Energy Metabolism: Calories and Metabolic Pathways

Complete guide to energy metabolism - calories, basal metabolic rate, total daily energy expenditure, thermic effect of food, energy balance equations, and metabolic pathways.

Nutrition 2 min read

Energy Balance

Component	Definition	Typical % of TDEE
Basal metabolic rate (BMR)	Energy at rest (life-sustaining)	60-75%
Thermic effect of food (TEF)	Energy to digest/absorb nutrients	5-10%
Physical activity (PA)	Voluntary movement	15-35%
Non-exercise activity thermogenesis (NEAT)	Fidgeting, posture, daily movements	5-15%

Energy In = Energy Out

Side	Components
Energy intake	Food and beverages (calories from carbs, protein, fat, alcohol)
Energy expenditure	BMR + TEF + PA + NEAT

Basal Metabolic Rate

Factor	Effect on BMR
Body size (lean mass)	↑ Lean mass = ↑ BMR (most significant)
Age	↓ 1-2% per decade after age 20
Sex	Male ↑ 5-10% vs female (more lean mass)
Thyroid hormone	Hyperthyroidism ↑, hypothyroidism ↓
Temperature	Heat/cold stress ↑
Fasting/starvation	↓ 15-30% (adaptive thermogenesis)

BMR Estimation (Mifflin-St Jeor)

Sex	Equation
Male	10 × weight(kg) + 6.25 × height(cm) - 5 × age(y) + 5
Female	10 × weight(kg) + 6.25 × height(cm) - 5 × age(y) - 161

Metabolic Pathways

Carbohydrate Metabolism

Pathway	Location	Net ATP	Description
Glycolysis	Cytosol	2 ATP	Glucose → 2 pyruvate
Krebs cycle (TCA)	Mitochondria	2 ATP	Acetyl-CoA → CO₂ + NADH/FADH₂
Electron transport chain	Mitochondria (inner membrane)	34 ATP	NADH/FADH₂ → ATP (oxidative phosphorylation)
Glycogenesis	Cytosol (liver, muscle)	—	Glucose → glycogen
Glycogenolysis	Cytosol (liver, muscle)	—	Glycogen → glucose
Gluconeogenesis	Cytosol (liver)	−6 ATP	Lactate/amino acids → glucose
Pentose phosphate pathway	Cytosol	—	NADPH + ribose production

Fat Metabolism

Pathway	Location	Description
Lipolysis	Adipose, cytosol	Triglycerides → free fatty acids + glycerol
β-oxidation	Mitochondria	Fatty acids → acetyl-CoA (for energy)
Ketogenesis	Liver mitochondria	Excess acetyl-CoA → ketone bodies (acetoacetate, β-hydroxybutyrate, acetone)
Lipogenesis	Liver, adipose cytosol	Excess glucose → fatty acids → triglycerides

Protein Metabolism

Pathway	Location	Description
Proteolysis	Lysosome, proteasome	Proteins → amino acids
Transamination	Cytosol	Amino group transfer
Deamination	Liver	Amino group removal (→ urea via urea cycle)
Ketogenic/glucogenic	Liver	Carbon skeletons → glucose, ketone bodies, or TCA intermediates

Energy Values

Nutrient	Energy Density	TEF
Carbohydrate	4 kcal/g	5-10%
Protein	4 kcal/g	20-30% (highest TEF)
Fat	9 kcal/g	0-3%
Alcohol	7 kcal/g	10-15%

Weight Management

State	Energy Balance	Result
Weight maintenance	Intake = Expenditure	Stable weight
Weight gain	Intake > Expenditure	Fat storage (3500 kcal surplus ≈ 0.45 kg)
Weight loss	Intake < Expenditure	Fat utilization (3500 kcal deficit ≈ 0.45 kg)

Metabolic Rate Measurement

Method	Description	Accuracy
Indirect calorimetry	O₂ consumption, CO₂ production	Gold standard
Doubly labeled water	Isotope dilution over 7-14 days	Research gold standard
Prediction equations	Mifflin-St Jeor, Harris-Benedict	Estimated (±200-300 kcal/day)
Bioelectrical impedance	Body composition estimation	Variable