A cornerstone of nutrition prescription is determining patients’ energy expenditure, that is, the number of kilocalories patients need to maintain their current nutritional status.
The literature provides little solid data related to the effect of chronic kidney disease (CKD) on energy expenditure. Both direct and indirect calorimetry have been used to measure energy expenditure in CKD patients with conflicting results. No studies to date have been conducted using doubly-labeled water, which is the preferred method of estimating energy metabolism.
Measuring energy expenditure
During the progression of CKD, metabolic changes are detected in energy expenditure (J Ren Nutr. 2001;11:202-206), glucose and insulin metabolism (Kidney Int Suppl. 1983(16)S121-S127), protein metabolism (J Am Soc Nephrol. 1999;10:1050-1058), and lipid metabolism (Nephrol Dial Transplant. 1994;9:1580-1585). The generic measurement of energy expenditure in CKD patients has been done in a number of studies using different methodologies.
Techniques that are available to measure energy expenditure include direct calorimetry, indirect calorimetry, and doubly-labeled water. Doubly-labeled water is quickly becoming the gold standard in energy expenditure measurement. This method provides an average energy expenditure for a period of five to 14 days. It has the advantages of being non-invasive and reflective of actual energy expenditure under normal living conditions. In direct calorimetry, a specially designed chamber is used to measure the heat/energy given off by the body. Indirect calorimetry is the measurement of oxygen utilization and carbon dioxide production. Energy expenditure is calculated from these values.
In a review of energy expenditure in CKD patients (J Ren Nutr. 2006;16:17-26), Laura Byham-Gray examined studies from 1986-2004 for methodology and outcomes. Of all the studies reviewed, none used doubly-labeled water and only one, using direct calorimetry, measured CKD patients prior to stage 5. In a study by Kuhlmann et al (J Ren Nutr. 2001;11:202-206), researchers measured energy expenditure and glomerular filtration rate in 51 non-dialyzed CKD patients using direct calorimetry three times over six months.
The results of this study showed a significant and strong negative correlation between creatinine clearance and resting metabolic rate indicating that energy expenditure increases as kidney function declines. These results are different from those reported by teams using indirect calorimetry, including Kopple et al (Kidney Int. 1986;29:734-742), O’Sullivan et al (Am J Kidney Dis. 2002;39:369-375), Panesar and Agarwal (Clin Neph. 2003;59:360-366), and Avesani et al (Am J Kidney Dis. 2004;44:1008-1006).
The studies conducted thus far have had small sample sizes and used a variety of methodologies, so it is difficult to elucidate the impact CKD has on energy expenditure prior to stage 5. Careful review of these papers makes it clear that more work needs to be done in this area and with precise tools of measurement.
The use of direct or indirect calorimetry is very expensive and time consuming. Clinically, these measurements are not truly feasible. However, to increase the accuracy of nutrition prescriptions and presumably improve patient outcomes, an accurate energy expenditure equation is needed.
Many energy expenditure equations appear in the literature, such as Schofield, Harris & Benedict, James & Lean and World Health Organization Resting Energy Expenditure, to name a few. These equations have been validated in a variety of populations; unfortunately, none of the equations have been validated for any of the stages of CKD.
Formulating a new equation
It is for this reason that the nutrition prescription as it relates to kilocalorie requirements in CKD patients is not as accurate as desired. Ideally, total energy expenditure of patients with CKD should be measured in enough patients that a mathematically-based equation could be determined.
This equation would allow clinicians to make accurate assessments of patients’ kilocalorie requirements and thus prevent over- or underestimation of nutrient needs. Until such a time as this occurs, clinicians need to understand the limitations of the equations available and not be too vested in the values provided by them. Whether or not patients are maintaining weight and visceral protein status is more important than the exact number of kilocalories estimated by these unvalidated equations.
In conclusion, the data on energy expenditure for CKD patients is mixed and no validated equation exists that accurately predicts kilocalorie needs for CKD patients. Thus, clinicians need to be aware of the current equation limitations and focus on patient outcomes.
Dr. Steiber is Coordinator of the Dietetic Internship/Master’s Degree Program at Case Western Reserve University in Cleveland.