Describe energy balance (meaning and components of energy expenditure)

Energy expenditure has three main components that together determine the total energy the body uses each day.

1. Basal Metabolic Rate (BMR)

BMR is the minimum amount of energy the body expends while at rest to maintain basic life functions. It accounts for 60–70% of total daily energy expenditure and is the largest component. BMR supports essential processes including heart function, brain activity, nerve transmission, cell production, and temperature regulation.

BMR is influenced by several factors:

• Age: Metabolism is highest during growth phases (infancy, puberty) and slows with age
• Sex: Males typically have higher BMR than females due to greater muscle mass
• Body size: Larger bodies require more energy to maintain basic functions
• Body composition: More muscle tissue increases BMR; more fat tissue decreases it
• Genetics: Inherited traits affect metabolic efficiency
• Physical activity: Regular exercise increases muscle mass, raising BMR
• Hormonal changes: Thyroid hormones, insulin, cortisol, and adrenaline affect metabolic rate
• Weather conditions: Cold weather increases BMR for thermogenesis; hot weather may slightly decrease it

2. Physical Activity

Physical activity encompasses all body movements, from light everyday tasks to vigorous exercise. It is the most variable component of energy expenditure and can be adjusted through conscious choice.

• Light activities: Walking, household chores, gardening, leisurely cycling (approximately 200–300 kcal/hour)
• Heavy/vigorous activities: Running, swimming, aerobics, strength training (approximately 400–800 kcal/hour)

The energy cost of activity depends on body weight, activity intensity, duration, and individual metabolism.

3. Thermic Effect of Food (TEF)

TEF (also called Specific Dynamic Effect or Diet-Induced Thermogenesis) is the energy the body uses to digest, absorb, metabolize, and utilize nutrients from food. It typically represents about 10% of total daily energy expenditure.

The thermic effect varies by macronutrient:

• Protein: 20–30% of calories from protein are used in digestion and metabolism
• Carbohydrates: 5–10% of calories from carbohydrates are used
• Fats: 0–3% of calories from fats are used

Factors affecting TEF include the form of food (refined vs. unrefined), meal composition, and meal size.

A male college student aged 32 years weighs 65 kg. His activities for one day are summarized below (time in minutes, energy in kcal):

Step 1: Calculate Total Energy Cost for Activities (TECA)

Using the formula: Energy cost = Time (min) × Energy (kcal) × Body weight (kg)

• Sweeping: 15 × 0.053 × 65 = 51.68 kcal
• Walking normal: 30 × 0.069 × 65 = 134.55 kcal
• Class work: 450 × 0.0245 × 65 = 716.63 kcal
• Sleeping: 480 × 0.0172 × 65 = 536.64 kcal

TECA = 1,439.50 kcal

Step 2: Calculate Basal Metabolism (BM)

Using the formula: BM = 70 × W^0.75^

BM = 70 × 65^0.75^ = 70 × 22.89 = 1,602.3 kcal

Step 3: Calculate Specific Dynamic Effect (SDE)

SDE = 10% × (BM + TECA)

SDE = 0.10 × (1,602.3 + 1,439.50) = 304.18 kcal

Step 4: Calculate Total Energy Expenditure (TEE)

TEE = BM + TECA + SDE

TEE = 1,602.3 + 1,439.50 + 304.18 = 3,345.98 kcal

This student needs approximately 3,346 kcal per day to maintain his weight. If he consumes more than this, he will gain weight; if he consumes less, he will lose weight.

In Tanzania, understanding energy balance helps families plan meals wisely. For example, a market vendor in Dar es Salaam who walks and stands for many hours selling vegetables needs more energy than someone with a sedentary office job. If she consumes the same amount of rice and beans as her sitting colleague without adjusting for her higher activity level, she may become underweight over time. Conversely, a taxi driver who sits most of the day but eats large portions of fried foods may gain weight and risk developing conditions like diabetes or hypertension, which are increasingly common in urban Tanzania. Knowing one's energy needs helps in budgeting food expenses while maintaining good health.