Nutrient timing revisited: is there a post-exercise anabolic window?: Post-exercise nutrient timing


Journal Title (Medline/Pubmed accepted abbreviation): J. Int. Soc. Sports Nutr.
Year: 2013
Volume: 10
First Page: 5
doi: 10.1186/1550-2783-10-5

Summary of the review article:
Introduction:
Protein and carbohydrates are deliberately consumed after a workout in order to maximize the effects of exercise, namely growth of muscle. Some sports scientists have proposed a "window of opportunity"- 30 minutes after the workout- during which to consume a large amount of protein in order to optimize muscle growth. This article reviews the most current body of literature to assess the validity of the "window of opportunity" and/or circumstances in which it holds true.

Key author assertions:
Glycogen resynthesis
  • Resistance training depletes glycogen, or carbohydrate stores in the muscle tissue.
  • Depletion of glycogen does not only reduce the pools from which glucose can be retrieved for energy production, but also can reduce the efficiency of signaling pathways. Therefore, it is important to begin a workout with replete glycogen stores.
  • It is important to consume carbohydrates immediately after the workout in order to quickly replenish glycogen, but only when performing more than one workout a day. One day is often a sufficient amount of time to restore glycogen.
  • When speed of recovery is important, immediate consumption of carbohydrates after a workout allows glycogen to be restored up to 2 times faster than if carbohydrates are consumed 2 hours later.
  • The combination of protein and carbohydrates accentuates the rate of glycogen restoration.
Protein breakdown
  • Increasing circulating concentrations of insulin will minimize protein breakdown after a workout.
  • The combination of carbohydrates with protein will minimize protein breakdown after a workout. Consumption of these ingredients will have maximal benefits immediately after the workout.
  • Currently, there is a lack of data that support the use of one type of carbohydrate over another.
Protein synthesis
  • Muscle protein synthesis is undoubtedly accentuated by post-workout protein ingestion.
  • Some, but not all studies, have shown that the combination of carbohydrates and protein potentiates muscle protein synthesis to a greater degree than protein alone.
  • Several studies have assessed the timing of protein ingestion around the workout, including before, immediately after, one hour after, or three hours after a workout. There is no consistency in the current data to recommended one time over another to promote the greatest rate of muscle protein synthesis.
Muscle growth
  • Seven studies were reviewed that tested the effects of a post-workout protein beverage on the growth of muscle. About half showed no differences between groups (groups varied, for example protein post-workout or during another time of day or protein vs. a placebo). About half did show that protein ingestion immediately post-workout accentuated gains in muscle size, but many studies had confounding factors such as the addition of another supplement to the post-workout beverage or the fact that consuming the beverage increased the total amount of protein ingestion in the diet. Therefore, due to the very limited experiments that have been performed regarding the post-workout "window of opportunity" for enhancing muscle growth, it is impossible at this time to confirm or deny its validity.

Key practice applications: One overall limitation of this review is that it focused primarily on the bodybuilder and strength athlete. There was little discussion of other types of athletes, for whom post-exercise recovery can be equally important. It would have been helpful if the article had been written from the broader athletic perspective, especially with regard to new findings indicating that nutrient timing can improve adaptation to training and body composition (e.g., Fergusson-Stegall L et al., J Nutr Metab 2011;doi:10.1155/2011/623182). The article does a good job of summarizing many of the positive study findings regarding nutrient timing, but then also raises questions about the practicality and significance of these findings.

There are a number of concerns regarding the authors' interpretation of research findings. For example, the authors assert, with respect to muscle glycogen repletion, that nutrient timing is only important when the recovery period between events is less than approximately 8 h. They further assert that, by 24 h post-exercise, rates of glycogen recovery, with or without immediate post-exercise feeding of carbohydrate and protein, are the same if the athlete is provided sufficient carbohydrate over the entire period. One issue with the first point is that, without immediate post-exercise feeding, the athlete spends a longer time with an emptier "glycogen gas tank". Howarth et al. (J Appl Physiol 2010;109:431-8) have shown that low glycogen availability contributes to reduced overall protein balance in muscle. An issue with the second point is that in couple of the studies cited by the authors to support their view (Jentjens et al., 2001; Parkin et al., 1997) and other similar studies (Van Hall et al., J Appl Physiol 2000;88:1631-6), the amounts of carbohydrate fed were unrealistic and/or impractical outside the laboratory. Jentjens et al. (2001) fed 35 g carbohydrate every 30 min for 2.5 h post-exercise. Athletes in the Parkin et al. (1997) study consumed a total of 875 g carbohydrate over the 24-h recovery period, a very large amount. Finally, in the Van Hall et al. (2000) study, athletes consumed an initial bolus of 74 g carbohydrate, followed by feedings of ~18 g carbohydrate every 15 min for 4 h post-exercise. These laboratory results are just not practical in the real world and it would have been helpful if the authors of this review had applied a more critical eye to the studies that were cited to support their points.

Another concern is the inappropriate extrapolation, by the authors, of research results collected in one population to an entirely different population. For example, the authors cited a study by Capaldo et al. (1999) to support the concept that, at 5 h after a meal of 75 g carbohydrate, 37 g protein, and 17 g fat, insulin level were still double the fasting level. What was not mentioned, however, is that subjects in this study were not athletes (no history of exercise was reported) and, in fact, were 40-58 years of age and were eating a 600-kcal pizza meal. While the subjects had normal glucose tolerance, it is difficult to imagine appropriately applying such results to a young adult, strength-training population, which was the clear focus of this article.

There were also issues regarding the authors' citation of studies by Tipton et al., regarding pre- vs. post-exercise protein feeding, to support their contention that pre-exercise feeding may stimulate muscle protein synthesis more than post-exercise feeding. The authors themselves pointed out that the methodology of these studies has been criticized and that the findings have been refuted by other groups. Thus, given these limitations, it is not clear why these findings carried so much weight in the authors' opinions.

One of the most important take home points from this review, however, is that the authors presented no evidence of potential harm from employing nutrient timing strategies. Many of the positive findings for nutrient timing were presented and there appears to be no detrimental effects to use of these principles in athletes.
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