The caloric intake of an individual must be equal to the calories expended over a given period of time if body weight is to remain constant. This state is called energy balance.

• When caloric intake exceeds expenditure → positive energy balance → weight gain
• When caloric intake is less than expenditure → negative energy balance → weight loss

Thus, calculation of caloric requirement is essentially the estimation of total energy expenditure of the body.

  Components of Total Energy Expenditure

There are four primary needs which have to be met by food:

1. Basal Metabolism (Basal Metabolic Rate – BMR)

These are calories utilized by an individual who is at complete physical and mental rest, in a fasting state, and in a thermoneutral environment. These calories are required for:

• Functioning of vital organs (brain, heart, liver, kidneys)
• Maintenance of body temperature
• Maintenance of ionic gradients and cellular activity

An adult utilizes approximately 1200–1800 kcal in 24 hours under basal conditions.

Factors Affecting BMR

BMR is not constant and is influenced by:

• Body composition: Lean body mass increases BMR
• Age: Higher in children, decreases with age
• Sex: Lower in females (due to relatively lower muscle mass)
• Hormones:
• Thyroid hormones increase BMR
• Catecholamines increase BMR
• Fever: Each 1°C rise increases BMR by ~10–13%
• Climate: Cold exposure increases heat production
• Nutritional status: Starvation decreases BMR

BMR accounts for about 60–70% of total daily energy expenditure.

2. Muscular Activity (Physical Activity Energy Expenditure)

The calories utilized for muscular activity vary widely depending on the type, intensity, and duration of activity.

• Sedentary individuals: low energy expenditure
• Moderate workers: moderate increase
• Heavy laborers/athletes: marked increase (Athletes: may require 3000–5000 kcal/day or more)

Physical activity is the most variable component of energy expenditure and includes:

• Exercise (planned activity)
• Non-exercise activity thermogenesis (NEAT) such as walking, standing, fidgeting

Mental activity does not significantly increase caloric expenditure, although it may increase perceived fatigue.

Important Concept: NEAT

Energy expenditure includes not only exercise but also Non-Exercise Activity Thermogenesis (NEAT):

• Walking
• Standing
• Routine daily activities
• Even small movements like fidgeting

This component is highly variable and explains differences between individuals with similar body weight.

3. Growth, Pregnancy, Lactation, and Tissue Repair

These conditions are associated with a positive energy balance, meaning energy intake must exceed expenditure.

• Growth: Energy for synthesis of new tissues
• Pregnancy: ~+300 kcal/day
• Lactation: ~+450–500 kcal/day
• Convalescence after illness also increases energy needs due to tissue repair.

4. Specific Dynamic Action of Food (Thermic Effect of Food – TEF)

Ingestion of food increases metabolic rate due to processes such as digestion, absorption, transport, and metabolism.

• Proteins: 20–30%
• Carbohydrates: 5–10%
• Fats: 0–3%

For a mixed diet, TEF is approximately 10% of total caloric intake.

This energy is released as heat and is not available for mechanical work.

5. Adaptive Thermogenesis

In addition to the above, modern physiology recognizes adaptive thermogenesis, which refers to changes in energy expenditure in response to:

• Cold exposure (increased heat production)
• Overfeeding
• Starvation
• Hormonal changes

This adjustment helps maintain energy balance but may oppose weight loss efforts.

Total Daily Energy Expenditure (TDEE)

The total caloric requirement of a person is given by:

TDEE = BMR + Physical Activity + TEF + Adaptive Thermogenesis

This provides a complete understanding of energy needs.

  Methods for Calculating Caloric Requirement

1. Traditional Method (Based on Body Weight and Tables)

As given in the table below.

2. Equation-Based Method (More Accurate)

BMR can be estimated using predictive equations such as the Harris–Benedict or Mifflin–St Jeor equations, and then multiplied by an activity factor to estimate total energy needs.

Mifflin–St Jeor Equation (more commonly used today):

• Men:
  10W + 6.25H − 5A + 5
• Women:
  10W + 6.25H − 5A − 161

Harris–Benedict Equation (older but still used):

• Men:
  66.5 + (13.75 × W) + (5.003 × H) − (6.755 × A)
• Women:
  655.1 + (9.563 × W) + (1.850 × H) − (4.676 × A)

Where:
W = weight (kg), H = height (cm), A = age (years)

After calculating BMR, Total Daily Energy Expenditure (TDEE) is estimated by multiplying BMR with an activity factor:

Activity Level	Factor
Sedentary	1.2
Light Activity	1.375
Moderate	1.55
Heavy	1.725

  Energy Yield of Nutrients

Different nutrients provide different amounts of energy:

• Carbohydrates: 4 kcal/g
• Proteins: 4 kcal/g
• Fats: 9 kcal/g
• Alcohol: 7 kcal/g

👉 This is essential for calculating caloric intake from diet.

  Significance of Thermic Effect of Food

The thermic effect must be considered in caloric calculations.

Example:

If energy requirement = 2000 kcal/day
With TEF ≈ 10%:
Required intake ≈ 2000 / 0.90 = 2222 kcal/day
If derived entirely from protein:
Required intake ≈ 2000 / 0.70 = 2857 kcal/day

  Mechanism of Thermic Effect

The thermic effect is due to:

• Energy required for digestion and absorption
• Metabolic processing of nutrients
• Oxidative deamination of amino acids

  Additional Important Concepts

1. Energy Storage in the Body

Excess energy is stored as:

• Glycogen (limited storage in liver and muscle)
• Fat (triglycerides) (major energy reserve)

👉 Approximately 7700 kcal ≈ 1 kg body fat

2. Respiratory Quotient (RQ)

RQ is the ratio of CO₂ produced to O₂ consumed and gives an idea about the type of nutrient being oxidized:

• Carbohydrates: RQ = 1.0
• Fats: RQ ≈ 0.7
• Proteins: RQ ≈ 0.8

👉 Useful in metabolic studies and clinical nutrition.

3. Recommended Dietary Allowance (RDA) vs Requirement

• Requirement: Minimum amount needed for normal function
• RDA: Intake sufficient for nearly all healthy individuals

👉 RDA is always slightly higher than requirement.

4. Body Composition and Caloric Needs

Two individuals of the same weight may have different caloric needs because:

• Muscle tissue is metabolically more active than fat
• Higher muscle mass → higher caloric requirement

  Caloric Requirements by Age and Sex (Approximate Values)

(Note: The following table reflects older standard estimates and should be interpreted cautiously; modern guidelines use individualized calculations based on BMR equations and activity levels.)

Notes

1. Kg = Body weight in kilograms
2. Pregnant and lactating women should consume approximately +300 kcal/day (pregnancy) and +450–500 kcal/day (lactation) in addition to usual requirements (updated values).
3. These requirements are meant for individuals engaged in moderate physical activity.

Conclusions Drawn from Above Table

• Caloric requirement increases during growth and peaks in adolescence
• Declines with age due to reduced BMR and activity
• Males generally require more energy than females due to greater lean body mass
• Children require more calories per kg body weight

Caloric Needs in Children

Children require more calories per kg body weight because:

• Higher basal metabolic rate
• Energy required for growth
• Greater physical activity

  Final Integrated Understanding

Thus, the caloric requirement of a person is determined by a combination of:

• Basal metabolic needs
• Physical activity
• Thermic effect of food
• Physiological state (growth, pregnancy, illness)
• Environmental and hormonal influences

A proper understanding of these factors allows accurate estimation of energy needs for health, disease, and nutrition planning.