Energy Balance: The Basics
Energy balance describes the relationship between kilojoules (or calories) consumed from food and beverages and kilojoules expended through bodily functions and physical activity:
- Intake ≈ Expenditure: Body weight tends to remain relatively stable.
- Intake > Expenditure: Surplus energy may be stored, potentially contributing to weight gain over time.
- Intake < Expenditure: The body draws on stored energy, which may contribute to weight loss over time.
This model is a simplification. In practice, energy balance is regulated by hormonal signals, metabolic adaptation, genetics, sleep quality, stress and the thermic effect of different foods. Two individuals with identical calorie intakes may respond differently.
Estimated Daily Energy Requirements (Australia)
The following values are derived from Australian Nutrient Reference Values. They represent population-level estimates and should not be used as individual prescriptions.
| Group | Sedentary (kJ/day) | Moderate (kJ/day) | Active (kJ/day) |
|---|---|---|---|
| Men 19–30 yrs | ~10,000 | ~11,500 | ~13,500 |
| Men 31–50 yrs | ~9,700 | ~11,200 | ~13,000 |
| Men 51–70 yrs | ~9,000 | ~10,500 | ~12,200 |
| Women 19–30 yrs | ~7,900 | ~9,100 | ~10,800 |
| Women 31–50 yrs | ~7,600 | ~8,800 | ~10,400 |
| Women 51–70 yrs | ~7,200 | ~8,400 | ~9,800 |
Unit conversion: 1 kilocalorie (kcal) = 4.184 kilojoules (kJ). Australian food labels use kilojoules. For reference, 8,700 kJ ≈ 2,080 kcal.
Macronutrients
The three macronutrients each serve distinct physiological roles. The Acceptable Macronutrient Distribution Ranges (AMDR) recommended for Australian adults are:
| Macronutrient | AMDR (% energy) | kJ per gram | Key Functions |
|---|---|---|---|
| Carbohydrates | 45–65 % | 17 kJ (4 kcal) | Primary energy source; brain fuel; fibre for digestive health |
| Protein | 15–25 % | 17 kJ (4 kcal) | Tissue repair and growth; enzyme and hormone production; immune function |
| Fat | 20–35 % | 37 kJ (9 kcal) | Energy storage; hormone synthesis; absorption of vitamins A, D, E, K |
Estimating Basal Metabolic Rate (BMR)
BMR represents the energy your body requires at complete rest for basic functions such as breathing, circulation and cellular repair. It typically accounts for 60–75 % of total daily energy expenditure.
Mifflin-St Jeor Equation
This is one of the more widely validated estimation methods in current literature:
- Men: BMR (kcal) = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) + 5
- Women: BMR (kcal) = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) − 161
Activity Multipliers (TDEE = BMR × Factor)
| Level | Description | Factor |
|---|---|---|
| Sedentary | Desk job, little or no exercise | 1.2 |
| Lightly active | Light exercise 1–3 days/week | 1.375 |
| Moderately active | Moderate exercise 3–5 days/week | 1.55 |
| Very active | Hard exercise 6–7 days/week | 1.725 |
| Extremely active | Very hard exercise, physical occupation | 1.9 |
Worked Example
A 35-year-old woman, 165 cm tall, weighing 65 kg, who exercises moderately 3–5 days per week:
- BMR = (10 × 65) + (6.25 × 165) − (5 × 35) − 161 = 650 + 1031.25 − 175 − 161 = 1,345 kcal
- TDEE = 1,345 × 1.55 = ~2,085 kcal (≈ 8,720 kJ)
This estimate is a starting point only. It does not account for individual metabolic variation, body composition, medications or medical conditions. For personalised energy planning, consult an accredited practising dietitian.
Practical Guidelines for Balanced Eating
- Prioritise nutrient-dense whole foods: vegetables, fruits, whole grains, lean proteins, legumes, nuts and seeds.
- Use portion awareness rather than rigid counting — paying attention to hunger and fullness cues.
- Include foods from all five food groups as recommended by the Australian Dietary Guidelines.
- Limit ultra-processed foods, added sugars and excessive sodium.
- Eat regularly and mindfully rather than following extreme restriction patterns.