Nutrition, Metabolism & Energy Balance
Nutrients, cellular respiration, absorptive and postabsorptive states, and body temperature
By the end of this unit you can…
- ✓Describe the dietary sources and uses of carbohydrates, lipids, proteins, vitamins and minerals
- ✓Distinguish catabolism and anabolism and explain oxidation–reduction and phosphorylation
- ✓Describe glycolysis, pyruvate oxidation, the Krebs cycle and the electron transport chain
- ✓Explain glycogenesis, glycogenolysis, gluconeogenesis, lipolysis, lipogenesis and ketogenesis
- ✓Compare the absorptive and postabsorptive states and their hormonal control
- ✓Describe the liver's metabolic roles, cholesterol transport, energy balance and thermoregulation
Key terms
Practice →1Nutrients
A nutrient is a substance in food that the body uses for growth, maintenance and repair. The major nutrients are carbohydrates, lipids and proteins (needed in large amounts); vitamins and minerals are needed in small amounts; water is the most important nutrient by volume. Essential nutrients can't be made by the body in adequate amounts and must come from the diet.
| Nutrient | Main sources | Main uses |
|---|---|---|
| Carbohydrates | Starches, sugars (grains, fruits, vegetables, milk) | Glucose is the main fuel — neurons and RBCs rely almost entirely on it. Fiber aids elimination |
| Lipids | Animal fats, oils; essential fatty acids linoleic and linolenic (omega-6, omega-3) | Concentrated energy (9 kcal/g); membranes (phospholipids); cholesterol → steroids, bile salts, vitamin D; absorbing fat-soluble vitamins |
| Proteins | Meat, eggs, milk (complete — all essential amino acids); legumes, grains (incomplete) | Structural materials, enzymes, hormones, antibodies; ~0.8 g/kg body weight/day |
| Vitamins | Varied | Mostly coenzymes. Fat-soluble (A, D, E, K — stored, can become toxic) vs water-soluble (B complex, C — excess excreted) |
| Minerals | Varied | Ca, P (bone), Na, K, Cl (fluid balance & nerve function), Fe (hemoglobin), I (thyroid hormone) |
Carbohydrates and proteins yield 4 kcal/g; fats 9 kcal/g. Nitrogen balance: protein intake = protein used. Positive in growth and pregnancy; negative in starvation and severe stress.
2Metabolism overview
Metabolism = all chemical reactions in the body. Anabolism builds larger molecules (needs energy); catabolism breaks them down (releases energy). Energy released by catabolism is captured as ATP.
- Oxidation–reduction
- Glucose is oxidized (loses electrons/H atoms); coenzymes are reduced (gain them). The key carriers are NAD⁺ → NADH and FAD → FADH₂, which carry high-energy electrons to the electron transport chain.
- Substrate-level phosphorylation
- A phosphate group is transferred directly from a metabolic intermediate to ADP (glycolysis, Krebs cycle).
- Oxidative phosphorylation
- Energy from electrons moving down the electron transport chain pumps H⁺; ATP synthase uses the H⁺ gradient to make ATP (chemiosmosis). Makes most ATP.
Cellular respiration: C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + ~32 ATP (+ heat). Three stages: glycolysis (cytosol), Krebs cycle (mitochondrial matrix) and the electron transport chain (inner mitochondrial membrane).
3Glucose oxidation
- 1Glucose is phosphorylated using 2 ATP (energy investment phase) and split into two 3-carbon molecules.
- 2These are oxidized: NAD⁺ is reduced to 2 NADH.
- 34 ATP are made by substrate-level phosphorylation → net 2 ATP.
- 4End product: 2 pyruvic acid (pyruvate) molecules.
Fate of pyruvate: with O₂, it enters the mitochondria. Without enough O₂, pyruvate is reduced to lactic acid (regenerating NAD⁺ so glycolysis can continue) — fast but inefficient. The liver later converts lactic acid back to pyruvate/glucose.
- 1Each pyruvate loses a CO₂ (decarboxylation), is oxidized (NAD⁺ → NADH) and joins coenzyme A → acetyl CoA.
- 2Acetyl CoA (2C) combines with oxaloacetic acid (4C) → citric acid (6C).
- 3Over eight steps, citric acid is oxidized and decarboxylated back to oxaloacetic acid.
- 4Per acetyl CoA: 2 CO₂, 3 NADH, 1 FADH₂, 1 ATP (via GTP). The cycle turns twice per glucose.
- 1NADH and FADH₂ deliver H atoms; electrons pass along protein complexes (cytochromes).
- 2Their energy pumps H⁺ into the intermembrane space, creating an electrochemical gradient.
- 3O₂ is the final electron acceptor, combining with electrons and H⁺ to form water.
- 4H⁺ flows back through ATP synthase, driving phosphorylation of ADP → most of the cell's ATP (~28 of 32).
| Stage | Location | Net ATP per glucose |
|---|---|---|
| Glycolysis | Cytosol | 2 |
| Krebs cycle | Mitochondrial matrix | 2 |
| Electron transport chain + chemiosmosis | Inner mitochondrial membrane | ~28 |
| Total | ~32 |
4Glycogen, fat & protein metabolism
| Process | What happens | When |
|---|---|---|
| Glycogenesis | Glucose → glycogen (liver, skeletal muscle) | After meals (high glucose, insulin) |
| Glycogenolysis | Glycogen → glucose | Between meals (glucagon, epinephrine) |
| Gluconeogenesis | New glucose from non-carbohydrates (glycerol, amino acids, lactic acid) — in the liver | Fasting; protects the brain's glucose supply |
| Lipogenesis | Glucose or amino acids → triglycerides (fat storage) | Excess calories |
| Lipolysis | Triglycerides → glycerol + fatty acids; fatty acids undergo β-oxidation → acetyl CoA → Krebs cycle | Fasting, exercise |
| Ketogenesis | Liver converts excess acetyl CoA → ketone bodies | When carbohydrate is unavailable (fasting, uncontrolled diabetes, low-carb diets) |
Protein metabolism: amino acids are used mainly for building proteins. When used for energy, they are deaminated — the amine group is removed as ammonia (NH₃), which is toxic. The liver converts ammonia to urea (urea cycle), excreted by the kidneys. Transamination moves amine groups between molecules to make nonessential amino acids.
5Absorptive & postabsorptive states
Absorptive (fed) state — ~4 hours after a meal: nutrients flood the blood; anabolism and storage dominate. Glucose is the main fuel; excess is stored as glycogen and fat; amino acids build proteins. Controlled mainly by insulin.
Postabsorptive (fasting) state — between meals: the goal is to keep blood glucose stable (70–110 mg/dL) for the brain. Sources: liver glycogenolysis, lipolysis, then gluconeogenesis. Other tissues switch to fatty acids (glucose-sparing). Controlled by glucagon, the sympathetic nervous system and epinephrine (also cortisol and GH).
| Absorptive | Postabsorptive | |
|---|---|---|
| Main hormone | Insulin | Glucagon (+ epinephrine) |
| Overall | Storage (anabolism) | Mobilization (catabolism) |
| Liver | Glycogenesis, lipogenesis | Glycogenolysis, gluconeogenesis, ketogenesis |
| Fuel for most cells | Glucose | Fatty acids (brain still uses glucose; ketones in prolonged fasting) |
6The liver & cholesterol
The liver is the body's main metabolic organ: it processes nearly every class of nutrient, makes plasma proteins, stores glycogen, iron and vitamins (A, D, B₁₂), converts ammonia to urea, detoxifies drugs and alcohol, and makes bile.
| Lipoprotein | Carries | Clinical meaning |
|---|---|---|
| LDL (low-density) | Cholesterol to tissues | “Bad” — high levels deposit cholesterol in artery walls (atherosclerosis) |
| HDL (high-density) | Excess cholesterol from tissues to the liver for disposal | “Good” — high levels are protective |
| VLDL | Triglycerides from the liver to adipose tissue | — |
| Chylomicrons | Dietary lipids from intestine (via lymph) | — |
7Energy balance & body temperature
Energy balance: energy intake (food) = energy output (heat + work + storage). The basal metabolic rate (BMR) — energy needed at rest just to stay alive — is increased by larger surface area, being male, youth, thyroid hormone (the main regulator), fever and stress. Total metabolic rate adds activity (muscle work is the biggest variable).
Food intake is regulated by the hypothalamus: leptin (from fat) and insulin, CCK, PYY signal satiety; ghrelin (from the stomach) signals hunger.
Core body temperature is kept near 37 °C (35.6–37.8 °C) by the hypothalamus. Heat is exchanged by radiation, conduction, convection and evaporation.
| Too cold → heat-promoting | Too hot → heat-loss |
|---|---|
| Constriction of cutaneous blood vessels | Dilation of cutaneous blood vessels |
| Shivering (involuntary muscle contraction) | Sweating (evaporative cooling) |
| ↑ metabolic rate (epinephrine, NE; thyroid hormone over time) | Behavioral: shade, less clothing |