21 Energy Expenditure
Energy expenditure is the rate at which the body converts the chemical energy of food (carbohydrates, fats and proteins) into work and heat. Almost all of this energy is released by oxidation in the mitochondria, where it is stored temporarily in the high-energy phosphate bonds of ATP. Energy expenditure can be expressed per day (kcal/day), or per kilogram of body weight per unit of time, which allows people of different size to be compared. For example, an adult of 70 kg who uses about 2300 kcal per day spends on average about 0.023 kcal/kg/min.
Components
- Basal / resting metabolism: about 60–70% of the daily total in sedentary people
- Thermic effect of food: about 10%
- Physical activity: the most variable part, from about 15% to more than 40%
Metabolic equivalents (METs)
The intensity of activity is often expressed in METs, multiples of resting oxygen consumption (1 MET = 3.5 mL O₂/kg/min). Quiet sitting is about 1 MET, walking at a normal pace 3–4 METs, jogging 7–8 METs, and fast running more than 10 METs. Exercise capacity below about 5 METs on a stress test is associated with worse prognosis in heart disease.
Energy (kcal) ≈ METs × body weight (kg) × time (h)
How it is measured
The reference methods are indirect calorimetry (oxygen consumption and carbon dioxide production, converted with the Weir equation) and, for free-living total daily expenditure, the doubly labelled water method. Wearable devices and heart-rate-based estimates are convenient but less accurate.
If intake exceeds expenditure over time, the excess is stored mainly as fat; if expenditure exceeds intake, body tissues are used. Increased expenditure without weight gain is typical of fever, hyperthyroidism, infection, burns and trauma; reduced expenditure occurs with hypothyroidism, inactivity and prolonged undernutrition.
References
- FAO/WHO/UNU. Human energy requirements. Report of a Joint FAO/WHO/UNU Expert Consultation. FAO Food and Nutrition Technical Report Series 1. Rome: FAO; 2004. fao.org/3/y5686e
- Jetté M, Sidney K, Blümchen G. Metabolic equivalents (METS) in exercise testing, exercise prescription, and evaluation of functional capacity. Clin Cardiol. 1990;13(8):555–565. doi:10.1002/clc.4960130809
- Weir JB de V. New methods for calculating metabolic rate with special reference to protein metabolism. J Physiol. 1949;109(1–2):1–9. doi:10.1113/jphysiol.1949.sp004363
This article is for general information and does not replace a consultation with a doctor. Measurements must be interpreted by a physician together with symptoms and other tests.
Published on 13 May 2024