite the fact that the caloric intake has decreased.
Beyond dieters, diabetics can also benefit from consuming a regular amount of dietary fiber. Once in the intestine, it slows the absorption of glucose to prevent a sudden increase in blood glucose levels. A relatively high intake of fiber will also decrease the absorption of cholesterol, a compound that is thought to contribute to atherosclerosis or scarring of the arteries. Serum cholesterol may be further reduced by a reduction in the release of insulin after meals. Since insulin is known to promote cholesterol synthesis in the liver, a reduction in the absorption of glucose after meals through the consumption of fiber can help to control serum cholesterol levels. Furthermore, dietary fiber intake may help prevent colon cancer by diluting potential carcinogens through increased water retention, binding carcinogens to the fiber itself and speeding the passage of food through the intestinal tract so that cancer-causing agents have less time to act.
Carbohydrates not only serve nutritional functions, but are also thought to play important roles in cellular recognition processes. For example, many immunoglobulins (antibodies) and peptide hormones contain glycoprotein sequences. These sequences are composed of amino acids linked to carbohydrates. During the course of many hours or days, the carbohydrate polymer linked to the rest of the protein may be cleaved by circulating enzymes or be degraded spontaneously. The liver can recognize differences in length and may internalize the protein in order to begin its own degradation. In this way, carbohydrates may mark the passage of time for proteins.