NAP: Recovery (2014)


Betts JA, Williams C, Boobis L, Tsintzas K. Increased carbohydrate oxidation after ingesting carbohydrate with added protein. Med Sci Sports Exerc. 2008; 40(5): 903–912.

Study Design:
Randomized Crossover Trial
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Quality Rating:
Neutral NEUTRAL: See Quality Criteria Checklist below.
Research Purpose:

To examine the metabolic impact of including protein in a post-exercise carbohydrate supplement when consumed between two bouts of prolonged running performed within the same day.

Inclusion Criteria:

 Not stated by authors.

Exclusion Criteria:

 Not stated by authors.

Description of Study Protocol:

In this randomized, double-blind crossover trial subjects performed two sets of running trials separated by 14 days. Preliminary tests were completed to familiarize subjects and to determine maximum oxygen consumption (VO2max). Each trial involved a 90-minute treadmill run at 70% VO2max, followed by four hours of recovery in the lab. During recovery subjects rested while consuming supplements that were either carbohydrate alone (CHO) or a mixture of carbohydrate and protein (CHO-PRO). At the beginning and end of the recovery period biopsies were done to assess the rate of muscle glycogen resynthesis. Immediately after the recovery period subjects completed a 60-minute run at 70% VO2max, and a third muscle biopsy was done. After a 14-day washout period subjects completed the running trials and biopsies again, receiving the alternate supplement during recovery.

Blinding Used

This study was double-blinded.


  • During the four-hour recovery period, all subjects received:
    • CHO solution: Relative to each subject's body weight, such that solution contained 0.8g per kg-1 per hour-1 and equated to total CHO intake of 236±21g. Total amount of energy available from solution was 13.4kJ per kg-1 per BM-1.
    • CHO-PRO solution: Contained same amount of CHO as CHO only solution plus an additional 0.3g PRO per kg-1per BM per hour-1 of whey protein isolate, providing a total of 89±8g of protein during recovery. Total amount of energy available from solution was 18.0kJ per kg-1per BM-1.
  • Solutions were provided in equal volumes for each treatment (590±53ml per hour-1). The prescribed solution was separated into eight volumes; the first volume was consumed immediately after the first muscle biopsy was removed and the remaining seven volumes were provided at 30-minute intervals throughout the four-hour recovery. Subjects had 15 minutes to consume each volume. The final volume was provided 30 minutes before the second muscle biopsy.

Statistical Analysis

  • Wilcoxon test was used to compare median values between treatments and other non-parametric variables
  • Paired T-tests were used to analyze other results involving a single comparison of two level means
  • Two-way general linear model for repeated measures (treatment x time) was used to identify overall differences between experimental conditions
  • A sample size of six was estimated to have a 99% power to detect differences.


Data Collection Summary:

Timing of Measurements

  • Within 14 days before the first trial subjects' submaximal and maximal oxygen uptakes were determined
  • Before beginning the first run on the trial day, subjects were weighed, provided a urine sample and had a resting blood sample taken. A five-minute resting expired gas sample was also collected.
  • During the first run one-minute expired gas samples, heart rates, ratings of perceived exertion and 10ml venous blood samples were taken every 30 minutes
  • Immediately after completion of the first run, subjects were weighed and had a muscle sample taken via biopsy from the vastus lateralis of the anterior thigh
  • During the recovery period expired gas samples, venous blood samples and subjective ratings of gut fullness and thirst were taken every hour prior to consumption of the experimental supplement. Urine was also collected during the recovery period.
  • A second muscle biopsy was taken exactly four hours after the first biopsy, at the end of the recovery period. Weight was again recorded.
  • During the second run all physiological measurements were taken at 15-minute intervals
  • Immediately after completion of the second run, weight was recorded and a final muscle biopsy was obtained.

Dependent Variables

  • Muscle glycogen concentrations: Determined by biopsy
  • Muscle glycogen degradation: Determined by biopsy
  • Serum insulin 
  • Plasma glucose 
  • Plasma urea 
  • Blood lactate 
  • Plasma free fatty acids 
  • Plasma glycerol 
  • Plasma myoglobin 
  • Protein oxidation: Determined by expired gas samples
  • Carbohydrate oxidation: Determined by expired gas samples
  • Fatty acid oxidation: Determined by expired gas samples.

Independent Variables

During the four-hour recovery period. subjects consumed:

  • CHO-only solution
  • CHO-PRO solution. 

Control Variables

  • Subjects recorded dietary intake for 48 hours before first trial and adhered to the same dietary intake the 48 hours before the second trial
  • All subjects had consumed at least 200g of CHO the 24 hours prior to each trial
  • Water intake was matched between trials for each subject
  • Exit interviews with subjects verified that participants could not distinguish between the CHO and CHO-PRO solutions
  • Both legs of subjects were sampled for biopsies
  • Lab temperature and humidity were similar during all trials.
Description of Actual Data Sample:
  • Initial N: Six males
  • Attrition (final N): None reported.
  • Age: 22±1 years
  • Other relevant demographics: VO2max was 61±6ml per kg-1 per minute-1
  • Anthropometrics: Body mass was 73.8±6.7kg
  • Location: Loughborough University, UK.


Summary of Results:

Key Findings

  • There were no differences for muscle glycogen concentrations between treatments at any time points; rates of muscle glycogen resynthesis did not differ between the CHO and the CHO-PRO supplements
  • Rate of muscle glycogen degradation during exercise following recovery was no greater with the CHO treatment than with the CHO-PRO treatment
  • Serum insulin concentrations were higher when the CHO-PRO supplement was taken during recovery than the CHO supplement (F=13.1, P=0.02)
  • The CHO-PRO supplement did not substantially increase plasma glucose concentrations during any time in recovery, while the CHO supplement resulted in an initial glucose peak approximately one hour after the start of the recovery period (P=0.08)
  • Plasma glucose decreased with the start of exercise following recovery for both treatments; however, the decrease was greater with the CHO supplement and the difference was maintained, with significant differences noted after 30 minutes of exercise (P=0.01)
  • Plasma urea concentrations increased immediately after the first intake of CHO-PRO supplement and gradually increased above the response to the CHO supplement as time progressed (treatment x time: F=32.8, P<0.001), with values significantly different between treatments throughout run two (P≤0.02)
  • Despite a significant interaction effect (treatment x time: F=2.38, P=0.015), lactate concentrations in the latter stages of recovery were not statistically different between treatments (P=0.08 at four hours of recovery)
  • Glycerol concentrations were elevated significantly more with the CHO treatment than with the CHO-PRO treatment (treat x time: F=5.8, P=0.02), with a specific treatment difference identified after 45 minutes of exercise (P=0.04)
  • There was no difference between treatments in plasma myoglobin response to exercise
  • The estimated rate of PRO oxidation during recovery was greater with ingestion of the CHO-PRO solution than with ingestion of the CHO solution (0.96±0.08mg per kg-1 per minute-1 and 0.17±0.15mg per kg-1 per minute-1, P=0.01)
  • Metabolic rate was higher with the CHO-PRO treatment during recovery (9.2±0.5kJ per minute-1  vs. 8.5±0.6kJ per minute-1, P=0.05)
  • There were no treatment differences in either CHO or fatty acid oxidation during recovery once corrected for the estimated PRO oxidation rate
  • Overall rates of metabolism were more similar between treatments during the run following recovery (CHO=64.1±1.6kJ per minute-1; CHO-PRO=66.6±2.1kJ per minute-1)
  • CHO ingestion resulted in higher rates of fatty acid oxidation than CHO-PRO (4.71±0.80mg per kg-1 per minute-1 vs. 2.56±0.35mg per kg-1 per minute-1, P=0.01); CHO-PRO intake produced higher rates of CHO oxidation than CHO intake alone (48.4±2.2mg per kg-1 per minute-1 and 41.7±2.6mg per kg-1 per minute-1, P=0.001)
  • There were no treatment differences in terms of hydration status
  • Subjective ratings of gut fullness were higher throughout the entire recovery period while taking the CHO-PRO supplement (13±1) than while taking the CHO supplement (10±1, P=0.03).
Author Conclusion:

Adding 0.3g per kg-1 per hour-1 of whey protein isolate to a recovery solution providing 0.8g per kg-1 per hour-1 of carbohydrate did not accelerate the rate of muscle glycogen resynthesis in the four hours following prolonged treadmill running. Addition of protein to a carbohydrate solution can increase the overall rate of carbohydrate oxidation during a second exercise bout subsequent to recovery without altering the rate of muscle glycogen degradation. 

Funding Source:
Pharmaceutical/Dietary Supplement Company:
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? No
  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? No
  2.2. Were criteria applied equally to all study groups? N/A
  2.3. Were health, demographics, and other characteristics of subjects described? No
  2.4. Were the subjects/patients a representative sample of the relevant population? ???
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) 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