The principal role of absorbed sugars is to provide a source of energy for body functions and activity and heat to maintain body temperature. Glucose is the preferred source of energy for the brain and central nervous system, red blood cells, and lens of the eye. Although many organs can use fats for energy, glucose is the preferred fuel. Carbohydrate, whether it was originally from a sugar or a starch, provides 4 kcal/g. Because of incomplete absorption, sugar alcohols contribute varying amounts of kilocalories. Glycogen stores are a readily available source of glucose for the tissues.
Caloric Values of Polyols
Sorbitol 2.6 kcal/g
Xylitol 2.4 kcal/g
Maltitol 2.1 kcal/g
Isomalt 2 kcal/g
Lactitol 2 kcal/g
Mannitol 1.6 kcal/g
Erythritol 0.2 kcal/g
Sugars in the blood ensure replenishing of glycogen stores; however, excessive intake of energy from any source results in converting glucose to fats in a process known as lipogenesis. When carbohydrates are eaten in excess of needs, lipogenesis results in increased fat stores.
CONVERSION TO OTHER CARBOHYDRATES
Monosaccharides are important constituents of many compounds that regulate metabolism. Examples include heparin, which prevents blood clotting; galactolipins, which are constituents of nervous tissue; and dermatan sulfate, which is present in tissues rich in collagen (especially the skin).
CONVERSION TO AMINO ACIDS
The liver can use part of the carbon framework from the sugar molecule and part of the protein molecule contributed by the breakdown of an amino acid to produce nonessential amino acids. These are essential to the body, but are not required in the diet.
NORMAL FAT METABOLISM
Oxidation of fats requires the presence of some carbohydrates. When carbohydrate intake is low, the body relies on energy from fat intake or stores. Fats are metabolized faster than the body can oxidize them; the resulting intermediate products are called ketone bodies. Ketones are normal products of lipid metabolism in the liver; muscles can use them for energy only if adequate amounts of glucose are available. An accumulation of ketones, or incompletely oxidized fatty products, results in ketosis.
PROTEIN – SPARERS
Carbohydrates, by furnishing energy in the diet, are said to be protein-sparing. Energy is an essential physiological requirement. With insufficient carbohydrate intake, the body burns protein for fuel. If carbohydrate intake is adequate, protein can be used to build and repair tissue.
Dietary fiber remains in the gastrointestinal tract longer than other nutrients. Undigestible fibers, such as lignin, cellulose, and hemicellulose, may be fermented by microflora in the large intestine. Fermentation produces gas and volatile fatty acids, which the cells lining the colon use for energy. This encourages the growth of bacteria that synthesize certain vitamins (some of the B-complex vitamins and vitamin K).
Dietary and certain functional fibers, particularly those that are poorly fermented, improve fecal bulk and laxation and ameliorate constipation in addition to other functions listed below.
Dietary fiber and functional fibers accelerate the transit rate (the time it takes for waste products to move through the intestine) in individuals with a slow transit time (constipation). Viscous fiber decreases the transit rate in individuals with a rapid transit time (diarrhea). The ability of fiber to bind water in the intestine and increase bulk from nondigestible substances decreases the length of time waste products are in the alimentary tract. An increased transit time causes tissue exposure to cancer-causing nitrogenous waste products for longer periods. An added benefit of fiber is its stool-softening ability, which helps prevent constipation. These fibers in the colon increase stool bulk, exercising the digestive tract muscles by increasing the radius of the colon and preventing the muscle from being chronically contracted. As muscle tone is maintained and colonic pressure is diminished, the gut is able to resist bulging out into pouches known as diverticula.
Viscous fibers, which also are referred to as soluble fiber, include pectins, gums, psyllium, mucilages, and algal polysaccharides. They influence the physiology of the upper gastrointestinal tract. Soluble fibers are physiologically important for their gel-forming ability, which results in increased viscosity of chyme and delays gastric emptying. Viscous fibers bind bile acids and decrease serum cholesterol levels. Viscous fibers also tend to improve glucose tolerance. Psyllium, added to many cereals and used as a laxative, is effective in reducing blood cholesterol levels.
Because fiber-rich foods are not calorie-dense and are retained longer in the stomach, they may cause one to feel full on a fewer number of kilocalories. Whether fiber plays a significant role in weight management has yet to be determined.
Carbohydrates are normally accompanied by other nutrients. Starchy foods are especially important for their contribution of protein, minerals, and B vitamins. Whole-grain products are superior because they contain fiber plus other nutrients; enriched products should always be used in preference to products that are processed but not enriched.
Dental Hygiene Considerations
- Use of carbohydrate requires an adequate supply of B vitamins and two minerals, phosphorus and magnesium. Usually, adequate amounts of these nutrients accompany the increased carbohydrate intake. However, this may not be true if refined sugars and breads are the predominant choices.
- Ketosis can occur in patients with uncontrolled diabetes or in individuals who have inadequate carbohydrate intake, such as individuals who are ill or are following a high-protein, very-low-carbohydrate regimen because they are burning fat rather than carbohydrate. Among other concerns, ketosis creates a disturbance in the acid-base balance of an individual. Patients with acetone or fruity-smelling breath should be questioned about their recent dietary intake.
- Increasing whole grains in the diet without increasing total energy intake may reduce risk of periodontal disease.
- Carbohydrates do no cause obesity. Excessive caloric intake and inadequate energy output are the primary causes of obesity.
- Fiber tends to regulate the transit rate of foods in the gastrointestinal tract. The best source of dietary fiber to relieve constipation is bran, but it must be initiated slowly to avoid severe gas and bloating.
- Enthusiastic patients who eat excessive amount of bran (50 to 60 g) gain no benefit from the surplus and expose themselves unnecessarily to hazards, such as decreased mineral and vitamin absorption.
- Some vegetables and fruits (e.g., bananas, white potatoes, and apples) are high in pectins, which bind water. They are frequently used to control diarrhea, but also can help relieve constipation by softening the stool.
- Even when an individual is trying to reduce caloric intake, consuming carbohydrates is important, especially vegetables, fruits, and whole-grain breads and cereals, to provide vital nutrients.
- Carbohydrates supply 4 kcal/g and are a less concentrated source of energy than fats (9 kcal/g).
To be continued…