Comprehensive Macronutrient Metabolism: Carbohydrates, Lipids, and Proteins

Essential vocabulary flashcards covering the biochemical classification, digestion, physiological functions, and metabolic health outcomes of carbohydrates, lipids, and proteins.

Glucose

The most abundant monosaccharide in the body and in foods, produced by plants during photosynthesis, which serves as the primary and preferred energy source for the brain and red blood cells ($4\,\text{kcal/g}$).

Fructose

The sweetest of all naturally occurring monosaccharides, found abundantly in fruits and as a major component of high-fructose corn syrup.

Galactose

A hexose monosaccharide that commonly joins with glucose to form the disaccharide lactose, rarely occurring as a free single unit in foods.

Condensation Reaction (Carbohydrates)

The chemical reaction that joins two monosaccharide units together to form a disaccharide, releasing a molecule of water ($\text{H}_2\text{O}$) in the process.

Sucrose

The most common dietary disaccharide (table sugar), composed of glucose and fructose, found naturally in sugar cane and sugar beets.

Lactose

A disaccharide composed of galactose and glucose, commonly referred to as milk sugar and found naturally in dairy products.

Maltose

The least common dietary disaccharide, composed of two linked glucose units and formed from the enzymatic breakdown and digestion of starches.

Oligosaccharide

A carbohydrate chain containing 3 to 10 monosaccharides that escapes small intestinal digestion, acts like fiber, and is found in human breast milk, legumes, beans, cabbage, and broccoli.

Amylose

A straight-chain polysaccharide found in plant starch that is more resistant to digestion than amylopectin and can act as a prebiotic to improve GI tract health.

Amylopectin

A branched-chain polysaccharide found in plant starch that is digested more easily and rapidly than amylose.

Cellulose

A straight-chain nondigestible plant structural polysaccharide composed of glucose units connected by beta-glycosidic bonds that human digestive enzymes cannot cleave.

Soluble Fiber

Dietary fiber that dissolves in water and is easily fermented by colonic bacteria (e.g., pectin, beta-glucan, gums, psyllium); it slows digestion, helps lower blood cholesterol, and helps control blood glucose.

Insoluble Fiber

Dietary fiber that does not dissolve in water and is not easily fermented by intestinal bacteria (e.g., cellulose, lignins, some hemicelluloses); it increases stool bulk and relieves constipation.

Functional Fiber

A nondigestible fiber that has been extracted or isolated from plants or manufactured by the food industry and demonstrated to have physiological health benefits.

Glycogen

The storage form of glucose in animals and humans, consisting of long, highly branched chains of glucose stored in liver and muscle tissue.

Diverticulosis

A condition in which increased intraluminal pressure in the colon causes weak spots along the wall to bulge outward, forming small pouches called diverticula.

Insulin

A peptide hormone produced by pancreatic beta cells that lowers blood glucose by facilitating cellular glucose uptake, stimulating glycogenesis and lipogenesis, and inhibiting lipolysis.

Glucagon

A peptide hormone produced by pancreatic alpha cells in response to low blood glucose; it stimulates liver glycogenolysis and gluconeogenesis to release glucose into the blood.

Hypoglycemia

A clinical state characterized by a drop in blood glucose below $70\,\text{mg/dL}$, presenting with symptoms such as hunger, dizziness, weakness, diaphoresis, and confusion.

Type 1 Diabetes

An autoimmune disorder accounting for 5% to 10% of diabetes cases, where the immune system destroys pancreatic beta cells, causing total insulin deficiency that mandates exogenous insulin therapy.

Type 2 Diabetes

The most common form of diabetes (90% to 95% of cases), characterized by progressive peripheral insulin resistance and relative secretory failure by the pancreas.

Hemoglobin A1C

A blood test measuring the average percentage of hemoglobin bound to glucose over the preceding 3 months; values between $5.7\%$ and $6.4\%$ indicate prediabetes, while $\ge 6.5\%$ confirms diabetes.

Anatomy of a Wheat Kernel

The structural composition of a grain kernel comprising the fiber- and mineral-rich outer bran, the nutrient-dense germ providing vitamin E and unsaturated fats, and the starchy, proteinaceous endosperm.

Lipids

A major class of hydrophobic macronutrients consisting of carbon, hydrogen, and oxygen that yield $9\,\text{kcal/g}$ and include triglycerides, phospholipids, and sterols.

Saturated Fatty Acid

A fatty acid chain in which every carbon is bound to hydrogen atoms with single bonds only, producing a straight chain that is solid at room temperature with a higher melting point.

Monounsaturated Fatty Acid (MUFA)

An unsaturated fatty acid possessing exactly one carbon-carbon double bond, which creates a bend in the chain and makes it liquid at room temperature (e.g., oleic acid).

Polyunsaturated Fatty Acid (PUFA)

An unsaturated fatty acid possessing two or more carbon-carbon double bonds, making the molecule flexible and liquid at room temperature with a lower melting point (e.g., linoleic acid).

Rancidity

The chemical spoilage of fats caused by the oxidation of unstable double bonds upon exposure to oxygen, heat, and light.

Triglyceride

The primary lipid in both foods and the body (making up approximately 95% of food fat), structurally consisting of three fatty acids attached to a glycerol backbone.

Phospholipid

A lipid molecule structured with a glycerol backbone, two hydrophobic fatty acid tails, and a hydrophilic phosphorus-containing head group, serving as an emulsifier and forming cellular bilayers.

Lecithin

A major phospholipid synthesized endogenously by the liver and found in cell membranes, widely used in food processing as an emulsifier in items like mayonnaise and salad dressings.

Sterols

A class of lipids containing four interconnected rings of carbon and hydrogen without glycerol or fatty acids (e.g., cholesterol, sitosterol) that provide no metabolic energy.

Micelle

A small transport carrier assembled in the intestinal lumen composed of monoglycerides, bile salts, and phospholipids that transports lipid digestion products across enterocyte membranes.

Chylomicron

A large lipoprotein produced in enterocytes that packages re-esterified triglycerides, cholesterol, and fat-soluble vitamins for transport into the lacteals of the lymphatic system.

Low-Density Lipoprotein (LDL)

A circulating lipoprotein ('bad cholesterol') that delivers cholesterol from the liver to peripheral cells; when elevated, it can undergo oxidation and contribute to arterial plaque formation.

High-Density Lipoprotein (HDL)

A protective circulating lipoprotein ('good cholesterol') produced by the liver that gathers excess cholesterol from peripheral tissues and returns it to the liver for excretion or recycling.

Alpha-Linolenic Acid (ALA)

An essential 18-carbon omega-3 polyunsaturated fatty acid with three double bonds, found in flaxseeds, walnuts, and canola oil, which the body can convert to EPA and DHA.

Linoleic Acid

An essential 18-carbon omega-6 polyunsaturated fatty acid with two double bonds, found primarily in plant oils, which serves as the precursor to arachidonic acid.

Eicosanoids

Biologically active, hormone-like compounds synthesized from EPA and arachidonic acid that help regulate inflammation, vascular tone, blood pressure, and platelet aggregation.

Atherosclerosis

The narrowing and stiffening of arteries caused by chronic endothelial injury, subendothelial LDL oxidation, foam cell accumulation, fatty streak development, and fibrous plaque formation.

Phytosterols

Plant sterols that share structural similarities with cholesterol and effectively reduce blood LDL cholesterol concentrations by competing for small intestinal absorption sites.

Amino Acid

The monomeric building block of proteins, structured around a central carbon atom bonded to an amine group ($\text{NH}_2$), a carboxylic acid group ($\text{COOH}$), a hydrogen atom, and a variable side chain.

Peptide Bond

The covalent linkage formed via a condensation reaction between the carboxylic acid group of one amino acid and the amine group of another, liberating a water molecule ($\text{H}_2\text{O}$).

Essential Amino Acids

The group of 9 amino acids (histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine) that the human body cannot synthesize and must obtain from the diet.

Conditionally Essential Amino Acids

Nonessential amino acids (such as tyrosine and glycine) that become essential when metabolic demands exceed synthetic capacity or when precursor availability is compromised by illness or trauma.

Denaturation

The structural alteration or unfolding of a protein's secondary, tertiary, or quaternary shape triggered by heat, acid, alkali, salts, or agitation, which destroys functionality while leaving the primary structure intact.

Pepsin

A gastric endopeptidase converted from the inactive zymogen pepsinogen by hydrochloric acid ($\text{HCl}$) that cleaves dietary proteins into shorter polypeptide chains.

Protein Turnover

The continuous cyclical biological process of degrading cellular proteins and resynthesizing new proteins, with more than $200\,\text{g}$ turned over each day in healthy adults.

Deamination

The biochemical removal of the nitrogenous amine group ($\text{NH}_2$) from an amino acid, yielding toxic ammonia that the liver converts to urea for subsequent renal excretion.

Positive Nitrogen Balance

A physiological condition in which dietary nitrogen intake exceeds nitrogen excretion, reflecting an anabolic state seen in growing children, pregnant women, and healing tissue.

Negative Nitrogen Balance

A catabolic state where urinary and fecal nitrogen output exceeds dietary nitrogen intake, occurring during starvation, severe trauma, surgery, or infection.

Complete Protein

A high-quality dietary protein source containing all 9 essential amino acids in proportions adequate to support human growth and protein synthesis (e.g., meat, fish, eggs, soy, quinoa).

Incomplete Protein

A dietary protein that is deficient or low in one or more essential amino acids, which are known as limiting amino acids; typical of plant-based foods.

Complementary Proteins

Two or more incomplete protein sources that, when consumed together (e.g., legumes paired with grains), supply the missing limiting amino acids of each other to provide a complete amino acid profile.

Kwashiorkor

A severe form of protein-energy malnutrition caused by an isolated protein deficiency despite adequate energy intake, characterized by abdominal and peripheral edema, brittle hair, apathy, and hepatomegaly.

Marasmus

A form of protein-energy malnutrition resulting from severe, chronic calorie deficiency, resulting in severe somatic wasting, an emaciated appearance, hypothermia, and loss of subcutaneous fat.

Vegan

A strict plant-based dietary pattern that excludes all animal flesh, poultry, fish, dairy foods, eggs, and animal-derived ingredients.

Lacto-Ovo-Vegetarian

A vegetarian dietary pattern that avoids meat, fish, and poultry but includes dairy products and eggs alongside grains, fruits, vegetables, legumes, seeds, and nuts.