Effect of increased dietary protein on tolerance to intensified training

Journal Title (Medline/Pubmed accepted abbreviation): Med Sci Sports Exerc
Year: 2011
Volume: 43
Page numbers: 598-607
doi (if applicable): 10.1249/MSS.0b013e3181f684c9

Summary of Background and Research Design

Background: Athletes attempting to improve endurance function often train with periods of high volume and limited recovery. An imbalance between training impulse and recovery often leads to fatigue and disturbances in biologic functioning and psychologic status. Nutrition is frequently used as a tool to help improve recovery from each intense exercise bout. For example, carbohydrate (CHO) feeding can lessen short-term decrements in endurance performance after intense training periods. There may also be some benefit of acute ingestion of protein for endurance exercise recovery; however, the effect of long-term protein feeding on exercise recovery from an intense training period has not been studied.

Hypothesis/purpose of study: The authors hypothesized that trained cyclists would experience improved tolerance to the stress of increased training volume when consuming a high-protein diet during and after intense training.

Subjects: Eight endurance-trained male cyclists participated in the study (mean age, 27 ± 8 yr; body mass, 73 ± 7 kg; maximal oxygen consumption [VO2max], 64.2 ± 6.5 mL/kg.min; maximal power output, 372 ± 21 W). All participants had a training history ≥ 5 years.

Experimental design: Counterbalanced, crossover

Treatments and protocol: Each participant completed 2 different 3-week long trials, both divided into normal (NOR), intensified (INT), and recovery (REC) training. Trials were separated by at least a 2-week washout period. During INT and REC training, participants received either a high-protein diet (PRO; 3 g protein/kg/day) or a normal diet (CON; 1.5 g protein/kg/day). Dietary CHO remained constant at 6 g/kg/day. Endurance performance was assessed with a VO2max test and a preloaded time trial, on Days 6 and 7, respectively, of each week. Alterations in blood metabolite responses were measured at rest, during, and after exercise. Each day, participants completed the Daily Analysis of Life Demands for Athletes (DALDA) questionnaire. The authors used a statistical technique that did not define results as significant or nonsignificant based on a specific P value (the approach that is typically used). Rather, the authors reported the probability of benefit or harm based on the means and confidence limits. In this approach, the probabilities were labeled as: < 1% = almost certainly no chance; 1% to 5% = very unlikely; 5% to 25% = unlikely; 25% to 75% = possible; 75% to 95% = likely; 95% to 99% = very likely; > 99% = almost certain. This method, although it does not yet have widespread scientific acceptance, was supported by several references that reported the findings of a group of expert statisticians.

Summary of research findings
  • There was no clear effect of PRO on weekly training volume during NOR, INT, or REC (all < 1%).
  • PRO blunted (4.3%; 90% confidence limits [CL] x/÷5.4%) the decrement in time trial performance after intense exercise (30% chance of benefit = “possible” benefit; < 1% chance of harm).
  • PRO enhanced (2.0%; 90% CL x/÷ 4.9%) restoration of endurance performance during recovery (48% chance of benefit = “possible” benefit; < 1% chance of harm).
  • PRO attenuated (17 score units; 90% CL ± 11 AUC of “worse than normal” scores part B, DALDA) increased symptoms of stress after INT (97% chance of benefit = “very likely” benefit).
  • PRO was not associated with any discernable changes in blood metabolite concentrations.

Interpretation of findings/Key practice applications

This study suggests that increased dietary protein intake may have a beneficial role in exercise recovery by attenuating impairments in endurance performance. It is not clear whether protein feeding per se, or subsequent maintenance of blood glucose or muscle glycogen, was the most important factor. Either way, a likely mediator of this potentially beneficial effect is perturbations in psychologic symptoms of stress. It should be noted that results may only be applicable in the context of relatively low CHO availability because cyclists in this study consumed less CHO (6 g/kg/d) than recommended (8 to 10 g/kg/d) for intense endurance training. Practical and logistical study limitations may have precluded tight scientific experimental control. For example, the training load prescribed during the experimental block was not supervised in a laboratory setting, and washout periods between trials were not standardized (range, 2 to 4 weeks).

Another potential limitation is that because CHO intake was fixed and the diets were isocaloric, both protein and fat had to vary. Thus, the higher protein diet also contained less fat than the lower protein diet. However, it seems unlikely that the decrease in fat on the higher protein diet would be associated with alterations of the variables mentioned above. Further research is needed to clarify the role of increased dietary protein in recovery from endurance training.

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