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Brush border enzymes include isomaltase, sucrase, and lactase in some adults; see Chapter An excess of enzyme is available for most oligosaccharide digestion, with the exception of lactose. Lactase availability limits the rate at which lactose is cleaved to glucose and galactose. Brush border enzymes are inhibited as levels of monosaccharides rise in the intestinal lumen, preventing an accumulation of sugars that could result in osmotic diarrhea. Dietary sucrose induces the enzymes sucrase and maltase. Lactase levels, however, are not influenced by the quantity of dietary lactose.

The fatty acids produced in the large bowel include butyric, isobutyric, propionic, and acetic acids. Cells of the large bowel derive energy from butyric acid and isobutyric acid in particular, and these molecules may play an important role in protecting the bowel mucosa from carcinogens. Monosaccharides are absorbed by simple diffusion, facilitated diffusion, and active transport. The L- isomers of glucose and galactose are absorbed exclusively by passive diffusion.

Passive diffusion is slowed by the movement of water into the gut lumen produced by the osmotic effect of ingested sugars. The D-stereoisomers of glucose and galactose are absorbed through protein cannels by active transport, facilitating more rapid uptake into the blood than passive diffusion. Fructose, a monosaccharide derived from sucrose, is absorbed via facilitated diffusion.

Osmotic diarrhea is induced by the acute ingestion of approximately g of fructose; more sugar is tolerated if ingested as sucrose, because digestion of the disaccharide shows the rate of absorption. Lactase deficiency, the most common enzyme deficiency affecting carbohydrate metabolism, is present in approximately half of all adults worldwide.

Besides humans, milk ingestion is typically limited to infancy in most animal; therefore, the lactase gene is expressed predominantly in infancy and deactivated thereafter. Lactose ingestion in adulthood favored the selection of genetic mutations that preserved lactase production into adulthood. Variation in adult lactose tolerance by ethnic background appears to correlate with the practice of dairying over millennia, although a causal association has not been elucidated.

Lactose-intolerant adults can generally tolerate about 5 g of lactose equal to approximately mL [3. Lactose tolerance can be assessed by administering 50 g of lactose and measuring the serum glucose. If glucose rises more than 1. Glucose is the principal source of nutrient energy. It is metabolized to carbon dioxide and water via the tricarboxylic acid cycle.

Alternatively, glucose can be stored as glycogen or converted to fatty acids for deposition in adipose tissue. Glycogen stores in muscle and the liver account for approximately g, or 1, kcal, sufficient to meet the energy needs of a fasting adult on a 2, kcal diet for approximately 14 hours. Nearly times as much energy, or , kcal, is stored in the adipose tissue of a lean adult. However, only a small portion of this energy is readily available, generally enough to support energy needs for up to 10 days.

Once glycogen stores are full, excess dietary carbohydrate is converted to fatty acids and stored in adipose tissue. The efficiency with which different sugars are converted to fat is variable. As an energy source, carbohydrate is intermediate between fat and protein with regard to both energy density and satiety induction. Carbohydrate provides roughly 4 kcal per g, which is slightly more than that of protein.

The satiety index of carbohydratemeaning the degree to which a given dose, measured in calories, induces a sense of fullnessis higher than that of fat and lower than that of protein see Chapter Complex carbohydrate is more satiating than simple carbohydrate, due largely to the fiber content.

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Viscous fibers have been shown to reduce appetite more readily than nonviscous fibers by slowing the emptying of the stomach and acting as a physical barrier that shields carbohydrates from digestive enzymes 9. Fiber adds volume but not calories to food, and soluble fiber may contribute to satiety by other mechanisms as well see Chapter 38 and Section VIIE. After carbohydrate ingestion, most of the glucose in the circulation escapes hepatic first-pass removal, whereas most fructose is absorbed by the liver, where it is used to produce glucose, lipid, or lactate.

Fructose ingestion raises serum levels of both lactic acid and uric acid. Galactose is metabolized principally in the liver and the rate of metabolism can serve as a marker of liver function. Galactose rises in serum in proportion to the dose ingested, although serum levels of galactose are blunted by concomitant administration of glucose, either orally or intravenously. Most tissues utilize a variety of nutrients for fuel, but the brain and red blood cells rely solely on glucose, except in periods of prolonged fasting, during which they can covert to ketone-body metabolism.

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Congenital deficiency of the enzyme glucosephosphate dehydrogenase principally affects the red blood cells, occurring in populations with historical exposure to malaria. These individuals are susceptible to hemolysis in the presence of drugs that disrupt glutathione reduction, such as sulfonamides.

The adult brain requires approximately g of glucose per day, accounting for kcal. Glucose needs increase during pregnancy and lactation, during which glucose is used in the production of lactose. Both amino acids and triglycerides can be used to synthesize glucose. Gluconeogenesis can produce approximately g of glucose per day in the absence of carbohydrate ingestion if other nutrients are abundant.

Although the glucose deficit can be compensated by ketone-body metabolism, fat oxidation also requires glucose. Once glycogen stores are depleted, therefore, a minimal intake of 50 g of glucose in any form appears to be necessary. Glucose can be produced endogenously, and it is thus not considered an essential nutrient. However, the recognition that a balanced diet requires carbohydrate has resulted in the establishment of a recommended dietary allowance RDA for adults of g of sugar or starch daily Conversely, a high-glucose diet results in elevated serum glucose and insulin levels, which are unaffected by a high-fructose diet High-carbohydrate diets lower levels of high-density lipoproteins, especially when compounded with high-fructose intake.

Consequently, a diet high in sucrose has deleterious effects on the lipid profile, whereas these effects are partially mitigated in a diet containing predominantly complex carbohydrates Polyunsaturated fat in the diet also blunts the fasting triglyceride rise induced by sucrose, and decreases LDL 14, Individuals with hypertriglyceridemia tend to have a particularly brisk rise in triglycerides in response to high-carbohydrate intake.

The main role of insulin is to promote energy entry and storage in cells when blood glucose levels are high, which is accomplished via several mechanisms: translocation of GLUT-4 glucose transporters to the plasma membrane, which facilitates glucose entry into liver, skeletal muscle, and adipose tissues; stimulation of glycogen and fat formation; inhibition of fat utilization for energy via suppression of glucagon release; inhibition of glyconeogenesis by the liver. Glucagon is released when blood glucose levels fall, and its actions are directly opposite of those produced by insulin, promoting glycogen breakdown to release glucose and synthesis of new glucose via gluconeogenesis in the liver and kidney.

The gut has emerged as a major regulator of carbohydrate metabolism with discovery of the role of incretins, peptide hormones released from the intestinal L cell in response to the presence of nutrients in the luman of the small intestine. GLP-1, one of the most well characterized incretins, acts to lower blood glucose via stimulation of insulin release, increasing insulin sensitivity in the tissues, promoting beta-cell mass, suppression of glucagon secretion, delaying gastric emptying, and increasing satiety in the brain.

The adrenal gland also plays a role in glucose homeostasis via release of epinephrine, which stimulate glycogenolysis in the liver.

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Epinephrine also stimulates glycogenolysis in skeletal muscle, whereas glucagon does not. According to the National Library of Medicines Medical Encyclopedia 16 , a simple carbohydrate is composed of mono- or disaccharides, while complex carbohydrates are composed of units containing three or more sugar molecules see Table This definition is structural rather than functional, and is oversimplifed in several important ways.

Thus, the actual food item is a mix of simple and complex carbohydrates based on a structural definition. Many fruits contain the monosaccharide fructose, and many vegetables contain the disaccharide maltose, facilitating a functionally misleading classification of fruits and vegetables as simple carbohydrates based solely on the structural definition.

A functional definition of carbohydrate complexity is based on the metabolic fate of ingested items. Foods that engender a brisk rise in blood glucose, and consequently blood insulin, are considered simple carbohydrates from a functional perspective. Foods that induce low and slow postingestive increases in glucose and insulin are functionally complex carbohydrates.

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In such a scheme, fruits and vegetables would be considered sources of complex, rather than simple, carbohydrate, which conforms better to prevailing views on their place in a healthful diet. This would not refute the presence in such foods of structurally simple carbohydrate but would take into account how nutrients are packaged in such foods and base characterization on the overall influence of the food on metabolic response rather than on the chemical structure of a given constituent.

A functional rather than structural approach to the characterization of carbohydrate complexity is generally quite consistent with glycemic load GL values and less so with glycemic index GI values 17 see The Glycemic Index and Glycemic Load on pages 89, Tables and Given the increasing evidence that low-GL diets may offer diverse health benefits ; see Chapters 5 and 6 , this would seem to lend support for functional categorization.

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Food Group Food Glycemic Index. In some applications, sucrose rather than white bread is used as the reference standard, and given a value of The glycemic index, fiber, and the dietary treatment of hypertriglyceridemia and diabetes. J Am Coll Nutr ; Revised international table of GI values. International table of glycemic index and glycemic load values.

Am J Clin Nutr ; Another wrinkle in the definition of carbohydrate complexity is the manner in which foods package nutrients.

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While sugar added to a whole grain breakfast cereal is structurally similar to that added to a candy bar, its metabolic fate is influenced by the company it keeps. Fiber in grain products, in particular soluble fiber see Section VIIE , slows the entry of glucose and lipids from the GI tract into the bloodstream, attenuating postprandial glycemia, lipemia, and insulinemia 18, Fiber added to food during processing, including inulin, polydextrose, and maltodextrin, share some laxative properties with natural fibers; however, their effect on blood glucose and cholesterol are quite modest For this reason, there is a practical rationale for classifying foods as sources of simple or complex carbohydrate based on their overall nutritional composition and the metabolic fate of the carbohydrate they provide.

While this dispute plays out, the clinician is encouraged to consider whole grains, vegetables, fruits, beans, and legumes as sources of complex carbohydrate based on the metabolic implications of their dietary intake In general, the wholesale rejection of a macronutrient class may facilitate weight loss in the short term by restricting choice and thus calories but is at odds with the nutrient balance required for optimal health and the dietary balance required for pleasure and sustainability Long-term carbohydrate restriction has not been shown to be superior to other dietary patterns for weight loss, and may actually precipitate adverse health outcomes The practice of wholesale carbohydrate restriction therefore not encouraged, while selective restriction of sugar and simple carbohydrate certainly is.

The restriction of simple sugars has been associated with improvements in nonalcoholic fatty liver The topic of carbohydrate restriction is addressed in greater detail in Chapters 5 and The FDA has granted approval for five nonnutritive sweeteners: acesulfame potassium, aspartame, neotame, saccharin, and sucralose, and a naturally occurring low-calorie sweetener stevia Nonnutritive sweeteners bind sweet taste receptors by mimicking the structural motifs of natural carbohydrates; however, they elicit an effective sweetness response to times stronger than table sugar.

Because of this, they can be added to food in such small quantities as to negate their caloric contribution For decades, nonnutritive sweeteners were considered an effective method of reducing caloric intake without sacrificing palatability of food and drink, however recent studies have produced evidence that their use may actually contribute to obesity in adults and children via dysregulation of energy balance Several hypotheses have emerged to explain the paradoxical association of nonnutritive sweeteners and weight gain.

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For example, nonnutritive sweeteners may alter the gut microbiological flora, triggering an inflammatory process that promotes insulin resistance and weight gain Another potential mechanism suggests that the GI tract utilizes sweet taste as a means of predicting a high-calorie meal, and will alter its absorptive properties to compensate Finally, the recent discovery of sweet taste receptors in the GI tract has provoked a new hypothesis that nonnutritive sweeteners inappropriately activate sugar receptors in the gut, leading to GLP-1 release and insertion of glucose transporters in intestinal epithelia David Jenkins et al.

Recently, the GL has gained increased popularity as a tool for dietary guidance and has been implicated in regulating food reward and cravings The GL is the GI of a food multiplied by the amount of carbohydrate per serving. For example, while the GI would require a comparison between a small amount of ice cream and a very large serving of carrots to fix the dose of carbohydrate at the same value in each case, the GL would be based on the amount of carbohydrate in a typical serving of carrot or ice cream see Tables and There is mounting evidence that a low-GL diet is generally healthful and of particular value in ameliorating insulin resistance or impaired glycemic responses As a practical matter, guiding patients toward a less processed diet abundant in vegetables, fruits, and whole grains, along with healthful oils from plant sources and lean protein, as is clearly warranted on general principles see Chapter 45 , will also direct them toward a diet relatively low in overall GL.

The converse is likely to be true as well i.

Advice to consume carbohydrate foods with a low-GL will correspond closely with advice to eat more whole grains, vegetables, and fruits. However, foods high in fat, including saturated fat, but low in sugar or starch will also have a low-GL but not necessarily warrant a prominent place in a healthful diet. Thus, the clinician is encouraged to offer guidance to patients in terms of foods and their place in a health-promoting diet see Chapters 45 and 47 rather than based on some isolated property of a food or food group. Modern nutrition in health and disease, 10th ed.