Objective: The rule of thumb (ROT) method is used to estimate energy expenditure (EE) at bedside. ROTs are fixed numbers of calories given daily per kilogram of body weight. Textbooks nevertheless indicate that age and body mass index (BMI) affect EE. This should also affect ROTs. We thus scrutinized the impact of BMI, age, and sex on ROTs, compared the results to the often used 25 kcal/kg ROT, and calculated a BMI-, age-, and sex-adjusted ROT table containing calories per kilogram in the basal state.
Methods: We based calculations on the Harris-Benedict equation corrected for systematic error in women and obesity obtained in previous validation studies and used age, weight, and height of 676 consecutively admitted patients from five hospitals.
Results: The calculated ROTs continuously decreased from normal weight (20.8 ± 2.2 kcal/kg) to overweight (18.9 ± 1.8 kcal/kg) and obese patients (15.5 ± 1.6 kcal/kg, P < 0.001). However, not only BMI but also increasing age reduced the ROT significantly within each BMI category (P < 0.01 except for BMI > 35 kg/m²), resulting in a BMI- and age-adjusted ROT spectrum of 12-27 kcal/kg in the total population. The 25-kcal ROT, even when used with normal ("ideal") body weight, overestimated calculated ROTs in more than 95% of patients.
Conclusion: We found that both BMI and age significantly impacted ROT estimates. Thus, using one single fixed ROT for all patients independent of age and BMI does not seem appropriate. We consequently suggest a calculated table of BMI-, age-, and sex-adjusted ROTs where the results of resting EE were multiplied with 1.1, 1.2, and 1.3 and separately listed in the table to account for activity/stress factors.
Copyright © 2012 Elsevier Inc. All rights reserved.