Energy and Energy Balance: Basal Metabolism, Physical Activity and Requirements
Components of energy expenditure
- Basal metabolic rate (BMR) - energy for involuntary functions measured at rest, awake, after a 12–14 hour fast, in a thermoneutral room. It rises with body size and lean mass, fever, hyperthyroidism, pregnancy and cold stress, and falls with age and starvation.
- Physical activity - the most variable component; expressed as a physical activity level (PAL) multiple of BMR.
- Thermic effect of food - roughly 10% of intake, highest for protein.
Units: 1 kcal = 4.184 kJ. Energy balance means intake equals expenditure; a surplus is stored as fat and a deficit causes loss, initially of glycogen and water, then fat and eventually lean tissue.
Worked example
A 30-year-old woman, 60 kg, 165 cm, lightly active. Mifflin–St Jeor BMR = (10 × 60) + (6.25 × 165) − (5 × 30) − 161 = 600 + 1031.25 − 150 − 161 = 1320 kcal/day. With PAL 1.4, requirement = 1320 × 1.4 ≈ 1848 kcal/day. Reasoning: the equation predicts BMR, PAL converts it to total expenditure, and the female constant (−161) must be used, not the male constant (+5).
Traps that cost marks
BMR and resting metabolic rate are close but not identical; exam answers should use the term the question uses. Do not add the thermic effect of food on top of a PAL figure - it is already embedded in the PAL value. Do not state protein as 9 kcal/g or fat as 4 kcal/g. Do not assume a fixed BMR per kilogram for all adults - age, sex and body composition change it.
Key relationships
Positive energy balance over months → overweight and obesity. Negative balance → weight loss, and in children, growth faltering. Requirements are set as RDA for groups of healthy people, not for sick individuals.