ESTIMATION OF ENERGY EXPENDITURES

Discussion
Since the early 1900s, various formulas have been employed to estimate energy expenditure.  Since the advent of the doubly labeled water (DLW) technique in the 1980s, scientists have begun to more accurately determine total energy expenditure (TEE) in free-living persons (1).  Unfortunately, due to its high cost and the limited number of laboratories that perform the DLW technique, this application is not currently accessible in the clinical setting.  Most recently, the American Dietetic Association (ADA) explored evidence that reported the accuracy and application of various methods used to measure energy expenditure, particularly indirect calorimetry and predictive formulas for various population groups (2,3).  These reports provide evidence that can be used by dietetic professionals to make informed clinical decisions regarding whether to measure or estimate resting energy expenditure (REE) in patients (2).  The predictive equations that have been evaluated include: the Harris-Benedict equation (4), Mifflin–St. Jeor equation (5), Owen equations (6,7), and equations used by the World Health Organization and the Dietary Reference Intakes (8). In 2006, the Swinamer equation, Ireton-Jones equations, Penn State equations, and other equations were evaluated for their application in estimating energy expenditure in critically ill patients (3). 

    The Dietary Reference Intakes for energy, which are based on studies using the DLW technique, are considered the most accurate references for estimating
TEE in free-living persons (2,9).  These values can serve as a resource for the assessment of patients who are not critically ill or do not have multiple disease processes (2,8).  (Refer to Estimated Energy Requirements (EER) for Male and Female Under 30 Years of Age and Estimated Energy Requirements (EER) for Men and Women 30 Years of Age.) The Mifflin–St. Jeor equation predicts REE with the most consistency and the least percentage of error in the ambulatory population (2).  Multiple studies have reported variable accuracy with the Harris-Benedict equation; this equation accurately predicts REE only 45% to 81% of the time in healthy non-obese subjects (2).  The accuracy of all predictive equations decreases when applied to the obese population.  In studies of obese patients, the Harris-Benedict equation accurately predicted REE only 33% to 64% of the time, while the Mifflin–St. Jeor equation accurately predicted REE 70% of the time (2).  Because of the variations reported with the use of the Harris-Benedict formula, evidence provides limited support for its use in estimating the energy expenditure of ambulatory or hospitalized population groups (2,9). Energy expenditure depends on factors including age, gender, height, weight, and physical activity.  In the hospital setting where patients generally have multiple complications and the potential for rapid changes in medical status, predictive formulas that include not only determinants of REE, but also modifiers for illness severity, inflammatory state, and respiratory demands may be needed (9,10).  Refer to the Ireton-Jones equations and Penn State equations discussed below (3,9,10).   The clinician should realize that any method used to estimate energy expenditure only provides an approximation (2). These equations should be used only as a guide or starting point, after which the patient must be closely monitored and interventions must be devised based on individual needs that promote the attainment of nutritional status.

Recommended Formulas to Calculate REE in Critical Care Patients
Indirect calorimetry is the standard for determination of REE in critically ill patients because REE based on measurement is more accurate than estimation using predictive equations (Grade I)* (3).  If predictive equations are needed in non-obese critically ill patients, the best prediction accuracy of equations studied (listed in order of accuracy) include the Penn State equation (2003a version) (79%), Swinamer (55%) and Ireton-Jones equation (1992 version), (Grade III) (3,9,11).  The Harris-Benedict equation (with or without activity and stress factors), Ireton-Jones equation (1997 version), and Fick equation should not be used to determine REE in critically ill patients, as these equations do not have adequate prediction accuracy (Grade I) (3).  In addition, the Mifflin–St. Jeor equation should not be used in critically ill patients, as it was developed for healthy people and has not been well researched in the critically ill population (Grade I) (3).  If predictive equations are needed for critically ill, mechanically ventilated individuals who are obese, consider using the Ireton-Jones equation (1992 version) or the Penn State equation (1998 version), as they have the best prediction accuracy of the equations that have been studied (Grade III) (3).  Refer to the Critical Illness Evidence-Based Nutrition Practice Guideline (2006) in the ADA Evidence Analysis Library for detailed information (3).  

*The American Dietetic Association has assigned grades, ranging from Grade I (good/strong) to Grade V (insufficient evidence), to evidence and conclusion statements. The grading system is described in Clinical Nutrition Management A Reference Guide.

    The Ireton-Jones equations were developed specifically for critically ill patients and hospitalized burn patients.  These formulas are the most widely used formulas for patients in the critical care and hospital setting (9, 10, 12-15).  These equations have easily measured variables (height, weight, age, gender, diagnosis, presence of obesity, and ventilatory status) that are used in the equation to estimate REE.  For the diagnosis variable, patients are assigned as burn, non-burn trauma, or other.  Different than most guidelines and definitions, the Ireton-Jones equation considers obesity to be present if the body mass index (BMI) is greater than 27 kg/m2 (12,14).  If a patient is mechanically ventilated, the ventilator equation should be used regardless of the presence of obesity.  The variables in these equations take into account the health and mobility status of a critical care patient.  The common practice of multiplying additional physical activity level (PAL) factors or injury factors is not validated with these formulas.

    With the exception of the Penn State Equation, all equations below were developed using actual weight.  For the Penn State equation, actual weight was adjusted using the following formula (ideal body weight x 125%) for patients with a BMI of > 30.  With the exception of the Penn State Equation, there is no evidence that substituting adjusted or ideal weight in these calculations results in improved accuracy (2,9). 

Penn State equations (3):
The 1998 version is recommended for mechanically ventilated, obese, critically ill patients (Grade III) (3): 
REE = BEE (1.1) + VE (32) + Tmax (140)- 5340

The 2003a version is recommended for mechanically ventilated, non-obese, critically ill patients (Grade III) (3):
REE = BEE (0.85) + VE (33) + Tmax (175)- 6433, where
BEE = basal energy expenditure calculated using the Harris-Benedict equationa (4)
VE = minute ventilation (L/min)
Tmax = maximum temperature (degrees Celsius) 

aHarris-Benedict equation (4) for use in Penn State equation only:
kcal/day (male) = 66 +13.8 (W) + 5.0(H) – (6.8 x A)
kcal/day (female) = 655 + 9.6 (W) + 1.8 (H) – (4.7 x A), where
W = actual weight (kg)
H = height (cm)
A = age (years)

A newer version of the Penn State equations (2003b), in which the Mifflin–St. Jeor equation is used to calculate BEE, is being evaluated (3).

Swinamer Equation (16)
Published in 1990, was based on observations in 112 mechanically ventilated, critically ill trauma, surgical, and medical patients within the first 2-days of admission to critical care unit (Grade III) (16).

Energy Expenditure = 945 (BSA) – 6.4 (age) +108 (T) + 24.2 (breaths/min) + 81.7 (VT) -4349

Equation includes body surface area (BSA) in squared meters (m2), temperature (T) in degrees Celsius, and tidal volume (VT) in liters per minute (L/min).

Ireton-Jones equations (1992 version) (3, 14):
IJEE (ventilator-dependent) = 1925 – 10 (A) + 5 (W) + 281 (S) + 292 (T) + 851 (B)
IJEE (spontaneous breathing) = 629 – 11 (A) + 25 (W) – 609 (O), where
IJEE = estimated energy expenditure (kcal/day)
A = age (years)                                                                 T = trauma (present = 1, absent = 0)
W = weight (kg)                                                               B = burns (present = 1, absent = 0)
S = sex (male = 1, female = 0)                                       O = BMI > 27 kg/m2 (present = 1, absent = 0)                             

Mifflin–St. Jeor Equation and Recommended Formulas to Calculate REE in Ambulatory Patients 
The Mifflin–St. Jeor equation was designed to estimate REE in the ambulatory population (5, 10).  Actual body weight is used in these equations, regardless of BMI (5).  It is generally recommended that a PAL factor be multiplied to obtain TEE.  It has been recommended that the REE be multiplied by a PAL factor of 1.3 for sedentary individuals; however, this recommendation has not been validated in studies (9). If needed, use a higher activity factor to correct for active individuals engaging in purposeful activity (9).  See the following table as a guideline, or refer to PAL described in Dietary Reference Intakes in Section A to determine appropriate PAL estimate (17).  Injury factors have not been validated for use with these equations and therefore are not recommended as part of TEE calculations.

The Mifflin–St. Jeor equation for men is:
REE = 10(weighta in kg) + 6.25(height in cm) – 5(age in years) + 5

The corresponding equation for women is:
REE = 10(weighta in kg) + 6.25(height in cm) – 5(age in years) – 161

aUse total body weight, regardless of BMI.

Estimating Energy Expenditure in the Obese Population
Ideally, the REE of an obese patient should be based on lean body mass that is determined by methods such as dual x-ray absorptiometry, underwater weighing, or gold-standard energy expenditure prediction models (eg, DLW technique) (9). However, these methods are not practical in most clinical settings.  The common clinical practice of using an adjusted body weight to estimate the metabolically active tissue mass does not improve the accuracy of predicting the metabolic rate in obese patients (10).  The Adjusted Body Weight for Obesity formula, a well-known formula developed by Cunningham (18,19), is not considered applicable in current clinical practice and therefore should not be used in any predictive equations (5,9,10,19).  The consensus of literature supports the use of actual body weight in predictive formulas for estimating REE in obese patients (19,20).  Formulas like the Mifflin–St. Jeor, Ireton-Jones (1992 version), and Penn State (1998 version) equations have utilized obese subjects in the validation of the equations and therefore are an option for predicting REE in obese patients (3,5,10). According to the ADA’s Weight Management Practice Guidelines, estimated energy requirements should be based on resting metabolic rate (RMR) (21).  If possible, RMR should be measured (eg, indirect calorimetry).  If RMR cannot be measured, then the Mifflin–St. Jeor equation using actual body weight is the most accurate formula for estimating RMR for overweight and obese healthy individuals (Grade I) (21).

Estimating Approximate Energy Requirements for Adults
As an alternative to the above calculations, a useful initial approximation of the energy needs of patients in the clinical setting can be obtained as follows (8,22):

Estimating Energy Requirements for Spinal Cord Injury
People with spinal cord injury tend to have reduced metabolic activity due to denervated muscle. Measured energy expenditure is at least 10% below predicted; therefore, caloric needs of spinal cord injured patients should be based on measured energy expenditure (Grade III) (23). If indirect calorimetry is not available during the acute phase (0 - 4 weeks post-injury using prediction equations based on critical care level using admission weight and  multiplying by an injury/stress factor of 1.2 has been suggested  (23).  During the rehabilitation phase, one study reports initial caloric needs can be estimated using 22.7 kcal/kg body weight for individuals with tetraplegia and 27.9 kcal/kg for those with paraplegia (23). When estimating caloric needs of individuals with spinal cord injury, acuteness of injury, level of injury, gender, and physical activity level should be taken into consideration (Grade III) (23).

Measuring REE by Indirect Calorimetry
Indirect calorimetry is an indirect measurement of REE based on quantification of an individual’s respiratory gas exchange (ratio of oxygen consumed to carbon dioxide produced).  From respiratory gas exchange measurements, a respiratory quotient can be obtained that can provide additional information about individual substrate utilization (10).  Many stress factors and kilocalorie ranges proposed for estimating energy expenditure for specific disease states are based on indirect calorimetry studies; however, the accuracy of these formulas for estimating expenditure for the individual patient can vary (9,10).  Factors that affect energy expenditure and impact the outcome of indirect calorimetry results include: changes in medications that act as a stimulant or a sedative, changes in the degree or type of ventilator support, and day-to-day variations in the metabolic stress level (10).  These factors should be considered when monitoring and interpreting measured REE.  Indirect calorimetry remains a viable option for estimating energy requirements in the critical care setting and can be useful in the prevention of overfeeding the critical care patient.  Precise guidelines and more in-depth considerations for the use of indirect calorimetry have been published (2,10-12).

   
References

1. Black AE, Coward WA, Cole TJ, Prentice AM.  Human energy expenditure in affluent societies: an analysis of 574 doubly-labelled water measurements.  Eur J Clin Nutr.  1996;50:72-92. 
2. Indirect Calorimetry Evidence Analysis Project.  American Dietetic Association Evidence Analysis Library.  American Dietetic Association; 2005. Available at: http://www.adaevidencelibrary.org.  Accessed October 15, 2007. 
3. Critical Illness Evidence-Based Nutrition Practice Guideline.  American Dietetic Association Evidence Analysis Library. American Dietetic Association; 2006. Available at: http://www.adaevidencelibrary.com. Accessed October 15, 2007.
4. Harris J, Benedict F.  A Biometric Study of Basal Metabolism in Man. Washington, DC: Carnegie Institute of Washington; 1919. Publication No. 279.
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6. Owen OE, Holup JL, D’Alessio DA, Craig ES, Polansky M, Smalley KJ, Kavle EC, Bushman MC, Owen LR, Mozzoli MA. A reappraisal of the caloric requirements of men. Am J Clin Nutr.  1987;46:875-885.
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8. Institute of Medicine’s Food and Nutrition Board.  Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids.  National Academy of Sciences; 2002:265-334. Preprint available at: http://www.nap.edu.books/0309085373/html/index.html. Accessed April 20, 2005.
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12. Ireton-Jones C. Indirect calorimetry.  In: Skipper A, ed. Dietitian’s Handbook of Enteral and Parenteral Nutrition. Gaithersburg, Md: Aspen Publishers; 1998:148-164.
13. Kemper MS. Indirect calorimetry equipment and practical considerations of measurement. In: Weissmen C, ed. Problems in Respiratory Care: Nutrition and Respiratory Disease. Philadelphia, Pa: JB Lippincott Co; 1989:479-490.
14. Ireton-Jones CS, Turner WW Jr, Liepa GU, Baxter CR.  Equations for estimation of energy expenditures in patients with burns with special reference to ventilatory status.  J Burn Care Rehabil.  1992;13:330-333.
15. Ireton-Jones CS, Jones JD.  Why use predictive equations for energy expenditure assessment?  J Am Diet Assoc.  1997;97(suppl):A44.
16. Swinamer DL, Grace MG, Hamilton SM, Jones R, Roberts P, King EG.  Predictive equation for assessing energy expenditure in mechanically ventilated critically ill patients.  Crit Care Med.  1990;18:657-661.
17. Shetty PS, Henry CJ, Black AE, Prentice AM.  Energy requirements of adults: an update on basal metabolic rates (BMRs) and physical activity levels (PALs).  Eur J Clin Nutr.  1996;50(suppl 1):S11.
18. Cunningham JJ.  An individualization of dietary requirements for energy in adults.  J Am Diet Assoc.  1982;80:335-338.
19. Frankenfield DC, Muth ER, Rowe WA.  The Harris-Benedict studies of human basal metabolism: history and limitations.  J Am Diet Assoc.  1998;98:439-445.
20. Ireton-Jones CS, Turner WW Jr.  Actual or ideal body weight; which should be used to predict energy expenditure?  J Am Diet Assoc.  1991;91:193-195.
21. Adult Weight Management Evidence-Based Nutrition Practice Guideline.  American Dietetic Association Evidence Analysis Library. American Dietetic Association; 2006.  Available at: http://www.adaevidencelibrary.com. Accessed October 15, 2007.
22. Franz MJ, Reader D, Monk A, eds.  Implementing Group and Medical Nutrition Therapy for Diabetes.  Alexandria, Va: American Diabetes Association; 2002:b4.
23. Spinal Cord Injury Evidence Analysis Project.  American Dietetic Association Evidence Analysis Library. American Dietetic Association; 2007. Available at: http://www.adaevidencelibrary.com. Accessed November 7, 2007.

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