Estimating Energy Requirements for an Obese Patient
Indirect calorimetry is the gold standard measure for estimating energy expenditure. However, because it is not feasible for patients in most settings, dietitians rely on predictive equations to determine energy needs. Several equations are commonly accepted by dietitians, including the Harris-Benedict equation, the Ireton-Jones equation, the Owen equation, and the Mifflin-St Jeor equation. These equations are available online at http://rd411.com/index.php?option=com_content&view=article&id=97:energy-requirement-for-adults&catid=74:nutrition-assessment&Itemid=353.
Many registered dietitians (RDs) face a dilemma when calculating resting metabolic rate (RMR) for patients who are overweight or obese. Most RDs and some medical centers have established their own protocols, using either actual or adjusted body weight and specific formulas to estimate energy needs. While in many cases a margin of error will not affect the immediate health of a patient, in critical care units and in cases where nutrition support is needed, an incorrect estimate of energy expenditure can affect the outcome of the patient’s treatment.
What is the most accurate method of estimating RMR for obese patients? Should dietitians use actual body weight for energy requirements on obese patients (body mass index [BMI] >30) or use an adjusted body weight?
This dilemma recently was solved when the American Dietetic Association (now known as the Academy of Nutrition and Dietetics [AND]) made a recommendation for the most accurate method of estimating energy needs in obese patients. However, a review of the background and issues regarding this topic is useful. Two articles that were printed in Nutrition in Clinical Practice in 2005 helped ignite a controversy about this subject. While one article suggests that the use of adjusted body weight prevents overfeeding in obese hospitalized patients (Krenitsky), another suggests that it is reasonable to use an equation that incorporates actual body weight (Ireton-Jones).
The controversy over use of actual or adjusted body weight to estimate RMR is in part related to fat mass (FM) and fat-free mass (FFM), components of body weight that are related to energy expenditure. Aging and gender differences influence resting metabolic rate through the variation in FM and FFM. For example, women have a lower metabolic rate because they have a great amount of fat (adipose) tissue. Both the FFM and FM contribute to RMR. The question is how much of the excess body weight (FM) is metabolically active. The adjustment previously mentioned (multiplying by a factor of .25) implies that 25% of the fat mass in an obese person is metabolically active. Because it is difficult to determine the percent of fat that is metabolically active, adjustments in actual body weight to reflect metabolically active FM are at best an estimate.
A number of studies have looked at the accuracy of various equations for calculating energy needs of patients. Boullata et al concluded that no equation accurately predicts resting energy expenditure in most hospitalized patients. In a recent study by Frankenfield et al, four predictive equations were studied to evaluate their accuracy in obese and nonobese patients. This article concluded that health care professionals should avoid using the Harris-Benedict equation for determining energy expenditure, particularly with an adjusted body weight, for obese patients (Frankenfield et al).
The AND Evidence Analysis Library has reviewed studies relating to various formulas and configurations (actual, adjusted, and ideal body weight) for estimating energy needs in obese patients. According to the library, the Mifflin-St Jeor formula using actual body weight is the most accurate method currently available to calculate RMR in obese individuals. The strength of this argument and references supporting it are available to AND members or by subscription at www.adaevidencelibrary.com. The AND Nutrition Care Manual® has adopted the Evidence Analysis Library’s conclusions, and states “estimated energy needs should be based on RMR. If possible, RMR should be measured (eg, indirect calorimetry). If RMR cannot be measured, then the Mifflin-St Jeor equation using actual weight is the most accurate for estimating RMR for overweight and obese individuals.”
Implications for practitioners
Estimating the energy expenditure of an obese person is difficult. Based on the information available at this time, the Mifflin-St Jeor equation using actual body weight will provide the most accurate assessment of RMR in the obese patient (BMI >30).
RMR = (9.99 × weight in kilograms) + (6.25 × height in centimeters) − (4.92 × age) + 5
RMR= (9.99 × weight in kilograms) + (6.25 × height in centimeters) – (4.92 × age) – 161
Total energy expenditure
Calculate by using the RMR × Activity Factor × Injury Factor
Currently online and handheld nutrition assessment programs may not have the Mifflin-St Jeor equation available, making use of the Mifflin-St Jeor equation more cumbersome for the dietetics practitioner.
Equations such as Harris-Benedict and Mifflin-St Jeor are simply estimates of energy expenditure. It is necessary to evaluate energy needs derived from equations for their accuracy based on the patient’s condition, improvement or decline, and changes in weight. Dietitians should use their clinical expertise and experience in assessing the obese patient and adjust energy provided to meet treatment goals.
References and recommended readings
Academy of Nutrition and Dietetics. AND Evidence Analysis Library.
Available to AND members or by subscription at: www.adaevidencelibrary.com.
Accessed October 4, 2011.
Academy of Nutrition and Dietetics. Nutrition Care Manual.
Available to subscribers at: www.nutritioncaremanual.org.
Accessed October 4, 2011.
Boullata J, Williams J, Cottrell F, Hudson L, Compher C. Accurate determination of energy needs in hospitalized patients. J Am Diet Assoc. 2007;107:393-401.
Frankenfield D, Roth-Yousey L, Compher C. Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults: a systematic review. J Am Diet Assoc. 2005;105:775-789.
Ireton-Jones C. Adjusted body weight, con: why adjust body weight in energy-expenditure calculations? Nutr Clin Pract. 2005;20:474-479.
Krenitsky J. Adjusted body weight, pro: evidence to support the use of adjusted body weight in calculating calorie requirements. Nutr Clin Pract. 2005;20:468-473.
Review Date 10/11