Introductory Nutrition: Lipids, Proteins, and Energy Metabolism

Comprehensive practice flashcards covering lipid classes and digestion, protein biochemistry and requirements, clinical nutrition topics, and cellular energy metabolism.

What are the primary roles of dietary lipids both in culinary applications and within the human body?

In cooking, lipids provide flavor, texture, appearance, mouthfeel, and function as emulsifiers. Within the body, lipids serve as a concentrated energy source, provide thermal insulation, cushion vital organs, constitute structural components of cell membranes, and serve as precursors for synthesized substances such as bile.

What are the three major chemical categories of lipids?

Triglycerides (fats and oils), phospholipids, and sterols (such as cholesterol).

What is the basic chemical composition of a triglyceride molecule?

A triglyceride is composed of one three-carbon glycerol backbone bonded to three fatty acids.

How do saturated fatty acids differ structurally and physically from unsaturated fatty acids?

Saturated fatty acids contain no carbon-carbon double bonds, holding the maximum possible number of hydrogen atoms, and are typically solid at room temperature. Unsaturated fatty acids contain one or more carbon-carbon double bonds, causing kinks in the chain, and are typically liquid at room temperature.

What distinguishes a monounsaturated fatty acid (MUFA) from a polyunsaturated fatty acid (PUFA)?

A monounsaturated fatty acid contains exactly one carbon-carbon double bond (e.g., oleic acid), whereas a polyunsaturated fatty acid contains two or more carbon-carbon double bonds (e.g., linoleic acid).

Which specific fatty acids belong to the omega-3 and omega-6 polyunsaturated fatty acid families?

Omega-3 fatty acids include alpha-linolenic acid, eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA). Omega-6 fatty acids include linoleic acid and arachidonic acid (AA).

What health benefits are linked to regular consumption of omega-3 fatty acids?

Omega-3 fatty acids help reduce the risk of cardiovascular disease, depression, dental disease, and arthritis, while enhancing immune function and promoting brain development in infants.

What is the dietary recommendation for fish intake, and what health risk is associated with excessive consumption?

Adults should aim for $2\text{--}3$ servings of a variety of fish per week. The primary safety concern with excessive consumption is mercury toxicity, which can adversely affect the central nervous system.

What are seed oils, and why has high consumption of the "Hateful 8" been scrutinized?

Seed oils (such as canola, corn, cottonseed, grapeseed, soybean, sunflower, safflower, and peanut oils) are rich in omega-6 fatty acids and heavily utilized in ultra-processed foods. Excessive intake can tilt the fatty acid balance and promote systemic inflammation.

How are industrial trans fatty acids produced, and what is their impact on cardiovascular health?

Industrial trans fatty acids are created through the partial hydrogenation of vegetable oils. They raise "bad" low-density lipoprotein (LDL) cholesterol and increase cardiovascular disease risk; dietary guidelines recommend keeping their intake as low as possible.

In the structural diagram of a phospholipid, what do the two distinct chemical regions represent and how do they function?

The top spherical portion represents the hydrophilic (water-soluble) head containing a phosphate group, while the two lower extended chains represent hydrophobic (fat-soluble) fatty acid tails. This amphipathic structure allows phospholipids to form cell membranes and serve as food emulsifiers.

Where is cholesterol produced in the body, and which foods contain it?

Approximately two-thirds ($\frac{2}{3}$) of the body's cholesterol is synthesized endogenously by the liver, while the remaining third comes from the diet. Cholesterol is found exclusively in animal-derived foods such as egg yolks, liver, beef, poultry, and dairy.

What are the major sequential steps of lipid digestion and absorption in the small intestine?

1. Bile released from the gallbladder emulsifies large fat droplets in the duodenum.
2. Pancreatic lipase hydrolyzes fatty acids off the glycerol backbone, producing free fatty acids, glycerol, and monoglycerides.
3. Digestion products are packaged into micelles for transport to the absorptive enterocytes.
4. Enterocytes absorb the lipids, resynthesize triglycerides, and package them with cholesterol, phospholipids, and proteins into chylomicrons.
5. Chylomicrons exit enterocytes and enter the lacteals of the lymphatic system.

How do low-density lipoproteins (LDL) and high-density lipoproteins (HDL) differ in composition and physiological function?

LDL is a concentrated source of cholesterol that transports lipids from the liver to peripheral tissues ("bad cholesterol"). HDL is a concentrated source of protein that scavenges excess lipids from peripheral tissues and transports them back to the liver for dismantling and excretion ("good cholesterol").

What are the clinical target values for fasting blood lipid panels in healthy adults?

• Total cholesterol: $< 200\,mg/dL$
• LDL cholesterol: $< 100\,mg/dL$
• HDL cholesterol: $> 50\,mg/dL$ for females, $> 40\,mg/dL$ for males
• Triglycerides: $< 150\,mg/dL$

What are the acceptable macronutrient distribution ranges and dietary guidelines for total fat and fatty acid intake?

• Total fat: $20\text{--}35\%$ of total daily calories
• Saturated fat: $< 10\%$ of total daily calories
• Linoleic acid (omega-6): $5\text{--}10\%$ of total daily calories
• Alpha-linolenic acid (omega-3): $0.6\text{--}1.2\%$ of total daily calories
• Trans fat and cholesterol: As little as possible while maintaining a balanced diet

What are the nonmodifiable versus modifiable risk factors for atherosclerosis and cardiovascular disease?

• Nonmodifiable: Family history of CVD (especially before $60$ years of age), advanced age, male sex, and race/ethnic background.
• Modifiable: Diabetes mellitus, diet lacking balance and nutrient density, elevated LDL cholesterol, excess body fat, hypertension, physical inactivity, tobacco exposure, and untreated sleep apnea.

What chemical elements constitute amino acids, and what are their four common structural components?

Amino acids are composed of carbon, hydrogen, oxygen, nitrogen, and sometimes sulfur. Each amino acid possesses a central carbon anchor bonded to: an amino group ($-NH_2$), a carboxylic acid group ($-COOH$), a hydrogen atom ($-H$), and a unique side chain ("R" group).

What is an essential amino acid, and which nine amino acids are classified as essential?

An essential amino acid is one that the body cannot synthesize (or cannot synthesize in sufficient quantities) and must be obtained from food. The nine essential amino acids are: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.

What is a conditionally essential amino acid?

A nonessential amino acid that becomes essential because the body's capacity to synthesize it is impaired due to specific physiological stresses or diseases, such as severe kidney or liver disease.

What type of chemical bond links amino acids together, and what is the primary structural conformation of most functional proteins?

Amino acids are joined together by strong covalent peptide bonds. While proteins range from primary to quaternary structures, most functional proteins in the body exist in a folded tertiary structure.

What genetic defect causes sickle cell anemia?

Sickle cell anemia is an inherited disorder caused by a faulty DNA code that inserts the incorrect amino acid into the hemoglobin chain, producing abnormal hemoglobin that distorts red blood cells into a sickle shape.

How are proteins digested and absorbed along the gastrointestinal tract?

• Mouth: Mechanical mastication only; no enzymatic digestion.
• Stomach: Hydrochloric acid ($HCl$) denatures protein structures, and pepsin hydrolyzes peptide bonds.
• Small intestine: Pancreatic and intestinal proteases (e.g., trypsin, exopeptidases) cleave remaining bonds into free amino acids and di-/tripeptides for enterocyte absorption.
• Large intestine: No enzymatic protein digestion occurs.

What is the metabolic fate of the nitrogen removed from catabolized amino acids?

During deamination, the amino group ($-NH_2$) is stripped from the carbon skeleton and converted into toxic ammonia. The liver converts ammonia into urea, which is transported through the bloodstream to the kidneys for urinary excretion.

What is the Recommended Dietary Allowance (RDA) for protein in healthy adults, and what do the 2025–2030 DGAs recommend?

The adult RDA is $0.8\,g/kg$ of body weight per day. The 2025–2030 Dietary Guidelines for Americans recommend consuming $1.2\text{--}1.6\,g/kg$ of body weight per day.

What are complementary proteins, and how do grains and legumes complement each other?

Complementary proteins combine two or more incomplete plant protein sources to supply all nine essential amino acids. Grains are limited in lysine (and threonine), while legumes are limited in methionine; eaten together (e.g., red beans and rice, peanut butter on whole wheat), they provide a complete amino acid profile.

What are the potential physiological advantages and health concerns of consuming a high-protein diet?

• Advantages: Enhanced satiety, spontaneous reduction in calorie intake, maintenance of lean muscle mass, improved glycemic control, and reduced blood triglycerides and blood pressure.
• Disadvantages: Increased renal nitrogen burden/kidney stress, potentially higher saturated fat intake, increased risk of colorectal cancer, and increased urinary calcium loss contributing to osteoporosis risk.

What causes food allergies, and what nine foods account for 90% of all food allergies in the United States?

Food allergies occur when the body's immune system mounts an inappropriate inflammatory response to harmless food proteins. The top nine allergens are cow's milk, crustacean shellfish, eggs, fish, peanuts, sesame, soybeans, tree nuts, and wheat.

What is celiac disease, how is it diagnosed, and which grains must be avoided?

Celiac disease is an autoimmune condition in which gluten ingestion causes immune-mediated destruction of the intestinal villi and mucosa. It is diagnosed using blood antibody tests and intestinal biopsies. Patients must strictly avoid wheat, barley, rye, triticale, and their derivatives (e.g., semolina, durum, farina).

How do nutrigenetics and nutrigenomics differ?

Nutrigenetics studies how an individual's genetic variations influence their nutrient requirements and metabolic responses (e.g., risk for obesity or diabetes). Nutrigenomics studies how specific dietary nutrients directly affect the expression of the genome.

How is food insecurity defined, what fraction of food in the U.S. is wasted, and what initiative operates at the University of Kentucky?

Food insecurity is the inability to consistently access enough nutritious food necessary for a healthy, active life. In the U.S., $40\%$ of food is wasted annually while 1 in 6 Americans is food insecure. The UK Campus Kitchen operates "Meals on Wings" to recover surplus campus food and deliver balanced meals to the community.

How do anabolic pathways differ from catabolic pathways?

Anabolic pathways are "uphill" synthetic reactions that consume energy (ATP) to build complex macromolecules from simple precursors (e.g., glucose to glycogen). Catabolic pathways are "downhill" degradative reactions that break down complex molecules into smaller units, releasing usable energy and heat (e.g., glycogen to glucose).

What is ATP, and how efficient is the human body at converting food energy into metabolic work?

Adenosine triphosphate (ATP) is the body's primary high-energy cellular energy currency. The human body is approximately $25\%$ efficient at converting food energy into physiological work; the remaining $\approx 75\%$ is released as heat.

What are the cellular location, oxygen dependency, and net ATP yield of glycolysis?

Glycolysis takes place in the cytosol of the cell, is an anaerobic process (requires no oxygen), is reversible, and produces a net gain of $2\text{ ATP}$ and $2\text{ NADH} + 2\text{ H}^+$ per molecule of glucose converted into two 3-carbon pyruvate molecules.

Where does the pyruvate dehydrogenase reaction occur, and which four B vitamins are required as coenzymes?

It occurs inside the mitochondria, converting pyruvate into acetyl CoA and $CO_2$ in an irreversible aerobic step. It requires thiamin (TPP), riboflavin (FAD), niacin ($NAD^+$), and pantothenic acid (CoA).

What metabolic shift occurs when pyruvate accumulates under anaerobic conditions?

Under anaerobic conditions (such as intense exercise), pyruvate cannot form acetyl CoA and is instead converted into lactic acid/lactate, leading to muscle pain and fatigue. Through the Cori cycle, lactate enters the bloodstream, travels to the liver, and is recycled into glucose.

Where does the citric acid (TCA) cycle take place, and how does acetyl CoA enter it?

The citric acid cycle occurs in the mitochondrial matrix under aerobic conditions. It begins when a 2-carbon acetyl CoA combines with 4-carbon oxaloacetate to form 6-carbon citrate.

How do the glycerol backbone and fatty acid chains of triglycerides enter energy metabolism?

The glycerol backbone enters glycolysis as a 3-carbon intermediate and can be converted into pyruvate or synthesized back into glucose. Fatty acid chains undergo beta-oxidation in the mitochondria, where 2-carbon units are cleaved into acetyl CoA; because acetyl CoA cannot be converted back into pyruvate, fatty acids cannot yield glucose.

What is the difference between transamination and deamination in amino acid catabolism?

Deamination strips the amine group from an amino acid to form ammonia and a carbon skeleton. Transamination is a coupled reaction transferring the amino group from an amino acid to a keto acid (carbon skeleton), catalyzed by transaminases (e.g., ALT) and requiring pyridoxal phosphate (PLP, from vitamin B-6).

Why does the body shift into ketosis during starvation or severe carbohydrate restriction, and what physiological risks are involved?

When glycogen stores and available carbohydrates are depleted, the liver breaks down fatty acids into excess acetyl CoA, which is converted into ketone bodies (acetoacetate, beta-hydroxybutyrate, and acetone) to provide alternative fuel for the brain and CNS. Because ketones are acidic, excessive accumulation lowers blood pH (ketoacidosis), which can cause coma or death if untreated.

According to metabolic fate tables, which macronutrients can yield glucose, and what are the respective ATP yields of alanine, glucose, and palmitic acid?

Glucogenic amino acids, glucose, and the glycerol backbone of triglycerides yield glucose; fatty acids and alcohol cannot yield glucose. Complete aerobic oxidation yields approximately $14\text{ ATP}$ from alanine, $30\text{--}32\text{ ATP}$ from glucose, and $106\text{ ATP}$ from palmitic acid.