EE: Number of Measurements (2005)

Citation:

Gasic S, Schneider B, Waldhausl W. Indirect calorimetry: Variability of consecutive baseline determinations of carbohydrate and fat utilization from gas exchange measurements. Horm Metab Res 1997;29:12-15.

 
Study Design:
Prospective Cohort Study
Class:
B - Click here for explanation of classification scheme.
Quality Rating:
Positive POSITIVE: See Quality Criteria Checklist below.
Research Purpose:
  1. To evaluate short-time intraindividual variability of two consecutive series of gas exchange measurements
  2. And of calculated data on total energy expenditure/24 h and carbohydrate and fat utilization.
Inclusion Criteria:
  1. Understand and give written consent
  2. Adults
  3. Healthy (did not define)
  4. Had not taken any drugs for at least 6 weeks before study
Exclusion Criteria:
  1. Refusal to consent
  2. Did not fit inclusion criteria
Description of Study Protocol:

Definitions:

Steady state”- equilibration of respiratory values (respiratory volumes, oxygen uptake, carbon dioxide production)

ANTHROPOMETRIC:

  • Ht measured ? yes (method not discussed)
  • Wt measured ? yes (method not discussed)
  • Fat-free mass measured? not discussed
  • BMI? yes

CLINICAL:

  • Monitored heart rate? not discussed
  • Body temperature? not discussed
  • Urine was collected during a 6 hr period for estimated urinary 24h nitrogen excretion (extrapolated for a 24h period).

Resting energy expenditure:

  • IC type: canopy method
  • Rest before measure: 30-45 min
  • Measurement length: up to 30 min in length
  • Fasting length: 12 hour overnight fast
  • Exercise conditioning 24 prior to test?: not mentioned
  • Room temp: 21 degree C; constant humidity 70%
  • No. of measures were they repeated? 2 measurement series of up to 30 min each were performed 15-20 min apart
  • Coefficient of variation? In each subject, the difference between the first and the second measurement series as well as the respective mean coefficient of variation (CV) were calculated.
  • Equipment of Calibration: yes for at least 10 min prior to test
  • Training of measurer? not mentioned
  • Subject training of measuring process?: yes

DIETARY:

  • Subjects were instructed to take an equilibrated mixed diet (60% carbohydrates, 25% fat and 15% proteins) containing 25-30 kcal/kg/d for 3 days prior to the study.

Statistical tests: 

  •  The mean values of multiple assessments obtained during the 2 consecutive measurement series were used. For each subject, the difference between the 1st and 2nd measurement series as well as the respective mean coefficient of variation (CV) was calculated. The individual differences were examined using the paired t-test, the paired Wilcox on test or the Sign test, as appropriate.
  • P<0.05 was considered significant.

Data Collection Summary:

Outcome(s) and other measures

  1. Total energy production/24 h as well as that obtained from CHO and fat utilization were calculated in both consecutive measurement series.
  2. Measured directly: O2 –consumption (VO2; ml/min) and CO2; VCO2; ml/min).
  3. Respiratory quotient (RQ) was calculated: RQ = VCO2 /VO2.
  4. Total energy production/24 h was calculated according to Wier’s equation.
  5. For calculation of substrate utilization the following equations were used:
      • -amount of CHO utilization/24h (g/C):
      • g/C = [-2.91 VO2 + 2.12 VCO2] 1440 – 2.56 UN
      • UN = urinary urea nitrogen (g/24h)
      • -amount of fat utilization/24h (g/F)
      • g/F = [1.69 VO2 – 1.69 VCO2] 1440 – 1.94 UN
      • -amount of protein utilization/24h (g/P)
      • g/P = 6.25 UN
  6. For calculation of energy obtained from substrate utilization, constants determined by Loewy et al. were used.
  7. Independent variables of weight, height, age, BMI,

Blinding used: No

Description of Actual Data Sample:
  • N = 24 healthy volunteers
  • N = 16 females; 8 males
  • (mean age: 34.7 +13.1 y)
Summary of Results:

ANTHROPOMETRIC

  • Only provided mean ht, wt and BMI for subjects as a group.
  • Mean ht: (cm): 170.2 + 8.2 (st.dev)
  • Mean wt: (kg): 66.8 + 13.4
  • Mean BMI: 22.9 + 3.5

MEASUREMENT PROCESS:

Number of measurements 2 measurement series of up to 30 min each were performed 15-20 min apart

Length of measurements

  • Up to 30 min in length
  • In each subject, the duration of the measurement series was equal in both tests

Steady state

Initial measurement followed equilibration of respiratory values for at least 10 min prior to the test; subsequent measurements were performed 15-20 apart; following achievement of “steady state” conditions

RQ

O2-consumption, CO2-production, the RQ and total energy production/24 h showed acceptable mean coefficients of variation (CV) of 3.7%, 4.6%, 3.5%, and 3.6%, respectively.

MEASUREMENT TIMING:

  • Sleep or rest—rested 30-45 min prior to test
  • Physical activity—not mentioned
  • Food intake Subjects were instructed to take an equilibrated mixed diet (60% carbohydrates, 25% fat and 15% proteins) containing 25-30 kcal/kg/d for 3 days prior to the study.
  • Various times in the day—measurements taken between 8 and 10 am

INDIVIDUAL CHARACTERISTICS

  • Circulatory hormones—not discussed
  • Breathing ability—spontaneously breathing
  • Medical tests/procedures-IC and special diet and urine collection during a 6 h period
  • Chemicals (medications/drugs/herbs, caffeine, nicotine, alcohol)—were not allowed to take any drugs for at least 6 weeks prior to the test.
  • Directly assessed parameters (VO2 and VCO2) showed slight intra-individual differences between the 2 tests; average values during test 2 differed by ~ 4% when compared to test 1 (p<0.01). Calculated kcal total/24 h was 4% lower during test 2 (p<0.0001).
  • Total energy production/24 h as well as that obtained from CHO and fat utilization was calculated in both measurement series.
  • Protein utilization was derived from estimated urinary 24 h nitrogen excretion.
  • In contrast, CHO and fat utilization values demonstrated a coefficient of variation of 21.2% and 17.4%, respectively, suggesting considerable variability of estimates of fuel utilization by IC.
  • For research purposes, particularly over short-time periods, IC provides sufficient accuracy only in estimating total resting energy production, while considerable uncertainty exists in using this method to assess CHO and fat utilization.
Author Conclusion:

The method of IC has been subject to severe doubts as to its applicability to substrate utilization. Nevertheless, IC has been widely used for clinical and research purposes as both hardware and software have been available for on-line calculation of energy expenditure (energy production) and substrate utilization from gas exchange measurements by using programmed equations. It has been stated that the accuracy of assessment of energy production and substrate utilization using IC should be within 5% of the respective true value.

Imperfections in the method have been known for a long time. The difficulty to strictly standardize resting conditions may jeopardize accurate measurements of VO2, particularly in critically ill patients.

This may not apply to our healthy subjects as their individual VO values during each of the 2 measurement series were generally stable. Comparing the 2 measurement series, acceptable CVs were obtained for VO2 (3.7%), VCO2 (4.6%), and RQ (3.5%) in keeping with previously published data. Similarly an acceptable CV of 3.6% was observed for total energy production corresponding to data obtained by others.

In contrast, calculated values for CHO and fat utilization demonstrate strikingly elevated mean CVs of 21.2% and 17.4%, respectively. This is in keeping with previously presented data that even small differences in gas exchange measurements can have great influence on calculated substrate utilization. Thus, a difference of 10% in VCO2 gives an error of only 2-3% in total energy production, but may cause an error in CHO and fat utilization of 50-100%. The present data demonstrate that, despite meticulously optimized and controlled test conditions, a difference of only 4% in average VO2 and VCO2 between the 2 measurement series resulted in a huge variability of substrate utilization estimates.

We conclude that in clinical research particularly over short-time periods, IC provides sufficient accuracy only in estimating total energy production/24 h while considerable uncertainty exists while using this method to assess CHO and fat utilization.

Funding Source:
University/Hospital: University of Vienna, Institute for Medical Statistics & Documentation
Reviewer Comments:

Limitations:

  • Small sample size
  • Limited generalizability; healthy (not defined) convenience sample ;mostly non-obese (BMI 22.9 + 3.5)
  • Extrapolating 24 h urinary nitrogen from 6 h collection period; measuring nitrogen excretion over short time periods might not be sufficient to minimize errors in estimating substrate utilization (may be more a concern with critically ill patient where urea concentration in body fluids and the amount of urea synthesized in the liver can change quickly)
Quality Criteria Checklist: Primary Research
Relevance Questions
  1. Would implementing the studied intervention or procedure (if found successful) result in improved outcomes for the patients/clients/population group? (Not Applicable for some epidemiological studies) Yes
  2. Did the authors study an outcome (dependent variable) or topic that the patients/clients/population group would care about? Yes
  3. Is the focus of the intervention or procedure (independent variable) or topic of study a common issue of concern to dieteticspractice? Yes
  4. Is the intervention or procedure feasible? (NA for some epidemiological studies) Yes
 
Validity Questions
1. Was the research question clearly stated? Yes
  1.1. Was (were) the specific intervention(s) or procedure(s) [independent variable(s)] identified? N/A
  1.2. Was (were) the outcome(s) [dependent variable(s)] clearly indicated? N/A
  1.3. Were the target population and setting specified? N/A
2. Was the selection of study subjects/patients free from bias? Yes
  2.1. Were inclusion/exclusion criteria specified (e.g., risk, point in disease progression, diagnostic or prognosis criteria), and with sufficient detail and without omitting criteria critical to the study? N/A
  2.2. Were criteria applied equally to all study groups? N/A
  2.3. Were health, demographics, and other characteristics of subjects described? N/A
  2.4. Were the subjects/patients a representative sample of the relevant population? N/A
3. Were study groups comparable? Yes
  3.1. Was the method of assigning subjects/patients to groups described and unbiased? (Method of randomization identified if RCT) N/A
  3.2. Were distribution of disease status, prognostic factors, and other factors (e.g., demographics) similar across study groups at baseline? N/A
  3.3. Were concurrent controls or comparisons used? (Concurrent preferred over historical control or comparison groups.) N/A
  3.4. If cohort study or cross-sectional study, were groups comparable on important confounding factors and/or were preexisting differences accounted for by using appropriate adjustments in statistical analysis? N/A
  3.5. If case control study, were potential confounding factors comparable for cases and controls? (If case series or trial with subjects serving as own control, this criterion is not applicable.) N/A
  3.6. If diagnostic test, was there an independent blind comparison with an appropriate reference standard (e.g., "gold standard")? N/A
4. Was method of handling withdrawals described? Yes
  4.1. Were follow-up methods described and the same for all groups? N/A
  4.2. Was the number, characteristics of withdrawals (i.e., dropouts, lost to follow up, attrition rate) and/or response rate (cross-sectional studies) described for each group? (Follow up goal for a strong study is 80%.) N/A
  4.3. Were all enrolled subjects/patients (in the original sample) accounted for? N/A
  4.4. Were reasons for withdrawals similar across groups? N/A
  4.5. If diagnostic test, was decision to perform reference test not dependent on results of test under study? N/A
5. Was blinding used to prevent introduction of bias? No
  5.1. In intervention study, were subjects, clinicians/practitioners, and investigators blinded to treatment group, as appropriate? N/A
  5.2. Were data collectors blinded for outcomes assessment? (If outcome is measured using an objective test, such as a lab value, this criterion is assumed to be met.) N/A
  5.3. In cohort study or cross-sectional study, were measurements of outcomes and risk factors blinded? N/A
  5.4. In case control study, was case definition explicit and case ascertainment not influenced by exposure status? N/A
  5.5. In diagnostic study, were test results blinded to patient history and other test results? N/A
6. Were intervention/therapeutic regimens/exposure factor or procedure and any comparison(s) described in detail? Were interveningfactors described? Yes
  6.1. In RCT or other intervention trial, were protocols described for all regimens studied? N/A
  6.2. In observational study, were interventions, study settings, and clinicians/provider described? N/A
  6.3. Was the intensity and duration of the intervention or exposure factor sufficient to produce a meaningful effect? N/A
  6.4. Was the amount of exposure and, if relevant, subject/patient compliance measured? N/A
  6.5. Were co-interventions (e.g., ancillary treatments, other therapies) described? N/A
  6.6. Were extra or unplanned treatments described? N/A
  6.7. Was the information for 6.4, 6.5, and 6.6 assessed the same way for all groups? N/A
  6.8. In diagnostic study, were details of test administration and replication sufficient? N/A
7. Were outcomes clearly defined and the measurements valid and reliable? Yes
  7.1. Were primary and secondary endpoints described and relevant to the question? N/A
  7.2. Were nutrition measures appropriate to question and outcomes of concern? N/A
  7.3. Was the period of follow-up long enough for important outcome(s) to occur? N/A
  7.4. Were the observations and measurements based on standard, valid, and reliable data collection instruments/tests/procedures? N/A
  7.5. Was the measurement of effect at an appropriate level of precision? N/A
  7.6. Were other factors accounted for (measured) that could affect outcomes? N/A
  7.7. Were the measurements conducted consistently across groups? N/A
8. Was the statistical analysis appropriate for the study design and type of outcome indicators? Yes
  8.1. Were statistical analyses adequately described and the results reported appropriately? N/A
  8.2. Were correct statistical tests used and assumptions of test not violated? N/A
  8.3. Were statistics reported with levels of significance and/or confidence intervals? N/A
  8.4. Was "intent to treat" analysis of outcomes done (and as appropriate, was there an analysis of outcomes for those maximally exposed or a dose-response analysis)? N/A
  8.5. Were adequate adjustments made for effects of confounding factors that might have affected the outcomes (e.g., multivariate analyses)? N/A
  8.6. Was clinical significance as well as statistical significance reported? N/A
  8.7. If negative findings, was a power calculation reported to address type 2 error? N/A
9. Are conclusions supported by results with biases and limitations taken into consideration? Yes
  9.1. Is there a discussion of findings? N/A
  9.2. Are biases and study limitations identified and discussed? N/A
10. Is bias due to study's funding or sponsorship unlikely? Yes
  10.1. Were sources of funding and investigators' affiliations described? N/A
  10.2. Was the study free from apparent conflict of interest? N/A