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NAP: Energy Balance and Body Composition (2014)


Mettler S, Mitchell N, Tipton KD. Increased protein intake reduces lean body mass loss during weight loss in athletes. Med Sci Sports Exerc. 2010; 42 (2): 326-337.

Study Design:
Randomized Controlled Trial
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Quality Rating:
Positive POSITIVE: See Quality Criteria Checklist below.
Research Purpose:

To examine the influence of dietary protein on lean body mass loss and performance during short-term hypoenergetic weight loss in athletes. 

Inclusion Criteria:
  • Healthy males 18 to 40 years of age
  • Body mass index (BMI) over 20kg/m2
  • Participation in regular resistance exercise training for at least the previous six months. The training had to include two or more resistance-training sessions per week. 
Exclusion Criteria:

Metabolic disorders determined by health questionnaire.

Description of Study Protocol:


Local sports facilities. 


Parallel design, RCT.


High protein diet

  • Week One: Habitual diet
  • Week Two: All food provided to match their habitual energy with 50% CHO, 15% protein and 35% fat
  • Weeks Three to Four: Calories reduced to 60% of habitual intake but in the Intervention Group, the macronutrient content was 50% CHO, 35% protein and 15% fat. The protein was distributed among the different meals and snacks throughout the day but not explicitly timed with training sessions. 

Statistical Analysis

ANOVA for repeated measure with Tukey adjustment. 

Data Collection Summary:

Timing of Measurements

Weeks One, Two, Three and Four.

Dependent Variables

  • Weight: After emptying bladder, in same shorts and T-shirt each time
  • Body composition: Dual energy X-ray absorptiometry
  • Diet: Three-day food report during Week One, to determine habitual energy intake
  • Energy expenditure: Physical activity questionnaire during Week One, to determine habitual energy expenditure. Three-dimensional accelerometer and a heart rate (HR) montior were worn throughout the study, except during sleep and training for the accelerometer.
  • Daily Analysis of Life Demands for Athletes (DALDA) questionnaire, training diary, general problem report: Completed each evening before bed. General problem report to include any problems the subjects felt may have impacted training or habitual activities. 
  • Satiety questionnaire: Two 100-mm visual analog scales anchored by the extreme values "extreme hunger" at zero mm and "very full" at 100mm. One scale was used to rate the present satiety in the evening and the other to estimate the satiety for the entire day.
  • Fatigue and muscle soreness: Rating between one (no fatigue and soreness) and 10 (training not possible anymore) for each training session
  • Fasting blood for glucose, non-esterified fatty acids (NEFA), glycerol, urea, free testosterone, cortisol, growth hormone and free IGF-1
  • Mood state: POMS-24 questionnaire
  • Performance assessment: Squat jump height, jump peak force, one-repetition maximum (IRM), muscle endurance test chest press and mean power and peak power of the 30-second Wingate test.  

Independent Variables

High-protein diet of 35% protein, 50% CHO and 15% fat.

Control Variables

  • 60% energy reduction from habitual diet
  • All food provided to the subjects during Weeks Two through Four 
  • Constant training load.
Description of Actual Data Sample:
  • Initial N: 22 males
  • Attrition (final N): 20
  • Age: 25.8±1.7 years for controls and 24.7±1.6 years for high-protein
  • Ethnicity: Not described
  • Other relevant demographics: 4.9±0.4 and 4.6±0.4 training sessions per week for controls and high-protein
  • Anthropometrics: No difference in BMI, body mass, body fat percentage. BMI, 24.2±0.9kg/m2 and 23.4±0.5kg/m2 for controls and high-protein.
  • Location: Birmingham, UK.
Summary of Results:


  • Body mass was not different between groups (P=0.178), but there was a significant effect of time (P=0.001) and a group-x-time interaction (P=0.001). Total body mass did not change from Week One to Week Two in Control (P>0.999) and High-Protein Groups (P>0.886). Both groups lost the same amount of fat mass, but the Control Group lost significantly more lean (P=0.006) and total (P=0.036) body mass than the High-Protein Group.
  • Relative lean body mass loss in arms, legs, trunk and head was statistically different between the groups (P=0.009), but did not differ between the segments
  • There were no statistically significant differences between the two groups for any of the performance tests
  • There was a statistically significant decrease of 2.8% in peak jump force (P=0.011) and a 7.3% increase (P=0.044) in muscle endurance between Weeks One and Four, with no significant difference between groups. Relative to body mass, IRM increased significantly (P<0.001) during Weeks Three and Four, compared with Weeks One and Two, but there was no statistical difference between the Control and High-Protein groups.
  • Glucose, free testosterone and free IGF-1 decreased while NEFA and urea increased significantly (P<0.05) with energy restriction (effect of time). Urea was significantly increased  (P=0.017) in the High-Protein Group, compared with the controls. Urea (P<0.001) and NEFA  (P=0.005) showed a significant group-x-time interaction.
  • There was no effect of group, time or group-x-time for the number of training sessions per week, training duration per week and the average exercise energy expenditure, as estimated with the HR monitor. There was no effect of the energy restriction per se, nor was there any difference between the groups with respect to total mood disturbance in the POMS-24 questionnaire. There was no difference between the groups in the A part of the DALDA. In contrast, results of the B part of the DALDA questionnaire were influenced by the energy restriction and showed a significant (P<0.05) increase of the "worse than normal" ratings during the weight loss weeks, including a significant (P=0.023) group-x-time effect, that is the High-Protein Group showed a larger rise of the "worse than normal" ratings in the weight weeks than the Control Group.
Author Conclusion:
  • Intake of approximately 2.3g per kg-1 or approximately 35% protein was significantly superior to approximately10g per kg-1 or approximately 15% energy protein for maintenance of lean body mass in young healthy athletes during short-term hypoenergetic weight loss
  • Performance was not affected by the energy restriction or the diet composition in this study.
Funding Source:
UK Sports Council and Swiss Federal Council of Sports
Reviewer Comments:
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? 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? 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? Yes
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) Yes
  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.) 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? Yes
  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.) No
  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? 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? 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)? 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? 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