Carbohydrates: Structures, Digestion, and Metabolism

Comprehensive vocabulary flashcards reviewing carbohydrate classifications, chemical structures, enzymatic digestion, intestinal absorption, hormonal blood glucose regulation, fiber properties, and sugar substitutes.

Monosaccharides

Carbohydrates with the general chemical formula $\text{C}_n\text{H}_{2n}\text{O}_n$ that typically form a single ring structure; the hexoses important in human nutrition ($\text{C}_6\text{H}_{12}\text{O}_6$) include glucose, fructose, and galactose.

Glucose

A monosaccharide known as blood sugar in the body or dextrose in food manufacturing; it accounts for 80% of carbohydrate absorbed by humans, serves as the preferred energy source for the brain and central nervous system, and forms a component of every disaccharide.

Fructose

The sweetest of all monosaccharides, also known as levulose or fruit sugar; it occurs naturally in fruits, honey, and saps, and is widely consumed in soft drinks and ready-to-eat cereals through high-fructose corn syrup.

Galactose

A monosaccharide sugar that is rarely found free in nature; commonly referred to as milk sugar, it primarily exists bound to glucose in the disaccharide lactose.

Disaccharides

Carbohydrates composed of pairs of monosaccharides chemically joined together; the three primary dietary disaccharides are sucrose, lactose, and maltose.

Condensation

A chemical reaction in which two monosaccharide units bond together to form a disaccharide, combining a hydroxyl group ($-\text{OH}$) from one unit and a hydrogen ($-\text{H}$) from the other to release a molecule of water ($\text{H}_2\text{O}$).

Hydrolysis

A chemical reaction that splits a disaccharide into two monosaccharides by inserting a molecule of water ($\text{H}_2\text{O}$), which provides an $-\text{H}$ atom to one monosaccharide and an $-\text{OH}$ group to the other.

Sucrose

A disaccharide composed of one glucose unit and one fructose unit linked together; commonly known as table sugar, beet sugar, or cane sugar, and found naturally in fruits, vegetables, and grains.

Lactose

A disaccharide composed of glucose and galactose linked together; known as milk sugar, it is the primary animal-derived carbohydrate found in dairy products.

Maltose

A disaccharide composed of two linked glucose units; also known as malt sugar, it is produced during starch breakdown and found in germinating seeds and fermented beverages like beer.

Glycogen

A storage polysaccharide found in animal liver and muscle tissue consisting of long chains of glucose held by alpha linkages in highly branched patterns; it is stored with water, is not a food carbohydrate source because it breaks down rapidly after slaughter, and allows rapid release of glucose on demand.

Starch

A plant storage polysaccharide composed of hundreds of glucose molecules linked by alpha bonds in either occasionally branched or unbranched chains; the richest sources are grains (such as rice, wheat, and corn), legumes, and potatoes.

Dietary Fibers

Nonstarch polysaccharides and structural components of plant foods linked by beta bonds that resist breakdown by human digestive enzymes, although some are fermented by intestinal bacteria.

Cellulose

The primary constituent of plant cell walls and an insoluble fiber, formed of unbranched glucose chains linked by beta linkages that human digestive enzymes cannot hydrolyze.

Soluble Fibers

Nonstarch polysaccharides that dissolve in water to form viscous gels and are easily fermented by bacteria in the colon (such as pectins and gums found in oats, barley, and fruit); they bind bile, slow nutrient absorption, and lower blood cholesterol.

Insoluble Fibers

Nonstarch polysaccharides that do not dissolve in water and are less fermentable (such as cellulose, hemicellulose, and lignins found in whole grains, celery, and corn skins); they provide bulk and promote intestinal motility.

Amylase

A carbohydrase enzyme that hydrolyzes amylose and breaks starch down into maltose and smaller polysaccharides; produced by salivary glands in the mouth and the pancreas for release into the small intestine.

Carbohydrate Digestion Enzymes

Enzymes that break down dietary carbohydrates in the GI tract, consisting of salivary and pancreatic amylase for starch hydrolysis, followed by small intestinal brush-border disaccharidases (maltase, sucrase, and lactase) that split disaccharides into absorbable monosaccharides.

Monosaccharide Absorption Mechanism

The process by which digestion end-products cross the intestinal villi into the capillaries: glucose and galactose are absorbed via active transport, while fructose is absorbed through facilitated diffusion before all enter the portal vein to the liver.

Lactose Intolerance

A clinical condition resulting from lactase deficiency in which undigested lactose draws water into the intestine and is fermented by colonic bacteria, causing bloating, gas, abdominal cramps, and diarrhea.

Gluconeogenesis

The metabolic process of synthesizing new glucose molecules in the liver and kidneys from noncarbohydrate precursors, primarily amino acids (proteins) and glycerol.

Ketone Bodies

Acidic organic compounds produced by the liver from incomplete fat breakdown when carbohydrate availability is insufficient, providing an alternative fuel source for tissues during prolonged fasting or starvation.

Protein-Sparing Action

The metabolic effect of consuming sufficient dietary carbohydrates ($50\text{--}100\,\text{g/day}$) to supply energy needs, preventing the body from degrading dietary or muscle protein for gluconeogenesis and preventing ketosis.

Insulin

A pancreatic hormone secreted by beta cells in response to elevated blood glucose; it stimulates glucose uptake by muscle and adipose tissue, promotes glycogen synthesis in the liver, and facilitates fat storage.

Glucagon

A pancreatic hormone secreted by alpha cells in response to low blood glucose; it signals liver cells to break down stored glycogen into individual glucose molecules for release into the bloodstream.

Blood Glucose Homeostasis

The physiological mechanism maintaining circulating glucose concentrations within a narrow, normal range through balanced actions of insulin (which reduces blood glucose) and glucagon or epinephrine (which raise blood glucose).

Epinephrine

An adrenal stress hormone that modulates the body's 'fight or flight' response; it mobilizes glucose rapidly by stimulating the breakdown of liver glycogen stores.

Glycemic Response

The rate at which a food raises blood glucose concentration, the magnitude of the glucose peak, and the duration required for blood glucose levels to normalize following ingestion.

Glycemic Index

A numeric rating scale classifying carbohydrate foods by their ability to raise blood glucose relative to a reference food; classified as low ($\le 55$), medium ($56\text{--}69$), or high ($\ge 70$).

Type 1 Diabetes

The less common form of diabetes mellitus characterized by autoimmune destruction of pancreatic beta cells, resulting in little to no insulin production and elevated blood glucose.

Type 2 Diabetes

The most common form of diabetes mellitus, typically linked with obesity, in which peripheral body cells become resistant to insulin and the pancreas exhibits insufficient insulin secretion.

Sugar Alcohols

Nutritive sweeteners and polyols (such as sorbitol, mannitol, maltitol, and xylitol) that yield $2\text{--}3\,\text{kcal/g}$, induce a lower glycemic response, and do not cause dental caries, but may cause gastrointestinal discomfort or diarrhea in large doses.

Nonnutritive Sweeteners

Intense sugar substitutes (such as aspartame, acesulfame potassium, sucralose, saccharin, and stevia) that provide negligible or zero energy and do not contribute to dental caries.

Aspartame

A synthetic nonnutritive sweetener approximately 200 times sweeter than sucrose made of phenylalanine and aspartic acid; products containing it must carry a warning label for people with phenylketonuria (PKU).

Sucralose

A nonnutritive artificial sweetener (Splenda) synthesized by substituting chlorine atoms for hydroxyl groups on sucrose molecules, making it the only artificial sweetener manufactured directly from sucrose.

High-Fructose Corn Syrup (HFCS)

A manufactured liquid sweetener made by treating cornstarch with enzymes to convert a portion of glucose to fructose, widely formulated to contain 42%, 55%, or 90% fructose with the remainder as glucose.

Invert Sugar

A liquid mixture containing equal parts glucose and fructose created by the chemical hydrolysis of sucrose; it is sweeter than table sugar and used commercially to maintain food moisture and prevent shrinkage.

Resistant Starches

Starches in legumes, unripe bananas, and whole milled grains that escape enzymatic breakdown and absorption in the small intestine, acting similarly to soluble fiber in the colon.

Nutritional Labeling Claims for Sugar and Energy

FDA labeling criteria requiring that foods claiming to be 'sugarless' or 'sugar-free' contain less than $0.5\,\text{g}$ of sugar per serving, and foods claiming to be 'reduced kcalories' provide at least $25\%$ fewer kcalories than the comparison product.