Effects of ingesting protein in combination with carbohydrate during exercise on endurance performance: a systematic meta-analysis
Journal Title (Medline/Pubmed accepted abbreviation):  J Strength Cond Res
Year:  2010
Volume: 24
Number: 8
Page numbers: 2192-2202
doi (if applicable): 

Summary of Background and Research Design

Hypothesis/purpose of study:This meta-analysis combined the data from 11different studies to evaluate if there are performance benefits associated with the ingestion of a combination of carbohydrate and protein versus protein alone during endurance exercise.

Subjects:In the 11 studies reviewed, there were 111 male and 16 female subjects

Experimental design:Meta-analysis

Methods: In January 2010, a literature search of several databases (Pubmed, SportDiscus, CINAHL, Scopus, Rehabilitation, Physical Medicine (Embase database), and Cochrane Library) was conducted to identify potential articles, using the following search terms in varying combinations: protein, amino acids, exercise, sport drink, hydration, sport, performance, running, cycling, and marathon. 
Studies were included if:
1) they had performance measures (e.g., time trial performance, time to exhaustion, or maximum power output in a particular time period) that were done in the same exercise bout as the consumption of protein.
2) subjects ingested any form of protein during activity or immediately before the start and during activity.
3) the effect of the protein ingestion could be isolated.
4) a crossover, randomized design was used with blinding of researchers and subjects if possible.
Studies were excluded if:
1) two bouts of exercise were performed more than 2 hours apart and protein was ingested during the recovery period from the first bout.
2) the protocol included multiple days of protein supplementation during testing.
A total of 11 studies met inclusion criteria. The studies were subgrouped in two different ways for separate analyses as well: 1) time trial vs. time to exhaustion end points; and 2) studies in which either carbohydrates or calories were consistent among all treatments. The amount of protein consumed during a trial ranged from 7-45 g

Summary of research findings:
  • Of the 11 total studies, 4 of them found a significant improvement in performance and the magnitude of the performance improvement was, on average, 9%.
  • Studies that were isocarbohydrate (e.g., equal carbohydrate, with protein added on, n = 7) showed similar improvement in performance (10.5%) compared with the entire 11 studies as a whole.
  •   Isocaloric studies (e.g., carbohydrate reduced to account for the protein in order to keep energy consistent among treatments, n = 3) showed a small improvement in performance (3.4%) that was not significantly different.
  • Studies in which time to exhaustion was measured (n = 8) showed a mean improvement of 5.14 min (95% confidence interval of 0.42-9.86).
  • The 3 studies that used a time trial to measure performance each observed no significant effect of protein on performance.
  • Only 2 of the 11 studies showed a detrimental effect of protein on performance and neither finding was statistically significant.

Interpretation of findings/Key practice applications:

Overall, protein consumption with carbohydrate did have a positive effect on endurance performance. This improvement was largely driven by the preponderance of studies that: 1) were isocarbohydrate; and 2) used a time to exhaustion test versus a time trial to evaluate performance. The increased caloric load of the protein-carbohydrate combination in isocarbohydrate studies is a potential explanation for the benefit in performance versus carbohydrate alone. In addition, it was noted that, because of their longer duration, protein ingestion was often greater in studies using time to exhaustion versus time trial protocols. There are arguments against using a time to exhaustion protocol because it is not as representative of most athletic events compared with a time-trial. However, the time to exhaustion protocol might reflect other types of physical exertion such as firefighting and other highly physical occupations.
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