Nutrition: Lipids, Proteins, and Energy Metabolism

Comprehensive practice flashcards covering lipid classification and digestion, protein biochemistry, dietary requirements, high-protein diets, celiac disease, and cellular energy metabolism.

What are the three major structural categories of lipids?

The three major categories are triglycerides (fats and oils), phospholipids, and sterols (such as cholesterol).

What is the chemical composition of a triglyceride molecule?

A triglyceride is composed of one glycerol molecule attached to three fatty acid chains.

How do saturated, monounsaturated, and polyunsaturated fatty acids differ in their chemical bonding and physical state at room temperature?

Saturated fatty acids have no double bonds and are typically solid at room temperature; monounsaturated fatty acids (MUFAs) contain one double bond and are typically liquid; polyunsaturated fatty acids (PUFAs) contain two or more double bonds and are typically liquid.

What structural feature distinguishes the monounsaturated fatty acid (oleic acid) from the polyunsaturated fatty acid (linoleic acid) shown in these diagrams?

Oleic acid contains one double bond along its 18-carbon chain, whereas linoleic acid contains two double bonds along its 18-carbon chain.

Which two polyunsaturated fatty acids are considered essential in the human diet?

Alpha-linolenic acid (an omega-3 fatty acid) and linoleic acid (an omega-6 fatty acid).

What are the "Hateful 8" seed oils often highlighted on social media, and what concern is associated with their excess consumption?

The "Hateful 8" are canola oil, corn oil, cottonseed oil, grapeseed oil, soybean oil, sunflower oil, safflower oil, and peanut oil. They are rich in omega-6 fatty acids, frequently used in ultra-processed foods, and excessive intake may have negative impacts such as increased inflammation.

What are the primary health benefits associated with consuming omega-3 fatty acids?

Omega-3 fatty acids reduce the risk of heart disease, depression, dental disease, and arthritis, while improving immune function and infant brain development.

What chemical process produces trans fatty acids, and what configuration do their double bonds assume?

Trans fatty acids are primarily produced by industrial hydrogenation, which alters the double bonds of unsaturated oils into a "trans" configuration.

How does the chemical structure and function of a phospholipid differ from a triglyceride?

In a phospholipid, one of the three fatty acid chains found in a triglyceride is replaced by a phosphorus-containing group. Functionally, phospholipids serve as structural components of cell membranes and act as emulsifiers to help water and oil mix.

Where is cholesterol synthesized in the body, and what dietary sources contain it?

Approximately two-thirds ($\frac{2}{3}$) of the body's cholesterol is synthesized in the liver, while the remainder is obtained through the diet exclusively from animal foods.

What role does bile play in lipid digestion, and what triggers its secretion?

When triglycerides are detected in the duodenum, bile is released from the gallbladder to emulsify large fat droplets into smaller particles for enzymatic digestion.

What are the major end products of pancreatic lipase digestion of triglycerides in the small intestine?

Pancreatic lipase cleaves fatty acids off the glycerol backbone, producing free fatty acids, monoglycerides, and glycerol.

What is a chylomicron, and through which circulatory vessel does it leave the absorptive cells of the small intestine?

A chylomicron is a lipoprotein assembled in absorptive cells containing reassembled triglycerides, cholesterol, phospholipids, and protein coat. It exits the cell and enters the lacteal before eventually draining into the bloodstream.

According to the lipoprotein composition chart shown, what is the largest single component of a low-density lipoprotein (LDL)?

Cholesterol, making up $45\%$ of the LDL particle (along with $25\%$ protein, $20\%$ phospholipid, and $10\%$ triglyceride).

What are the primary functional differences between LDL and HDL cholesterol in lipid transport?

LDL ("bad" cholesterol) carries cholesterol and lipids from the liver to peripheral tissues like muscle and adipose tissue. HDL ("good" cholesterol) picks up excess cholesterol and lipids from tissues and returns them to the liver for dismantling.

What are the desirable fasting blood lipid thresholds for total cholesterol, LDL cholesterol, HDL cholesterol, and triglycerides?

Desirable levels are: Total cholesterol $< 200\,\text{mg/dL}$, LDL cholesterol $< 100\,\text{mg/dL}$, HDL cholesterol $> 50\,\text{mg/dL}$ for females and $> 40\,\text{mg/dL}$ for males, and triglycerides $< 150\,\text{mg/dL}$.

What are the recommended dietary intake ranges for total fat, saturated fat, and essential fatty acids as percentages of total daily calories?

Total fat: $20\text{--}35\%$ of total calories; Saturated fat: less than $10\%$; Linoleic acid: $5\text{--}10\%$; Alpha-linolenic acid: $0.6\text{--}1.2\%$.

What chemical elements make up protein molecules, and what functional groups are present on each amino acid?

Proteins are composed of carbon, hydrogen, oxygen, nitrogen, and sometimes sulfur. Each amino acid contains an amine group ($-\text{NH}_2$), a carboxylic acid group ($-\text{COOH}$), a hydrogen atom, and a unique side chain ("R" group).

What are the 9 essential amino acids that must be supplied by the diet?

Threonine, Valine, Tryptophan, Isoleucine, Leucine, Lysine, Phenylalanine, Methionine, and Histidine.

What is a conditionally essential amino acid?

A normally non-essential amino acid that becomes essential and must be supplied by the diet under specific pathological conditions, such as kidney disease or liver disease.

What type of chemical bond joins amino acids together, and what molecule is released during its formation?

A peptide bond (a strong covalent bond) joins amino acids together, releasing one molecule of water ($\text{H}_2\text{O}$).

What are the four structural levels of protein organization shown in this diagram?

(a) Primary structure (linear amino acid chain), (b) Secondary structure (alpha-helix or beta-pleated sheet), (c) Tertiary structure (three-dimensional folding), and (d) Quaternary structure (association of multiple polypeptide subunits, such as in hemoglobin).

What molecular abnormality causes sickle cell anemia?

A faulty DNA code causes the wrong amino acid to be inserted into the hemoglobin polypeptide chain, resulting in abnormal, crescent-shaped red blood cells.

Describe the chemical and enzymatic processes of protein digestion in the stomach and small intestine.

In the stomach, hydrochloric acid ($\text{HCl}$) denatures protein structures and pepsin breaks peptide bonds. In the small intestine, pancreatic and brush-border enzymes (such as trypsin and exopeptidases) cleave the remaining peptide bonds into absorbable amino acids. No enzymatic digestion occurs in the mouth or large intestine.

What happens to the nitrogen-containing amine group when amino acids are catabolized for energy?

The amine group is converted to ammonia, which the liver converts to urea. Urea enters the bloodstream (measured as blood urea nitrogen, BUN) and is excreted by the kidneys into urine (measured as urine urea nitrogen, UUN).

What are the Recommended Dietary Allowance (RDA) and 2025–2030 Dietary Guidelines for Americans (DGA) recommendations for daily protein intake in healthy adults?

The RDA is $0.8\,\text{g/kg}$ of body weight, whereas the 2025–2030 DGA recommends $1.2\text{--}1.6\,\text{g/kg}$ of body weight.

What are the primary limiting amino acids in grains, seeds, tree nuts, and legumes?

Grains are primarily limited in lysine and threonine (corn is also limited in tryptophan); seeds and tree nuts are limited in lysine; legumes are primarily limited in methionine.

What are the primary advantages and potential health disadvantages of following a high-protein diet?

Advantages include increased satiety, spontaneous caloric reduction, lean muscle preservation, improved blood glucose regulation, and reduced triglycerides/LDL. Disadvantages include increased nitrogen burden/stress on the kidneys, potentially elevated saturated fat intake, increased colorectal cancer risk, and elevated urinary calcium excretion increasing osteoporosis risk.

What 9 foods account for $90\%$ of all diagnosed food allergies in the United States?

Cow's milk, crustacean shellfish, eggs, fish, peanuts, sesame, soybeans, tree nuts, and wheat.

What is celiac disease, and which specific grains must be strictly avoided by affected individuals?

Celiac disease is an autoimmune condition where gluten cannot be absorbed and ingestion damages the intestinal mucosa. Foods to avoid include wheat, barley, rye, and triticale (along with derived flours like durum, semolina, and graham).

How do the fields of nutrigenetics and nutrigenomics differ?

Nutrigenetics studies how genetic variations influence an individual's physiological responses to nutrients and diet, whereas nutrigenomics examines how nutrients and dietary bioactive compounds affect gene expression.

According to the Food Insecurity Experience Scale (FIES), what conditions define moderate versus severe food insecurity?

Moderate food insecurity is characterized by insufficient money/resources, uncertainty about obtaining food, compromising on food quality/variety, and skipping meals or running out of food occasionally. Severe food insecurity occurs when a person has completely run out of food and gone an entire day or more without eating at times during the year.

What is the definition of metabolism, and which three nutrient classes are energy-producing?

Metabolism is the sum total of all chemical reactions that occur in living cells. The three energy-producing macronutrients are carbohydrates, lipids, and proteins.

How do anabolic and catabolic metabolic pathways differ in energy requirement and product size?

Anabolic pathways are "uphill" reactions that require energy to synthesize larger macromolecules from smaller precursors (e.g., glucose to glycogen). Catabolic pathways are "downhill" reactions that break down complex molecules into smaller units, releasing usable energy and heat.

What high-energy molecule is depicted here, and approximately what percentage of food energy is successfully converted into biological work?

Adenosine Triphosphate (ATP); the human body is approximately $25\%$ efficient at converting food energy into biological work, with the remainder lost as heat.

What metabolic pathway is depicted in this diagram, where does it occur, and what are its starting substrates and net end products?

Glycolysis; it takes place in the cytosol under anaerobic conditions, converting one 6-carbon glucose molecule into two 3-carbon pyruvate molecules with a net yield of $2\text{ ATP}$ and $2\text{ NADH} + 2\text{ H}^+$.

What conversion is catalyzed by the pyruvate dehydrogenase complex, where does it take place, and which four B-vitamins are required as coenzymes?

It converts pyruvate to Acetyl CoA in the mitochondria under aerobic conditions. The reaction requires thiamin (TPP), riboflavin (FAD), niacin (NAD+), and pantothenic acid (CoA).

What is the metabolic fate of pyruvate under anaerobic conditions in human muscle cells, and how is accumulated lactate handled by the body?

Under anaerobic conditions, pyruvate is converted to lactic acid/lactate, leading to muscle pain and fatigue. Lactate enters the bloodstream and travels to the liver, where it is recycled back into glucose via the Cori cycle.

What process catabolizes fatty acids into 2-carbon units, and why cannot fatty acids be converted into glucose?

Beta-oxidation cleaves fatty acid chains into 2-carbon Acetyl CoA molecules. Fatty acids cannot yield glucose because the conversion of pyruvate to Acetyl CoA is irreversible, precluding Acetyl CoA from entering gluconeogenesis.

What is the biochemical distinction between deamination and transamination reactions during amino acid metabolism?

Deamination is the complete removal of the amine group from an amino acid (forming ammonia). Transamination is the transfer of an amine group from an amino acid to a carbon skeleton (such as $\alpha$-ketoglutarate) to form a new amino acid and keto acid, catalyzed by enzymes like ALT and AST using the vitamin B-6 cofactor PLP.

Under what physiological conditions does ketosis occur, how are ketone bodies synthesized, and what health risk is associated with their accumulation?

Ketosis occurs during starvation, prolonged low-carbohydrate diets, or untreated type 1 diabetes when carbohydrate stores are depleted. Excess Acetyl CoA from fatty acid breakdown is converted into ketone bodies (acetoacetate, beta-hydroxybutyrate, and acetone). Because ketones are acidic, their accumulation lowers blood pH (ketoacidosis), which can cause coma and death.

How do the complete catabolic ATP yields compare among alanine, glucose, and palmitic acid?

Alanine yields $14\text{ ATP}$, glucose yields $30\text{--}32\text{ ATP}$, and palmitic acid (a 16-carbon fatty acid) yields $106\text{ ATP}$.