Metabolic Rate and Basal Metabolism

Metabolic Rate and Basal Metabolism - Energy Essentials

• Metabolic Rate (MR): Energy liberated/unit time (kcal/hr or kcal/day).
• Basal Metabolic Rate (BMR): Min. energy for vital functions at rest.
  • Conditions: Awake, 12-18h fast, supine, thermoneutral (20-25°C).
  • $1$ L O₂ consumed ≈ $4.8$ kcal.
• Resting Metabolic Rate (RMR): Less strict; ~10% > BMR.
• Factors ↑ BMR: ↑Surface area, thyroxine, adrenaline, fever, cold climate, pregnancy.
• SDA (Specific Dynamic Action): Post-meal heat; Proteins ~30%.
• BMR values: Males ≈ $1$ kcal/kg/hr; Females ≈ $0.9$ kcal/kg/hr.

⭐ Average BMR differs by sex, being generally higher in males than females for adults an

Metabolic Rate and Basal Metabolism - Baseline Burn

• Metabolic Rate (MR): Total energy expenditure per unit time.
• Basal Metabolic Rate (BMR): Minimum energy for vital functions at rest.
  • Constitutes 60-75% of daily energy expenditure.
  • 📌 Conditions (REST): Rested (awake, physical/mental), Environment (thermoneutral 20-25°C), State (post-absorptive 12-18h), Temperature (afebrile).
• Factors ↑ BMR: Thyroxine, fever (13% per 1°C), male, ↑BSA, cold, pregnancy.
• Factors ↓ BMR: Age, starvation, female.

⭐ BMR is lowest in the morning after a good night's sleep and 12-18 hours of fasting. oka

Metabolic Rate and Basal Metabolism - Rate Shifters

Basal Metabolic Rate (BMR) is dynamic, influenced by:

⭐ Specific Dynamic Action (SDA) or Thermic Effect of Food (TEF) is highest for proteins (20-30% of ingested energy), then carbohydrates (5-10%), then fats (0-3%).

Metabolic Rate and Basal Metabolism - Calorie Counters & Diet Impact

• Direct Calorimetry: Measures total heat produced by the body in a calorimeter.

• Indirect Calorimetry: Calculates energy expenditure from $VO_2$ & $VCO_2$.

  • Commonly used: Douglas bag, Benedict-Roth spirometer (measures $VO_2$).

• Respiratory Quotient (RQ)

  • Ratio of $CO_2$ produced ($VCO_2$) to $O_2$ consumed ($VO_2$): $RQ = VCO_2 / VO_2$.
  • Indicates primary metabolic fuel utilized.
  • RQ Values:

    Substrate	RQ
    Carbohydrates	1.0
    Proteins	0.8
    Fats	0.7
    Mixed Diet	~0.85
    Alcohol	0.67

  📌 Mnemonic: "Clean (CHO) 1.0 Plate (Protein) 0.8 For (Fat) 0.7 All (Alcohol) 0.67."

• Specific Dynamic Action (SDA) / Thermic Effect of Food (TEF)

  • ↑ Increased metabolic rate (heat production) following food ingestion.
  • Energy cost for digestion, absorption, processing, storage of nutrients.
  • SDA Percentages (of ingested calorie value):

    Macronutrient	SDA (%)
    Proteins	20-30%
    Carbohydrates	5-10%
    Fats	0-5%
    Mixed Diet	~10%

⭐ RQ > 1.0 indicates net lipogenesis (e.g., from excess carbohydrate intake) or overfeeding; RQ < 0.7 can indicate ketogenesis or prolonged fasting.

High‑Yield Points - ⚡ Biggest Takeaways

• Basal Metabolic Rate (BMR): Minimum energy expenditure at rest, post-absorptive, thermoneutral.
• Thyroid hormones (T3/T4) are key BMR regulators; ↑ T3/T4 significantly ↑ BMR.
• BMR is proportional to Body Surface Area (BSA), not solely body weight.
• Specific Dynamic Action (SDA) is highest for proteins (energy cost of food processing).
• Respiratory Quotient (RQ): Carbohydrates = 1.0; Fats ≈ 0.7; Proteins ≈ 0.8.
• Starvation: ↓ BMR; brain adapts to use ketone bodies for energy.
• BMR typically measured by indirect calorimetry (O2 consumption).