Resting Metabolic Rate and Thermic Effect of Food

Resting metabolic rate is the energy the body expends at rest to sustain basic physiological functions, and it is the largest single component of total daily energy expenditure in most people. The thermic effect of food is the additional energy spent digesting, absorbing, and processing a meal. Together they describe two of the three main components of energy expenditure, alongside physical activity.

Definition

Resting metabolic rate (often used interchangeably with basal metabolic rate under standardised conditions) is the rate of energy expenditure of an awake person at physical and mental rest in a thermoneutral environment, typically after an overnight fast. The thermic effect of food (diet-induced thermogenesis) is the increase in energy expenditure above resting that follows ingestion of a meal.

Scope

This topic covers how resting metabolic rate is defined and measured, what determines its magnitude, and how the thermic effect of food adds to total expenditure and varies with meal composition. It is presented as a reference and educational account of energy expenditure rather than as advice on individual energy needs.

Core questions

• What proportion of total daily energy expenditure is accounted for by resting metabolism?
• Which tissues and body-composition factors most influence resting metabolic rate?
• How large is the thermic effect of food, and how does it differ between protein, carbohydrate, and fat?
• How are these components measured by indirect calorimetry?

Key concepts

• Resting metabolic rate
• Basal metabolic rate
• Thermic effect of food (diet-induced thermogenesis)
• Total daily energy expenditure
• Fat-free mass as a determinant of resting expenditure
• Indirect calorimetry
• Predictive equations (e.g., Harris-Benedict)

Mechanisms

Resting metabolic rate reflects the energy cost of maintaining cellular and organ function and is closely related to fat-free (lean) mass, so body composition is a primary determinant; predictive equations such as the Harris-Benedict equation estimate it from body size, sex, and age (Roza & Shizgal, 1984). The thermic effect of food arises from the energy required to digest, absorb, transport, metabolise, and store ingested nutrients; it is proportionally greatest for protein and least for fat, and typically accounts for roughly a tenth of ingested energy on a mixed diet (Westerterp, 2004). Both components, and the partitioning of fuels that accompanies them, are quantified by indirect calorimetry from oxygen consumption and carbon dioxide production (Frayn, 1983).

Clinical relevance

Estimates of resting metabolic rate and the thermic effect of food inform how energy requirements and expenditure are understood in nutrition and metabolic research. The material here is descriptive and educational and does not provide individualised energy targets or dietary prescriptions.

History

Quantitative study of human energy expenditure by calorimetry developed through the nineteenth and early twentieth centuries, and Passmore and Durnin's 1955 review consolidated knowledge of energy expenditure in humans. Predictive equations for resting needs, exemplified by the Harris-Benedict equation reexamined by Roza and Shizgal (1984), made estimation practical, while later work characterised diet-induced thermogenesis as a distinct, macronutrient-dependent component of expenditure.

Key figures

• Klaas Westerterp
• Reginald Passmore
• John Durnin

Seminal works

• roza-shizgal-1984
• westerterp-2004
• passmore-1955

Frequently asked questions

Why does resting metabolic rate vary so much between people?

Much of the variation tracks differences in fat-free (lean) mass, since metabolically active tissue is the main driver of resting energy expenditure; age, sex, and body size also contribute, which is why predictive equations include these factors.

Does the thermic effect of food differ by macronutrient?

Yes. Protein has the highest thermic effect because of the energy cost of its metabolism and storage, carbohydrate is intermediate, and fat is lowest, as reviewed by Westerterp (2004).