Study Notes on Human Energy Expenditure During Rest and Physical Activity

Chapter 9: Human Energy Expenditure During Rest and Physical Activity

Metabolism

• Definition: Metabolism encompasses all the life-sustaining chemical reactions in organisms that include:   - Conversion of Energy: Energy in food is converted to energy available for cellular processes, enabling the organism to carry out vital functions and activities.   - Biosynthesis: Metabolism allows the conversion of food into building blocks for proteins, lipids, and carbohydrates in the body.   - Waste Elimination: It is also responsible for the elimination of metabolic waste products.

Components of Metabolism

• Anabolism (Synthesis):
    - Set of metabolic pathways that construct molecules from smaller units (building up).

• Catabolism (Breakdown):
    - Set of metabolic pathways that breakdown molecules into smaller units (breaking down), such as carbohydrates (CHO).

Total Daily Energy Expenditure (TDEE)

• Definition: Total Daily Energy Expenditure (TDEE) is the total amount of energy expenditure over a 24-hour period.

• Components of TDEE:   - Resting Metabolic Rate (RMR): ~60-75% of TDEE   - Thermogenic Effect of Food (TEF): ~10%   - Energy Expended in Physical Activity: ~15-30%

Components of Total Daily Energy Expenditure

• TDEE or Metabolic Rate depends on three factors:   - Basal Metabolic Rate (BMR): ~60-75%   - Physical Activity: ~20%   - Thermic Effect of Food: ~10%

Resting Metabolic Rate vs. Basal Metabolic Rate

• Basal Metabolic Rate (BMR):   - Defined as the minimum level of energy required to sustain essential vital functions within a 24-hour span.   - Reflects the body’s total energy expenditure (EE) at rest, measured under specific conditions: supine position, thermoneutral environment, ≥ 8 hours of sleep, and ≥ 12 hours of fasting.   - Oxygen Uptake for BMR: $3.5 ext{ mL O}_2 / ext{kg} / ext{min}$
    - Factors affecting BMR include biological sex, body size, muscle mass, age, fitness status, hormonal status, and body temperature.

• Resting Metabolic Rate (RMR):   - It is the most commonly measured metabolic rate, similar to BMR (within 5-10%) but measured under less rigorous conditions.

BMR and RMR: Age and Biological Sex Effects

• Females generally have a 5-10% lower RMR than males of the same age, due to:   - A decrease in fat-free mass (muscle) and an increase in body fat percentage, which lowers metabolic activity.

• BMR decreases approximately 2-3% per decade in adults due to changes in body composition (decrease in fat-free mass, increase in body fat, and decrease in physical activity).

BMR and RMR: Effect of Physical Activity (PA) or “Training”

• Regular endurance and resistance exercise can counteract the decrease in RMR that comes with aging:   - An 8-week endurance training program can increase RMR by about 10% in older men.   - Older men performing resistance training showed an 8% increase in RMR.   - Each 1-pound gain in fat-free mass (FFM) is associated with an increase in RMR of approximately 7 to 10 kcals daily.

Estimating BMR from Harris Benedict Equation

• The Harris-Benedict Equation, established in 1919, is the standard method for estimating BMR and considers body mass, stature, age, and sex.

Estimation of Total Daily Energy Expenditure (TDEE)

• To estimate TDEE, multiply the estimated BMR by a physical activity coefficient:   - Physical Activity Coefficients:     - 1.2 = Sedentary (little or no exercise, desk job)     - 1.375 = Lightly active (light exercise/sports 1-3 days/week)     - 1.55 = Moderately active (moderate exercise/sports 3-5 days/week)     - 1.725 = Heavy exercise (hard exercise/sports 6-7 days/week)

Physical Activity (PA)

• PA contributes to 15% - 30% of TDEE.

• It has a profound effect on human energy expenditure:   - For a TDEE of 3,000 kcal/day, this equates to an energy expenditure during PA of approximately 450-900 kcals.   - During continuous exercise, metabolic rates can increase up to 10 times.   - World-class athletes can potentially double their TDEE through 3 hours of intense training.

Thermic Effect of Food (TEF)

• TEF refers to the energy required for the following processes:   1. Digestion   2. Absorption   3. Transportation   4. Assimilation of foods

• It varies between 7-10% of TDEE and reaches its maximum within approximately 1 hour following a meal.

• Factors affecting TEF include meal size and macronutrient composition, with a pure protein meal potentially increasing TEF to about 25% of the meal's caloric value.

Environmental Factors and Energy Expenditure

• Hot Weather Effects: Exercise in hot conditions can lead to about 5% higher energy expenditure due to:   - The thermogenic effect of elevated core temperature.   - Energy expenditure for sweat gland activity.   - Altered circulatory dynamics.

• Cold Weather Effects: Cold environments can enhance energy metabolism during rest and exercise, influenced by:   - Body fat content and effectiveness of clothing.   - Metabolic rates can increase up to 5 times at rest during extreme cold stress, with shivering enhancing body heat to maintain core temperature.

Effect of Body Mass on Energy Expenditure

• Increases in body mass lead to heightened energy expenditure during various activities, particularly in weight-bearing exercises:   - For weight-supported exercise, the cost of energy decreases significantly.   - For overweight individuals, weight-bearing activities can lead to substantial caloric expenditures due to the increased energy cost of transporting a heavier body weight.

Practical Measurement of Energy Expenditure

METs (Metabolic Equivalent of a Task)

• Definition: A MET is a physiological measure that expresses the energy cost of physical activities. One MET equals the resting oxygen consumption of 3.5 mL O2·kg·min.

• Physical activity measures:   - At 2 METs, it requires twice the resting metabolism, and similarly, 3 METs equates to three times resting metabolism.

• Conversions:   - $1 ext{ MET} = 1 ext{ kcal} imes ext{kg} imes ext{hr}$

• Examples:   - 2 METs = 7.0 mL O2·kg·min; 3 METs = 10.5 mL O2·kg·min.

Caloric Expenditure Calculation Using METs

• Example 1:   - If an individual performs an activity at 3 METs for 1 hour weighing 220 pounds:     - Conversion from pounds to kilograms: $220 ext{ pounds} / 2.2 = 100 ext{ kg}$.     - Calculation: $3 ext{ METs} = 3 ext{ kcal} imes (100 ext{ kg}) imes 1 ext{ hr} = 300 ext{ kcals}$.

• Example 2:   - If an individual performs an activity at 6 METs for 30 minutes weighing 110 pounds:     - Weight conversion: $110 ext{ pounds} / 2.2 = 50 ext{ kg}$.     - Calculation: $6 ext{ METs} = 6 ext{ kcal} imes (50 ext{ kg}) imes 0.5 ext{ hr} = 150 ext{ kcals}$.

• Example 3:   - Performing an activity at 9 METs for 15 minutes weighing 150 pounds:     - Weight conversion: $150 ext{ pounds} / 2.2 = 68.2 ext{ kg}$.     - Calculation: $9 ext{ METs} = 9 ext{ kcal} imes (68.2 ext{ kg}) imes 0.25 ext{ hr} = 153.5 ext{ kcals}$.

Using METs to Classify Exercise Intensity

• Classification of exercise intensity based on METs:   - < 3 METs: Light intensity   - 3 - 5.9 METs: Moderate intensity   - ≥ 6 METs: Vigorous intensity

TABLE 1.1: Metabolic Equivalents (METs) Values of Common Physical Activities

• Light Activities (<3 METs):   - Walking at home/store/office: 1.5 METs   - Sitting while using a computer: 1.5 METs

• Moderate Activities (3-<6 METs):   - Walking slowly: 2.0 METs   - Sweeping floors: 3.0 METs   - Cleaning windows: 3.0 METs

• Vigorous Activities (≥6 METs):   - Jogging at 5 mi/h: 8.0 METs   - Basketball: 8.0 METs   - Stair climbing: 8.5 METs\n

• Summary of METs Values in Different Categories:
    - The table provides a detailed overview of various physical activities classified by their metabolic equivalence and is pivotal for understanding energy expenditure during different types of activity.