Energy Expenditure in Illness

Energy expenditure in illness is the rate at which the body uses energy when disease, injury, or critical illness alter metabolism. Because the host response can drive metabolic rate up (hypermetabolism) or, in some phases, down, this topic explains how energy needs are measured and why estimates often diverge from healthy-person predictions.

Definition

Energy expenditure in illness is the total rate of energy use by an acutely or chronically ill person, comprising resting energy expenditure plus the thermic effect of feeding and any activity, and frequently altered from healthy norms by the inflammatory and neuroendocrine response to disease.

Scope

The topic covers the components of energy expenditure, how illness changes them through the metabolic response to injury, and how expenditure is measured — chiefly by indirect calorimetry — and estimated by predictive equations. It is reference material on measurement and physiology, not a source of caloric targets for individual patients.

Core questions

• What are the components of total energy expenditure, and which dominates in bedbound patients?
• How does the metabolic response to injury change resting energy expenditure over time?
• Why is indirect calorimetry preferred over predictive equations in critical illness?
• How does the Weir equation turn gas exchange into a metabolic rate?

Key concepts

• Resting energy expenditure (REE)
• Total energy expenditure (TEE)
• Indirect calorimetry
• Weir equation
• Respiratory quotient
• Hypermetabolism
• Ebb and flow phases of injury
• Predictive equations and their error

Mechanisms

Total energy expenditure is dominated, in ill and bedbound people, by resting energy expenditure, which the metabolic response to injury can raise through sympathetic and inflammatory activation — the hypermetabolic 'flow' phase that follows an early, depressed 'ebb' phase first described by Cuthbertson (Cuthbertson, 1942). Indirect calorimetry measures oxygen consumption and carbon-dioxide production and converts them to energy expenditure through the relationships formalised by Weir, with the respiratory quotient indicating which substrates are being oxidised (Weir, 1949). Because illness shifts these quantities unpredictably, predictive equations developed in healthy people often misestimate needs, which is why critical-care guidance favours measurement where feasible (Singer et al., 2019) and why over- and under-feeding energy each carry consequences in the acute phase (Casaer & Van den Berghe, 2014).

Clinical relevance

Knowing how energy expenditure is measured and why it changes in illness underpins nutrition assessment and the appraisal of feeding studies. This entry describes the physiology and measurement of energy needs; it is educational and does not specify caloric prescriptions or feeding rates for individuals.

Evidence & guidelines

Measurement practice rests on the Weir relationships for indirect calorimetry (Weir, 1949) and on critical-care guidelines that favour measured over predicted expenditure (Singer et al., 2019); evidence on how closely to match intake to expenditure in acute illness remains contested (Casaer & Van den Berghe, 2014).

History

Indirect calorimetry has roots in the respiratory-physiology work of the late nineteenth and early twentieth centuries, but the bedside interpretation of gas exchange was consolidated by Weir's 1949 equations. Cuthbertson's earlier description of the ebb-and-flow metabolic response to injury explained why expenditure in the injured and septic departs from healthy predictions, and later critical-care guidelines turned this understanding toward measured rather than assumed energy targets.

Debates

Measured versus predicted energy expenditure

Predictive equations are convenient but frequently inaccurate in critical illness, so guidelines favour indirect calorimetry where available; access and practicality keep the question of how best to set energy targets alive.

Key figures

• David Cuthbertson
• Joseph Weir
• Pierre Singer

Seminal works

• cuthbertson-1942
• weir-1949
• singer-2019

Frequently asked questions

Why measure energy expenditure instead of estimating it from an equation?

Illness can raise or lower metabolic rate unpredictably, so equations developed in healthy people often misjudge needs; indirect calorimetry measures actual gas exchange and is preferred in critical illness when available.

What does the Weir equation do?

It converts measured oxygen consumption and carbon-dioxide production into a rate of energy expenditure, providing the standard basis for calculating metabolic rate from indirect calorimetry.