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    REE

    Resting energy expenditure: indirect calorimetry, the Weir equation, respiratory quotient and clinical significance.

    Resting energy expenditure (REE) is the number of calories the body uses over 24 hours at rest, awake and without physical activity. It is close to the basal metabolic rate but is measured under less strict conditions (no overnight stay or full fast), so it is usually slightly higher. REE is used to plan nutrition in healthy people, in weight management and especially in hospitalised and critically ill patients, in whom both under- and overfeeding are harmful.

    MetabolismIndirect calorimetryClinical nutrition
    Unitkcal/day
    Reference methodindirect calorimetry (VO₂, VCO₂)
    Measured vs predicted≈ 90–110% usually taken as normal
    Respiratory quotient≈ 0.7 (fat) to 1.0 (carbohydrate)
    Measured resting energy expenditure as a percentage of predicted: below 90 percent hypometabolic, 90 to 110 percent normal, above 110 percent hypermetabolic Measured REE, % of predicted Hypometabolic below 90% 90% Normal 90–110% 110% Hypermetabolic above 110%
    Figure 1. Measured REE as a percentage of the value predicted from an equation. The 90–110% band is a commonly used convention, not a strict norm.

    How REE is measured

    With indirect calorimetry, a hood, face mask or ventilator circuit collects exhaled air for 15–30 minutes while the person rests quietly. Oxygen consumption (VO₂) and carbon dioxide production (VCO₂) are converted into energy using the Weir equation:

    REE (kcal/day) = [3.941 × VO₂ (L/min) + 1.106 × VCO₂ (L/min)] × 1440

    Before the test, the person should not eat for several hours, avoid caffeine, smoking and exercise, and rest for 10–20 minutes. The ratio VCO₂/VO₂ (respiratory quotient) shows which fuels are being burned. Values outside about 0.67–1.3 suggest a measurement error.

    When calorimetry is not available, REE is estimated from equations (for example Mifflin–St Jeor), which are less accurate in individuals.

    Clinical significance

    Low REE

    Hypothyroidism, prolonged undernutrition or very-low-calorie diets, loss of muscle mass, sedation. May make weight loss harder.

    High REE

    Hyperthyroidism, fever, sepsis, burns, major trauma or surgery, COPD (work of breathing), cancer, heart failure with cachexia.

    Why it matters

    In critically ill patients, guidelines prefer indirect calorimetry to equations for setting energy targets, because predicted values are often wrong by several hundred kilocalories per day.

    References

    1. 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
    2. Mifflin MD, St Jeor ST, Hill LA, Scott BJ, Daugherty SA, Koh YO. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr. 1990;51(2):241–247. doi:10.1093/ajcn/51.2.241

    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 2 June 2019