Is there a post-workout “anabolic window”?

Anabolic window or metabolic window is a concept that refers to a limited time period right after resistance training. It is claimed that at that short period of time muscles are prepared for growth given that adequate and suitable protein source is consumed. This strategy is supposed to produce dramatic improvements in body composition and some go so far as to say that the timing of nutritional intake is more important that whole daily intake of nutrients. This, almost dogmatic, post-exercise period is considered the most critical part of nutrient timing. However, recent evidence has directly challenged the classical view of the relevance of post-exercise nutritional intake. Therefore, we decided to critically evaluate this popular nutritional strategy.

In this article we will review scientific literature and cover some of the aspects of nutrient timing and show whether there is some merit to such claims or is this yet another marketing scheme.

Spiking Insulin Post-Workout. Is it Necessary and Does it Really Prevent Protein Breakdown?

Prevention of muscle catabolism or muscle protein breakdown is one of the aspects of post-workout nutrient timing. Increasing insulin post-exercise seems to be the most important factor for reducing muscle protein breakdown [1,2], even more than increased amino acid availability [2]. Studies show that insulin reduces muscle protein breakdown independently of the presence of amino acids after resistance training [3,4,5,30], although amino acids tend to enhance the effect [6,7]. A simple way to increase insulin concentrations after exercise is by ingesting simple carbohydrates [31]. Børsheim et al. [5] have shown that 100g of carbohydrates ingested after resistance exercise improved protein turnover primarily due to a progressive decrease in muscle protein degradation. Same was noted by Roy and colleagues [30] where consumption of a 1 g/kg carbohydrate supplement immediately and 1 hour after resistance training significantly decreased myofibrillar protein breakdown and urinary urea nitrogen excretion. However, effect of carbohydrates on protein turnover is minor compared to previously reported effects of ingestion of essential amino acids [8,9].

Some controversy exists regarding the effect of insulin on muscle protein synthesis. Anabolic effect of insulin is closely related to the concurrent amino acid delivery [3,10]. Insulin has been shown to have a positive effect on protein synthesis, if amino acid delivery is maintained or increased [10].

So to answer the question, it seems reasonable to consume protein-carbohydrate solution after resistance exercise as these two nutrients have been shown to elevate insulin even further than carbohydrate alone [11]. However, studies are showing that rising insulin beyond 3-4 times above normal fasting levels (15–30 mU/L) provides no further benefits [12,13]. To put this in perspective, a classical mixed meal (75 g carbohydrate, 37 g protein, and 17 g fat) was reported to increase insulin 3 times above fasting levels within 30 minutes of consumption and 5 times after 1 hour [14]. This suggests that it may be more important to have a well-planned pre-workout meal.

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Increasing Protein Synthesis and Hypertrophy

Another aspect of purported benefits of nutrient timing is that it increases muscle protein synthesis. Appropriate resistance training causes significant skeletal muscle hypertrophy which occurs due to increased muscle protein synthesis, a decrease in muscle protein degradation or both. Resistance training alone has a potent and acute effect on mTOR signaling [15] and protein synthesis, which is counterbalanced by the accelerated rate of protein degradation [16]. Therefore, the degree of hypertrophy seems to be very much dependent on nutrient availability [17]. Studies are showing that muscle protein synthesis stimulated by essential amino acids is further potentiated by previous exercise [18,19]. The increment in muscle protein synthesis is maximally stimulated at a dose of protein of approximately 25 g or 10 g essential amino acids in rested and exercised muscle [20,21]. Some studies show that carbohydrate added to amino acids can have additional effect on muscle protein synthesis [22,25,28] while some studies failed to demonstrate any such benefit [20,23].

Esmarck et al. [24] were the first to provided evidence in support of the presence of a post-exercise “anabolic window”. In the study thirteen men (age, 74 ± 1 years) were receiving oral protein in liquid form for 12 weeks (3 times per week) immediately after or 2 h after each training session. Results of the study suggested that delayed nutrient intake after workout hinders muscular gains. Same was reported by Levenhagen et al. [25] and Miller et al. [22] who demonstrated that combination of carbohydrate and either protein or amino acids post-exercise can in fact have a synergistic effect on whole body protein homeostasis in healthy adults. Subjects in Levenhagen study [25] consumed 10 g protein, 8 g carbohydrate, 3 g fat either immediately or 3 hours after exercise. Researchers reported greater amino acid and glucose uptake if supplement was ingested immediately after workout. Also, protein synthesis was 3 times greater for subjects who received supplement immediately after exercise compared to later ingestion. There was no much difference in protein breakdown between groups. A study in 10 dogs by Okamura [26] also reported that early infusion of glucose and amino acids increased leg protein synthesis by 35% but did not alter protein breakdown. Long-term animal study noted that rats that were fed a mixed meal immediately after exercise gained more muscle in posterior limb and decreased more adipose tissue weight than rats ingesting a mixed meal after 4 hours [27].

In contrast to these findings Rasmussen et al. [28] concluded that timing of the essential amino acid drink consumption does not affect the response of muscle net balance or muscle protein synthesis. To further aggravate the post-exercise “anabolic window” theory, Tipton and colleagues [29] found that consumption of an essential amino acid-carbohydrate solution immediately before resistance exercise resulted in a significantly greater and more sustained muscle protein synthesis response than that when the solution is consumed after exercise.

Furthermore, in a more comprehensive study by Hoffman and associates [32] a well-trained males were randomly assigned to receive a protein supplement either in the morning and evening or immediately before and immediately after weight training. In addition, 7 subjects served as control and did not use any protein. No changes in body mass or percent body fat were seen in any of the groups after 10 week trial. However, the study was critiqued for using DAX to assess body composition, which lacks sensitivity compared to MRI and CT [33].

Currently available data lacks consistent indication of ideal post-exercise timing for maximal muscle protein synthesis. According to Alan Albert Aragon and Brad Jon Schoenfeld [33]: “…the utility of acute studies is limited to providing clues and generating hypotheses regarding hypertrophic adaptations; any attempt to extrapolate findings from such data to changes in lean body mass is speculative, at best.”

Restoring Glycogen Stores

Glycogen is considered essential to optimal resistance training performance. Medbo and Tabata [34] reported that 82,7% of ATP production is provided by glycolysis. During resistance-training muscle glycogen stores can be significantly depleted [35], and in order to ensure the quality of your next workout, this depleted stores should be replenished. For example, 3 sets of 12 reps performed to muscular failure resulted in a 26.1% reduction of glycogen stores [36]. Also, Tech et al. [37] reported that high intensity, heavy resistance exercise of leg muscles reduced glycogen content of the vastus lateralis by 26%. So, a typical high volume workout with multiple exercises for the same muscle group would deplete most of glycogen storage. It is important to realize that following such exercise, there is little to no glycogen increase until adequate carbohydrate is ingested [38,31].

Studies suggest that delaying the ingestion of a carbohydrate supplement after exercise results in a reduced rate of muscle glycogen storage [31]. Therefore, it is adventous to ingest carbohydrates early after exercise as it provides an immediate source of substrate for muscle to use for glycogen resynthesis. Early intake of carbohydrates also takes advantage of increased muscle insulin sensitivity that is caused by muscle contraction [39,40]. This is in part so because decreased glycogen storage [41] and muscle contraction [48] both increase the translocation of GLUT4 from the intracellular storage sites to the surface membrane, which increases influx of glucose into the cell.

Suggestions have been made that combination of carbohydrate-protein supplement is more effective for the rapid replenishment of muscle glycogen after exercise than an equal carbohydrates alone or carbohydrates of same caloric content [42]. However, studies are equivocal [43,44]. The discrepancies among studies are explained by lower protein concentrations and smaller but more frequent doses of carbohydrates used [42]. It has been previously reported that large doses of carbohydrate provided at frequent intervals such as every 15 min have promoted glycogen storage rates considerably higher than those seen when supplementing at 2 hour intervals [45,46].

The theory behind glycogen replenishment is sound, however, it remains under question whether there is the need to accelerate glycogen resynthesis in practice. Without a doubt there are a few occasions where accelerated glycogen resynthesis is required. For example in endurance sports where the duration between glycogen-depleting events is less than 8 hours [47] or those who tend to perform two workouts per day. However, if that is not the case than the need to accelerate glycogen resynthesis is greatly diminished. Studies are showing that delayed ingestion of carbohydrates has no effect on muscle glycogen storage at 8 and 24 hours post-exercise [49].

Conclusion

Regardless of marketing claims that immediate nutrient ingestion post-exercise is required to maximize muscle gains, evidence to support such “anabolic window” is far from definitive. In a review of literature, Aragon and Schoenfeld [33] concluded that there is a lack of evidence to support a narrow “anabolic window of opportunity” whereby protein need to be consumed immediately after workout routine to maximize muscular adaptations. More recently, same authors carried out a comprehensive meta-analysis of 23 studies evaluating the effect of protein timing on muscle strength and hypertrophy [50]. They concluded that current evidence does not appear to support the claim that immediate (≤ 1 hour) consumption of protein pre- and/or post-workout significantly enhances strength- or hypertrophic-related adaptations to resistance exercise.

There are several limitations in this body of evidence. Some studies with null findings have employed small sample sizes which makes them underpowered, thus increasing the chance of type II error. Furthermore, lack of matched studies makes it difficult to draw firm conclusions in this regard and majority of studies on the topic have been carried out in untrained individuals.

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