Metabolism & Energy Balance
1️⃣ Human Energy Balance
⚙️ Definition
Energy balance is the relationship between energy intake (calories consumed) and energy expenditure (calories burned).
Positive Energy Balance: intake > expenditure → weight gain
Negative Energy Balance: intake < expenditure → weight loss
Neutral Energy Balance: intake = expenditure → maintenance
⚖️ Energy Intake
Comes from macronutrients:
Carbohydrate = 4 kcal/g
Protein = 4 kcal/g
Fat = 9 kcal/g
Alcohol = 7 kcal/g
Determined by total calories consumed in 24 h.
Excess = stored as fat or glycogen. Deficit = body draws from stored fuel.
🔥 Energy Expenditure Components
ComponentDescriptionApprox. % of totalResting Metabolic Rate (RMR)Energy to maintain vital body functions (heart, breathing, temperature, cellular repair)~ 65–70 %Thermic Effect of Food (TEF)Energy required to digest, absorb, and metabolize food~ 10 %Physical ActivityMovement, exercise, daily activity~ 20–30 %Growth & RepairTissue growth (youth, pregnancy, training adaptation)Variable
🧮 Harris-Benedict Formula (BMR Estimation)
Men: 66.5 + (13.75 × kg) + (5.0 × cm) – (6.75 × age)
Women: 655 + (9.56 × kg) + (1.85 × cm) – (4.68 × age)
💡 Small daily adjustments (±200–300 kcal) can correct long-term imbalance.
2️⃣ Energy Systems of the Body
Energy systems supply ATP (cellular energy) through aerobic or anaerobic processes.
They overlap continuously, with dominance shifting by intensity & duration.
⚡ A. Phosphagen (ATP-PC) System
Fuel: Stored ATP + phosphocreatine
Duration: 0–30 s of max effort (sprint, jump)
Location: Sarcoplasm of muscle cell
Key Enzyme: Creatine kinase → ADP + PC → ATP + creatine
Oxygen Needed: No (anaerobic)
By-product: H⁺ (proton build-up → fatigue)
Purpose: Immediate power, short bursts
🔸 B. Anaerobic Glycolysis (Lactic Acid System)
Fuel: Glucose or glycogen
Duration: ~ 30 s – 2 min (high intensity)
Process: Glucose → 2 pyruvate → lactate (+ 2 ATP)
Key Enzymes: LDH (lactate dehydrogenase), NAD⁺/NADH system
Oxygen: Not required
By-products: Lactate + H⁺ (acid accumulation → fatigue)
Purpose: Short-term energy without oxygen
🔹 C. Aerobic System (Oxidative Phosphorylation)
Fuel: Carbohydrates, fats, lactate, ketones
Duration: > 2 min to hours (low to moderate intensity)
Location: Mitochondria
Stages:
Glycolysis → pyruvate
Krebs Cycle → CO₂, NADH, FADH₂
Electron Transport Chain → O₂ → H₂O + ~ 32 ATP
ATP Yield: Glucose ≈ 30–32 ATP | Fatty acid ≈ 106 ATP
By-products: CO₂ + H₂O (less fatigue than anaerobic systems)
Purpose: Sustained energy production
🏃♂️ All systems work together — phosphagen dominates first seconds, glycolysis supports short bursts, aerobic powers long-term work.
3️⃣ Body’s Energy Currency — ATP
💥 Adenosine Triphosphate (ATP)
The universal energy molecule for all cellular work.
Made of adenine + ribose + 3 phosphate groups.
Energy is released when the bond between the 2nd and 3rd phosphate breaks.
🔄 ATP–ADP Cycle
StepProcessEnzymeDescription1ATP → ADP + Pi + energyATPaseDephosphorylation (releases energy)2ADP + Pi + energy → ATPATP synthaseRephosphorylation (recharges ATP)
Water (H₂O) is essential for ATP hydrolysis.
Hydrogen ions (H⁺) are released → excess = metabolic acidosis → fatigue.
ATP turnover rate increases with training & hydration.
⚙️ ATP Production Summary
System Oxygen Speed Duration ATP YieldPhosphagenNoFastest0–30 s1 ATP/PC Anaerobic GlycolysisNoFast30 s–2 min2 ATP/glucose Aerobic OxidativeYesSlowest2 min–hrs30–106 ATP (depends on fuel)
⚖️ Coach’s Key Takeaways
Energy balance = foundation of metabolism: calorie in vs calorie out.
RMR is the largest portion of energy expenditure — protect it with lean mass.
ATP is the body’s energy currency, continuously recycled via the ATP-ADP cycle.
Anaerobic systems fuel short bursts; aerobic system sustains endurance.
Training and nutrition directly affect how efficiently the body produces & uses ATP.
Hydration is critical — water enables ATP hydrolysis and energy release.
