Effect of Increased Dietary Protein on Tolerance to Intensified Training
 
 
Journal Title (Medline/Pubmed accepted abbreviation): Med Sci Sports Exerc
Year: 2010
Volume: Epub August 2010
Number:
Page numbers:
doi: 10.1249/MSS.0b013e3181f684c9

Summary of Background and Research Design

Background:Athletes often combine high training volume with limited recovery time as a means to improve endurance; however, this approach can result in fatigue and reduced physical and psychological performance. Preliminary data suggest that protein supplementation is beneficial for exercise recovery in acute-based studies. These benefits may be applicable to the chronic effect of protein feeding on recovery from a period of intensified training.

Hypothesis:Trained cyclists following a high-protein diet will experience improved exercise tolerance (as determined by fewer perturbations in mood and markers of endocrine function), ultimately resulting in better performance.

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

Experimental design:Proof-of-concept; counter-balanced, crossover experimental design

Treatments and protocol: Subjects completed two 3-week trials, divided into 1 week each of normal (NOR), intensified (INT) and recovery (REC) training. Cyclists completed a daily training diary and were equipped with a downloadable heart rate (HR) monitor for the duration of each trial. Cyclists received either a high-protein diet (PRO; 3g/kg body mass daily) or a normal diet (CON; 1.5 g/kg body mass daily) during INT and REC. Dietary carbohydrate (CHO) content remained constant at 6 g/kg body mass daily. All participants confirmed their awareness of which dietary intervention was being administered during each trial. Weekly energy balance and diet were controlled for the duration of the study. Endurance performance was assessed with a VO2max test and a preloaded time trial. Alterations in blood metabolite responses to exercise were measured at rest, during, and following exercise. Cyclists completed the Daily Analysis of Life Demands for Athletes (DALDA) questionnaire each day and the Profile of Mood State-65 (POMS-65) questionnaire weekly, prior to exercise. Normally distributed time trial percentage change data were presented as ± 90% confidence limits (CL) and data requiring log transformation were presented as
 x/¸ 90% CL.

Summary of research findings:
  • No clear effect of additional protein intake on weekly training volume was observed during NOR, INT, or REC
  • Additional protein did not affect the time spent in each training zone during INT
  •   Increased dietary protein intake led to a possible lessening (4.3%, 90% CL = 0.8-23.2% ) of weakened time trial performance resulting from a block of high-intensity training compared with NOR (PRO = 2,639 ± 350 sec; CON = 2,555 ± 313 sec) when data were log-transformed.
  •   Restoration of endurance performance during recovery training possibly benefited (2.0%, 90% CL = 0.4-9.8%) from additional protein intake when data were log-transformed.
  • Frequency of symptoms of stress described as “worse than normal” reported following a block of high-intensity training was very likely (97%) attenuated (17 ± 11 area under the curve [AUC] of “a” scores from DALDA part B, for INT+REC) by increasing the protein content of the diet
  • No clear effect of diet was observed for feelings of fatigue, vigor, depression, anger, or confusion following INT or REC compared with NOR
  •   No discernable changes in blood metabolite concentrations were observed in PRO

Interpretation of findings/Key practice applications:

Protein supplementation reduced symptoms of psychological stress and may prevent reduced performance during a block of high-intensity training. These novel findings provide preliminary evidence that increased dietary protein during high-intensity exercise may minimize loss of endurance performance. These data were evaluated for the likelihood of meaningful change; a ~30% (possible) chance of a beneficial effect and a < 1% chance of a harmful effect on time trial performance was observed. However, these results may be applicable only in the context of relatively low CHO availability. Until further research is available, caution is urged before adoption of any dietary changes discussed in this study. CHO remains the most important fuel to maximize endurance performance during both normal and intensified periods of training.
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