NAP: Energy Balance and Body Composition (2007)


Zachwieja JJ, Ezell DM, Cline AD, Ricketts JC, Vicknair PC, Schorle SM, Ryan DH. Short-term dietary energy restriction reduces lean body mass but not performance in physically active men and women. Int J Sports Med. 2001; 22: 310-316. 

PubMed ID: 11414677
Study Design:
Randomized controlled trial
A - Click here for explanation of classification scheme.
Quality Rating:
Neutral NEUTRAL: See Quality Criteria Checklist below.
Research Purpose:
To determine the effect of two weeks of moderate energy restriction (750kcal per day) on exercise performance in physically active men and women.
Inclusion Criteria:
  • Men and women who regularly performed aerobic exercise
  • All were healthy as determined by medical history and physical exam, including routine blood and urine chemistries.
Exclusion Criteria:
  • None had experienced significant weight loss or gain (over five kg) in previous six months
  • None were routinely consuming dietary supplements such as creatine monohydrate.
Description of Study Protocol:
  • Recruitment: Recruitment methods not specified
  • Design: Randomized controlled trial
  • Blinding used: Subjects and primary investigators blinded to dietary treatment during intervention period
  • Intervention: Subjects randomly assigned to either energy restriction for two weeks (N=16) or energy balance (N=8).

Statistical Analysis

  • Change from baseline was calculated for body composition, urinary nitrogen excretion and performance results and statistical differences were determined by an unpaired T-test
  • To establish main effects, performance measures were also analyzed by repeated-measures ANOVA
  • Body weight data were analyzed by repeated-measures using a mixed model with an unstructured covariance matrix.
Data Collection Summary:

Timing of Measurements

  • 24-day study: 10-day baseline period followed by either two weeks of dietary energy restriction (750kcal per day, N=16) or energy balance (control, N=8)
  • Pre- and post-intervention measurements of body composition, muscular strength and endurance, running performance, anaerobic capacity and urinary nitrogen excretion.

Dependent Variables

  • Body weight determined each morning prior to breakfast, weighed on a calibrated electronic scale in street clothes after removing shoes, wallets, coins, keys from pockets, not allowed to weigh themselves during the intervention period
  • Body composition measured by DEXA
  • Muscle strength measured by leg and shoulder press, one repetition maximum
  • Muscle endurance measured by leg squats to fatigue
  • Running performance measured by five-mile run time
  • Anaerobic capacity measured by Wingate anaerobic capacity test
  • Urinary nitrogen excretion measured by chemiluminescence.

Independent Variables

  • Energy restriction (750kcal per day) for two weeks. Habitual energy and protein intake determined by three-day food record completed before study.
  • Subjects were fed diets designed and prepared by metabolic kitchen
  • Control diet: 51% CHO, 35% fat, 14% protein
  • Energy restriction diet: 47% CHO, 34% fat, 19% protein
  • Adjustments made for daily 500kcal per day treadmill exercise energy expenditure
  • Subjects were interviewed for food allergies or intolerances
  • Five-day cycle menu designed for all study participants.

Control Variables

Description of Actual Data Sample:
  • Initial N24 subjects, 13 men and 11 women
  • Attrition (final N): 24, 16 in experimental group, eight controls
  • Age: Mean, 24±4 years
  • Ethnicity: Not mentioned.

Other Relevant Demographics

  • Mean weight: 71.7±11.9kg
  • Mean body fat: 22.9±7.8%
  • Mean BMI: 24.1±2.8
  • Mean VO2max: 51.5±5.0ml per kg per minute.


Differences between men and women described, but not between experimental and control groups.


Pennington Biomedical Research Center, Louisiana.


Summary of Results:

 Results During Baseline and Following Intervention

  Restriction-Baseline Restriction-After Control-Baseline Control-After
Lean Body Mass (kg) 51.9±1.8 51.1±2.7 55.7±4.1 55.6±3.9
Fat Mass (kg)





Nitrogen Excretion (grams per 24 hours) 10.4±1.0 13.2±1.0 10.6±1.0 11.2±1.0
Creatinine Excretion (grams per 24 hours) 1.56±0.12 1.54±0.11 1.48±0.06 1.59±0.13
Nitrogen / Creatinine (per 24 hours) 6.83±0.48 8.50±0.41 7.19±0.51 7.21±0.41
Muscle Endurance (repetitions) 73±16 88±19 69±10 95±29
Five-Mile Run Time (seconds) 2,566±72 2,380±54 2,612±88 2,475±77
Leg Muscle Strength (kg) 135.5±9.1 140.9±9.5 147.3±13.2 150.0±12.7
Shoulder Muscle Strength (kg)
47.7±6.4 48.2±6.8 64.1±11.4 64.1±12.3

Anaerobic Capacity (joules)





Other Findings

  • Baseline energy intake was 3,151±118 kcal/day. Energy intake in the control group was constant averaging about 3,200 kcal per day and the restriction group consumed on average approximately 725kcal per day, compared to baseline (P<0.0001). Compliance to the intervention was high.
  • Exercise energy expenditure averaged 470±6 kcal per day in all subjects, accomplished through treadmill running at a self-selected pace
  • Body weight maintained in control group (-0.36±0.24kg), but energy restriction resulted in weight loss of -1.29±0.16kg (P<0.001). Gender had no influence.
  • There was a trend for lean body mass to decline more in the energy restriction group (P=0.093), accounting for 61% of the weight loss. Urinary nitrogen excretion also tended to be higher in the energy restriction vs. control group (13.2±1.1 vs. 11.2±1.0g per day, P=0.089).
  • Muscle strength was maintained or increased during the energy restriction period
  • Muscle endurance improved following two weeks of energy restriction or balance
  • Anaerobic capacity increased slightly in the restriction (+368±219 joules), but declined in the control group (-649±288 joules, P<0.05).
Author Conclusion:
  • The results of this study demonstrate that in exercising men and women, two weeks of moderate energy restriction does not impair skeletal muscle strength, endurance or five-mile run time and may even improve anaerobic capacity
  • This implies that physically active men and women can become accustomed to, over the short-term, a moderate imbalance between energy intake and expenditure without sacrificing capacity to perform aerobic as well as anaerobic tasks
  • However, energy restriction did result in modest weight loss, with a majority coming from the lean body mass compartment.
Funding Source:
Government: Dept. of Defense
Reviewer Comments:
  • Experimental groups and control group not equally sized
  • Differences between groups were not discussed
  • Recruitment methods were not specified
  • Inclusion criteria and exclusion criteria were not well-defined
  • High dietary compliance to intervention through metabolic kitchen.
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? Yes
  1.2. Was (were) the outcome(s) [dependent variable(s)] clearly indicated? Yes
  1.3. Were the target population and setting specified? Yes
2. Was the selection of study subjects/patients free from bias? ???
  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? Yes
  2.2. Were criteria applied equally to all study groups? Yes
  2.3. Were health, demographics, and other characteristics of subjects described? Yes
  2.4. Were the subjects/patients a representative sample of the relevant population? ???
3. Were study groups comparable? ???
  3.1. Was the method of assigning subjects/patients to groups described and unbiased? (Method of randomization identified if RCT) No
  3.2. Were distribution of disease status, prognostic factors, and other factors (e.g., demographics) similar across study groups at baseline? ???
  3.3. Were concurrent controls or comparisons used? (Concurrent preferred over historical control or comparison groups.) Yes
  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? Yes
  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%.) Yes
  4.3. Were all enrolled subjects/patients (in the original sample) accounted for? Yes
  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? Yes
  5.1. In intervention study, were subjects, clinicians/practitioners, and investigators blinded to treatment group, as appropriate? Yes
  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.) Yes
  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? Yes
  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? Yes
  6.4. Was the amount of exposure and, if relevant, subject/patient compliance measured? Yes
  6.5. Were co-interventions (e.g., ancillary treatments, other therapies) described? Yes
  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? Yes
  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? Yes
  7.2. Were nutrition measures appropriate to question and outcomes of concern? Yes
  7.3. Was the period of follow-up long enough for important outcome(s) to occur? Yes
  7.4. Were the observations and measurements based on standard, valid, and reliable data collection instruments/tests/procedures? Yes
  7.5. Was the measurement of effect at an appropriate level of precision? Yes
  7.6. Were other factors accounted for (measured) that could affect outcomes? Yes
  7.7. Were the measurements conducted consistently across groups? Yes
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? Yes
  8.2. Were correct statistical tests used and assumptions of test not violated? Yes
  8.3. Were statistics reported with levels of significance and/or confidence intervals? Yes
  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)? Yes
  8.6. Was clinical significance as well as statistical significance reported? Yes
  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? Yes
  9.2. Are biases and study limitations identified and discussed? Yes
10. Is bias due to study's funding or sponsorship unlikely? Yes
  10.1. Were sources of funding and investigators' affiliations described? Yes
  10.2. Was the study free from apparent conflict of interest? Yes