Nutrition & Metabolism

Core Definitions

• Metabolism — ALL the chemical reactions that occur in the body. Includes both catabolism and anabolism.
• Energetics — the study of the flow of energy and its transformation from one form to another.
• Catabolism — the breakdown of organic molecules, releasing energy that can be captured as ATP. First steps occur in the cytosol; most energy is generated in mitochondria.
• Anabolism — the synthesis of new organic molecules, requiring energy (ATP).
• Nutrients — essential substances absorbed by the digestive tract: water, vitamins, mineral ions, carbohydrates, lipids, and proteins.

Energy Capture Efficiency

Mitochondria capture roughly 40% of the energy released during catabolism as ATP. The remaining 60% escapes as heat that warms the cell and surrounding tissues.

Four Reasons Cells Synthesize New Organic Components

Fuel Priority Order

Glycolysis

Glycolysis = breakdown of one 6-carbon glucose molecule into two 3-carbon pyruvate molecules. Occurs in the cytosol. Anaerobic (does not require oxygen).

Requirements: (1) glucose, (2) cytoplasmic enzymes, (3) ATP and ADP, (4) NAD (nicotinamide adenine dinucleotide) — a coenzyme that removes hydrogen atoms.

Net gain: 2 ATP + 2 NADH per glucose molecule.

The Citric Acid Cycle (TCA Cycle / Krebs Cycle)

Occurs in the mitochondrial matrix. Requires oxygen (aerobic).

Preparatory step: Each pyruvate + NAD + Coenzyme A → acetyl-CoA (2-carbon) + CO₂ + NADH. This reaction is irreversible — acetyl-CoA cannot be converted back to pyruvate.

The acetyl group (2C) combines with a 4-carbon molecule → citric acid (6C). One complete revolution removes the 2 added carbons as CO₂, regenerating the 4-carbon starting molecule.

Primary function: Remove hydrogen atoms from organic molecules and transfer them to coenzymes NAD and FAD. The hydrogen-loaded coenzymes (NADH, FADH₂) carry electrons to the ETS.

Per turn: 1 GTP (→ 1 ATP) + 3 NADH + 1 FADH₂ + 2 CO₂. Two turns per glucose molecule.

The Electron Transport System (ETS)

Located in the inner mitochondrial membrane. Consists of cytochromes (protein-pigment complexes). Provides roughly 95% of the ATP cells need.

Total ATP Yield Per Glucose

Gluconeogenesis

Gluconeogenesis = synthesis of glucose from non-carbohydrate precursors (lactate, glycerol, some amino acids). Uses different enzymes than glycolysis. Acetyl-CoA cannot be used to make glucose because the pyruvate → acetyl-CoA step is irreversible. Fatty acids and many amino acids break down to acetyl-CoA, so they cannot be used for gluconeogenesis either.

Glycogen is an important energy reserve stored in liver and skeletal muscle — compact, insoluble granules of glucose.

Lipid Catabolism (Lipolysis)

Lipolysis = lipid breakdown. A triglyceride is first split by hydrolysis into 1 glycerol + 3 fatty acids.

• Glycerol → converted to pyruvate in the cytosol → enters the citric acid cycle
• Fatty acids → enter mitochondria → beta-oxidation clips them into 2-carbon fragments (acetyl-CoA), also generating NADH and FADH₂

One 18-carbon fatty acid yields approximately 144 ATP — almost 1.5 times the energy from three 6-carbon glucose molecules (3 × 36 = 108 ATP).

Fuel switching: Resting skeletal muscle uses fatty acids. Active skeletal muscle switches to glucose (faster ATP).

Lipid Synthesis (Lipogenesis)

Lipogenesis begins with acetyl-CoA. Almost any organic molecule (carbs, lipids, amino acids) can be converted to acetyl-CoA and used to build lipids. Essential fatty acids (linoleic acid, linolenic acid) cannot be synthesized by human cells and must come from the diet. They are needed for prostaglandin and phospholipid synthesis.

Lipid Transport

Free fatty acids (FFA) bind to albumin in blood. Released from adipose tissue during fasting/starvation.

Lipoproteins = lipid-protein complexes (triglycerides + cholesterol inside a coating of phospholipids + proteins):

Amino Acid Catabolism

The first step is removal of the amino group, requiring a coenzyme derived from vitamin B₆ (pyridoxine). Two pathways:

Three Reasons Protein Catabolism Is Impractical for Quick Energy

Essential vs. Nonessential Amino Acids

Of 20 amino acids, 10 are essential (must come from diet). Eight cannot be synthesized at all (isoleucine, leucine, lysine, threonine, tryptophan, phenylalanine, valine, methionine). Two more (arginine, histidine) can be synthesized but in insufficient amounts for growing children. The remaining 10 are nonessential (synthesized on demand via amination or transamination).

If even ONE essential amino acid is missing, protein synthesis halts completely — all amino acids must be present at the ribosome simultaneously.

DNA vs. RNA Catabolism

• DNA is NEVER catabolized for energy, even during starvation. The genetic information is absolutely essential to cell survival.
• RNA is regularly broken down and replaced. mRNA lasts minutes to hours; rRNA lasts about 5 days; tRNA is also replaced regularly.

Nucleotide Breakdown

When nucleotides are broken down, only sugars, cytosines, and uracils can enter the citric acid cycle for ATP production. Adenine and guanine cannot be catabolized — they are deaminated and excreted as uric acid.

Nitrogenous Wastes

Definition

Nutrition = the absorption of essential nutrients from food. A balanced diet contains all the nutrients needed to maintain homeostasis: adequate energy substrates, essential amino acids and fatty acids, minerals, vitamins, and sufficient water. It prevents malnutrition (unhealthy state from inadequate OR excessive intake of one or more nutrients).

Five Basic Food Groups (MyPlate)

Complete vs. Incomplete Proteins

• Complete proteins contain ALL essential amino acids in sufficient quantities. Sources: beef, fish, poultry, eggs, milk.
• Incomplete proteins lack one or more essential amino acids. Sources: most plants. Vegetarians must carefully combine foods to get all essential amino acids.

The average U.S. diet contains too much sodium and too many calories, with saturated fats providing too great a proportion. This increases incidence of obesity, heart disease, atherosclerosis, hypertension, and diabetes.

Minerals

Minerals are inorganic ions. The body cannot synthesize them — they must come from the diet. Three categories of importance:

Iron is a critical trace mineral — component of hemoglobin (blood O₂ transport), myoglobin (muscle O₂ storage), and cytochromes (ETS electron carriers).

Fat-Soluble Vitamins (A, D, E, K)

Absorbed with dietary lipids. Stored in body fat and liver → reserves can last months. Excess can cause hypervitaminosis (toxicity, especially fat-soluble).

Avitaminosis = vitamin deficiency disease. Rare for fat-soluble vitamins from diet alone (body stores them) but can occur from malabsorption or excessive demand.

Water-Soluble Vitamins (B Complex, C)

Most serve as components of coenzymes. Rapidly exchanged and excreted in urine, so toxicity is uncommon (except with mega-doses).

Intestinal bacteria produce five water-soluble vitamins plus fat-soluble vitamin K.

Water

Daily requirement: ~2500 mL (~40 mL/kg body weight). Sources: food (~48%), drinking (~40%), metabolic water from the ETS (~12%, about 300 mL/day). For each °C above normal body temperature, daily water loss increases by 200 mL.

Units of Energy

A calorie (lowercase) = energy to raise 1 g of water by 1°C. A Calorie (uppercase) = kilocalorie (kcal) = energy to raise 1 kg of water by 1°C. Food labels list Calories (kilocalories).

A calorimeter measures food energy by completely burning a food sample in an oxygen-filled chamber and measuring the temperature rise of surrounding water.

Energy Content by Nutrient

Lipids yield roughly twice the energy per gram as carbohydrates or proteins. This makes lipids the most energy-dense nutrient and explains why excess calories are stored as fat (triglycerides in adipose tissue).

Definitions

• Metabolic rate = the sum of ALL anabolic and catabolic processes at a given time. Varies widely with activity.
• Basal metabolic rate (BMR) = the minimum, resting energy expenditure of an awake, alert person under standardized conditions. Average: ~70 Cal/hour (~1680 Cal/day).

Factors That Influence BMR

Even under standardized conditions, BMR varies based on: age, sex, physical condition, body weight, and genetic differences.

Activity and Energy Expenditure

Sedentary activities add minimal energy demands. One hour of competitive swimming can add 500+ Cal to daily requirements.

Recommended Caloric Proportions

For all ages: carbohydrates 55–60%, fats < 30%, proteins 11–12% of daily caloric intake. These proportions do not change with age — only total caloric amount changes.

Why Thermoregulation Matters

Enzymes operate within a narrow temperature range. Below 36°C (97°F): disorientation. Above 40°C (104°F): disorientation. Above 42°C (108°F): convulsions and permanent cell damage.

Four Mechanisms of Heat Transfer

The Hypothalamus — Body Thermostat

Key Changes

• Caloric requirements decrease by ~10% per decade after age 50. Associated with reduced metabolic rates, body mass, activity levels, and exercise tolerance.
• The recommended proportions of carbs/fats/proteins do NOT change. Only the total caloric amount decreases.

Specific Nutritional Risks in the Elderly