Aragon and Schoenfeld Journal of the International Society of Sports Nutrition 2013, 10:5
                http://www.jissn.com/content/10/1/5
                 REVIEW                                                                                                      Open Access
                Nutrient timing revisited: is there a post-exercise
                anabolic window?
                Alan Albert Aragon1 and Brad Jon Schoenfeld2*
                  Abstract
                  Nutrient timing is a popular nutritional strategy that involves the consumption of combinations of
                  nutrients–primarily protein and carbohydrate–in and around an exercise session. Some have claimed that this
                  approach can produce dramatic improvements in body composition. It has even been postulated that the
                  timing of nutritional consumption may be more important than the absolute daily intake of nutrients. The
                  post-exercise period is widely considered the most critical part of nutrient timing. Theoretically, consuming the
                  proper ratio of nutrients during this time not only initiates the rebuilding of damaged muscle tissue and
                  restoration of energy reserves, but it does so in a supercompensated fashion that enhances both body
                  composition and exercise performance. Several researchers have made reference to an anabolic “window of
                  opportunity” whereby a limited time exists after training to optimize training-related muscular adaptations.
                  However, the importance - and even the existence - of a post-exercise ‘window’ can vary according to a
                  number of factors. Not only is nutrient timing research opentoquestionintermsofapplicability,butrecent
                  evidence has directly challenged the classical view of the relevance of post-exercise nutritional intake with
                  respect to anabolism. Therefore, the purpose of this paper will be twofold: 1) to review the existing literature
                  on the effects of nutrient timing with respect to post-exercise muscular adaptations, and; 2) to draw relevant
                  conclusions that allow practical, evidence-based nutritional recommendations to be made for maximizing the
                  anabolic response to exercise.
                Introduction                                                       well as causing damage to muscle fibers. Theoretically, con-
                Over the past two decades, nutrient timing has been the            suming the proper ratio of nutrients during this time not
                subject of numerous research studies and reviews. The              only initiates the rebuilding of damaged tissue and restor-
                basis of nutrient timing involves the consumption of combi-        ation of energy reserves, but it does so in a supercompen-
                nations of nutrients--primarily protein and carbohydrate--in       sated fashion that enhances both body composition and
                and around an exercise session. The strategy is designed to        exercise performance. Several researchers have made refer-
                maximize exercise-induced muscular adaptations and facili-         ence to an “anabolic window of opportunity” whereby a
                tate repair of damaged tissue [1]. Some have claimed that          limited time exists after training to optimize training-
                such timing strategies can produce dramatic improvements           related muscular adaptations [3-5].
                in body composition, particularly with respect to increases          However, the importance – and even the existence –
                in fat-free mass [2]. It has even been postulated that the tim-    of a post-exercise ‘window’ can vary according to a num-
                ing of nutritional consumption may be more important than          ber of factors. Not only is nutrient timing research open
                the absolute daily intake of nutrients [3].                        to question in terms of applicability, but recent evidence
                  The post-exercise period is often considered the most            has directly challenged the classical view of the relevance
                critical part of nutrient timing. An intense resistance train-     of post-exercise nutritional intake on anabolism. There-
                ing workout results in the depletion of a significant propor-      fore, the purpose of this paper will be twofold: 1) to re-
                tion of stored fuels (including glycogen and amino acids) as       view the existing literature on the effects of nutrient
                                                                                   timing with respect to post-exercise muscular adapta-
                                                                                   tions, and; 2) to draw relevant conclusions that allow
                * Correspondence: brad@workout911.com                              evidence-based nutritional recommendations to be made
                2
                 Department of Health Science, Lehman College, Bronx, NY, USA      for maximizing the anabolic response to exercise.
                Full list of author information is available at the end of the article
                                                 ©2013 Aragon and Schoenfeld; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the
                                                 Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use,
                                                 distribution, and reproduction in any medium, provided the original work is properly cited.
                  Aragon and Schoenfeld Journal of the International Society of Sports Nutrition 2013, 10:5                                            Page 2 of 11
                  http://www.jissn.com/content/10/1/5
                  Glycogen repletion                                                         or muscle protein synthesis (MPS) during the early (4 h)
                  Aprimary goal of traditional post-workout nutrient timing                  postexercise recovery period. The discrepancy between
                  recommendations is to replenish glycogen stores. Glycogen                  studies is not clear at this time.
                  is considered essential to optimal resistance training per-                  Glycogen availability also has been shown to mediate
                  formance, with as much as 80% of ATP production during                     muscle protein breakdown. Lemon and Mullin [19] found
                  such training derived from glycolysis [6]. MacDougall et al.               that nitrogen losses more than doubled following a bout
                  [7]demonstratedthatasinglesetofelbowflexionat80%                           of exercise in a glycogen-depleted versus glycogen-loaded
                  of 1 repetition maximum (RM) performed to muscular fail-                   state. Other researchers have displayed a similar inverse
                  ure caused a 12% reduction in mixed-muscle glycogen con-                   relationship between glycogen levels and proteolysis [20].
                  centration, while three sets at this intensity resulted in a               Considering the totality of evidence, maintaining a high
                  24% decrease. Similarly, Robergs et al. [8] reported that 3                intramuscular glycogen content at the onset of training
                  sets of 12 RM performed to muscular failure resulted in a                  appears beneficial to desired resistance training outcomes.
                  26.1% reduction of glycogen stores in the vastus lateralis                   Studies show a supercompensation of glycogen stores
                  while six sets at this intensity led to a 38% decrease, primar-            whencarbohydrate is consumed immediately post-exercise,
                  ily resulting from glycogen depletion in type II fibers com-               and delaying consumption by just 2 hours attenuates the
                  pared to type I fibers. It therefore stands to reason that                 rate of muscle glycogen re-synthesis by as much as 50%
                  typical high volume bodybuilding-style workouts involving                  [21]. Exercise enhances insulin-stimulated glucose uptake
                  multiple exercises and sets for the same muscle group                      following a workout with a strong correlation noted be-
                  would deplete the majority of local glycogen stores.                       tween the amount of uptake and the magnitude of glyco-
                    In addition, there is evidence that glycogen serves to me-               gen utilization [22]. This is in part due to an increase in
                  diate intracellular signaling. This appears to be due, at least            the translocation of GLUT4 during glycogen depletion
                  in part, to its negative regulatory effects on AMP-activated               [23,24] thereby facilitating entry of glucose into the cell. In
                  protein kinase (AMPK). Muscle anabolism and catabolism                     addition, there is an exercise-induced increase in the activ-
                  are regulated by a complex cascade of signaling pathways.                  ity of glycogen synthase—the principle enzyme involved in
                  Several pathways that have been identified as particularly                 promoting glycogen storage [25]. The combination of these
                  important to muscle anabolism include mammalian target                     factors facilitates the rapid uptake of glucose following an
                  of rapamycin (mTOR), mitogen-activated protein kinase                      exercise bout, allowing glycogen to be replenished at an
                  (MAPK), and various calcium- (Ca2+) dependent pathways.                    accelerated rate.
                  AMPK, on the other hand, is a cellular energy sensor that                    There is evidence that adding protein to a post-workout
                  serves to enhance energy availability. As such, it blunts                  carbohydrate meal can enhance glycogen re-synthesis.
                  energy-consuming processes including the activation of                     Berardi et al. [26] demonstrated that consuming a protein-
                  mTORC1 mediated by insulin and mechanical tension, as                      carbohydrate supplement in the 2-hour period following a
                  well as heightening catabolic processes such as glycolysis,                60-minute cycling bout resulted in significantly greater
                  beta-oxidation, and protein degradation [9]. mTOR is con-                  glycogen resynthesis compared to ingesting a calorie-
                  sidered a master network in the regulation of skeletal                     equated carbohydrate solution alone. Similarly, Ivy et al.
                  muscle growth [10,11], and its inhibition has a decidedly                  [27] found that consumption of a combination of protein
                  negative effect on anabolic processes [12]. Glycogen has                   and carbohydrate after a 2+ hour bout of cycling and
                  been shown to inhibit purified AMPK in cell-free assays                    sprintingincreasedmuscleglycogencontentsignificantly
                  [13], and low glycogen levels are associated with an                       more than either a carbohydrate-only supplement of
                  enhanced AMPK activity in humans in vivo [14].                             equal carbohydrate or caloric equivalency. The synergis-
                    Creer et al. [15] demonstrated that changes in the phos-                 tic effects of protein-carbohydrate have been attributed
                  phorylation of protein kinase B (Akt) are dependent on                     toamorepronouncedinsulinresponse[28],althoughit
                  pre-exercise muscle glycogen content. After performing 3                   should be noted that not all studies support these find-
                  sets of 10 repetitions of knee extensions with a load equat-               ings [29]. Jentjens et al. [30] found that given ample
                  ing to 70% of 1 repetition maximum, early phase post-                      carbohydrate dosing (1.2 g/kg/hr), the addition of a pro-
                  exercise Akt phosphorylation was increased only in the                     tein and amino acid mixture (0.4 g/kg/hr) did not in-
                  glycogen-loaded muscle, with no effect seen in the                         crease glycogen synthesis during a 3-hour post-depletion
                  glycogen-depleted contralateral muscle. Glycogen inhib-                    recoveryperiod.
                  ition also has been shown to blunt S6K activation, impair                    Despite a sound theoretical basis, the practical signifi-
                  translation, and reduce the amount of mRNA of genes re-                    cance of expeditiously repleting glycogen stores remains
                  sponsible for regulating muscle hypertrophy [16,17]. In                    dubious. Without question, expediting glycogen resynth-
                  contrast to these findings, a recent study by Camera et al.                esis is important for a narrow subset of endurance sports
                  [18] found that high-intensity resistance training with low                where the duration between glycogen-depleting events is
                  muscle glycogen levels did not impair anabolic signaling                   limited to less than approximately 8 hours [31]. Similar
                  Aragon and Schoenfeld Journal of the International Society of Sports Nutrition 2013, 10:5                                            Page 3 of 11
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                  benefits could potentially be obtained by those who per-                   time. It has been theorized that insulin-mediated phos-
                  form two-a-day split resistance training bouts (i.e. morn-                 phorylation of PI3K/Akt inhibits transcriptional activity
                  ing and evening) provided the same muscles will be                         of the proteolytic Forkhead family of transcription fac-
                  worked during the respective sessions. However, for                        tors, resulting in their sequestration in the sarcoplasm
                  goals that are not specifically focused on the perform-                    away from their target genes [44]. Down-regulation of
                  ance of multiple exercise bouts in the same day, the ur-                   other aspects of the ubiquitin-proteasome pathway are
                  gency of glycogen resynthesis is greatly diminished.                       also believed to play a role in the process [45]. Given
                  High-intensity resistance training with moderate volume                    that muscle hypertrophy represents the difference be-
                  (6-9 sets per muscle group) has only been shown to re-                     tween myofibrillar protein synthesis and proteolysis, a
                  duce glycogen stores by 36-39% [8,32]. Certain athletes                    decrease in protein breakdown would conceivably en-
                  are prone to performing significantly more volume than                     hance accretion of contractile proteins and thus facilitate
                  this (i.e., competitive bodybuilders), but increased vol-                  greater hypertrophy. Accordingly, it seems logical to
                  ume typically accompanies decreased frequency. For ex-                     conclude that consuming a protein-carbohydrate supple-
                  ample, training a muscle group with 16-20 sets in a                        ment following exercise would promote the greatest re-
                  single session is done roughly once per week, whereas                      duction in proteolysis since the combination of the two
                  routines with 8-10 sets are done twice per week. In sce-                   nutrients has been shown to elevate insulin levels to a
                  narios of higher volume and frequency of resistance                        greater extent than carbohydrate alone [28].
                  training, incomplete resynthesis of pre-training glycogen                    However, while the theoretical basis behind spiking in-
                  levels would not be a concern aside from the far-fetched                   sulin post-workout is inherently sound, it remains ques-
                  scenario where exhaustive training bouts of the same                       tionable as to whether benefits extend into practice.
                  muscles occur after recovery intervals shorter than 24                     First and foremost, research has consistently shown that,
                  hours. However, even in the event of complete glycogen                     in the presence of elevated plasma amino acids, the ef-
                  depletion, replenishment to pre-training levels occurs                     fect of insulin elevation on net muscle protein balance
                  well-within this timeframe, regardless of a significantly                  plateaus within a range of 15–30 mU/L [45,46]; roughly
                  delayed post-exercise carbohydrate intake. For example,                    3–4 times normal fasting levels. This insulinogenic effect
                  Parkin et al [33] compared the immediate post-exercise                     is easily accomplished with typical mixed meals, consid-
                  ingestion of 5 high-glycemic carbohydrate meals with a                     ering that it takes approximately 1–2 hours for circulat-
                  2-hour wait before beginning the recovery feedings. No                     ing substrate levels to peak, and 3–6 hours (or more) for
                  significant    between-group differences were seen in                      a complete return to basal levels depending on the size
                  glycogen levels at 8 hours and 24 hours post-exercise. In                  of a meal. For example, Capaldo et al. [47] examined
                  further support of this point, Fox et al. [34] saw no sig-                 various metabolic effects during a 5-hour period after
                  nificant reduction in glycogen content 24 hours after de-                  ingesting a solid meal comprised of 75 g carbohydrate
                  pletion despite adding 165 g fat collectively to the post-                 37 g protein, and 17 g fat. This meal was able to raise
                  exercise recovery meals and thus removing any potential                    insulin 3 times above fasting levels within 30 minutes of
                  advantage of high-glycemic conditions.                                     consumption. At the 1-hour mark, insulin was 5 times
                                                                                             greater than fasting. At the 5-hour mark, insulin was still
                  Protein breakdown                                                          double the fasting levels. In another example, Power
                  Another purported benefit of post-workout nutrient tim-                    et al. [48] showed that a 45g dose of whey protein isolate
                  ing is an attenuation of muscle protein breakdown. This                    takes approximately 50 minutes to cause blood amino
                  is  primarily achieved by spiking insulin levels, as                       acid levels to peak. Insulin concentrations peaked 40
                  opposed to increasing amino acid availability [35,36].                     minutes after ingestion, and remained at elevations seen
                  Studies show that muscle protein breakdown is only                         to maximize net muscle protein balance (15-30 mU/L,
                  slightly elevated immediately post-exercise and then                       or 104-208 pmol/L) for approximately 2 hours. The in-
                  rapidly rises thereafter [36]. In the fasted state, muscle                 clusion of carbohydrate to this protein dose would cause
                  protein breakdown is significantly heightened at 195                       insulin levels to peak higher and stay elevated even
                  minutes following resistance exercise, resulting in a net                  longer. Therefore, the recommendation for lifters to
                  negative protein balance [37]. These values are increased                  spike insulin post-exercise is somewhat trivial. The clas-
                  as much as 50% at the 3 hour mark, and elevated prote-                     sical post-exercise objective to quickly reverse catabolic
                  olysis can persist for up to 24 hours of the post-workout                  processes to promote recovery and growth may only be
                  period [36].                                                               applicable in the absence of a properly constructed pre-
                    Although insulin has known anabolic properties                           exercise meal.
                  [38,39], its primary impact post-exercise is believed to                     Moreover, there is evidence that the effect of protein
                  be anti-catabolic [40-43]. The mechanisms by which in-                     breakdown on muscle protein accretion may be over-
                  sulin reduces proteolysis are not well understood at this                  stated. Glynn et al. [49] found that the post-exercise
                  Aragon and Schoenfeld Journal of the International Society of Sports Nutrition 2013, 10:5                                            Page 4 of 11
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                  anabolic response associated with combined protein and                     the post-exercise ‘window’ is the finding by Tipton et al.
                  carbohydrate consumption was largely due to an eleva-                      [63] that immediate pre-exercise ingestion of the same
                  tion in muscle protein synthesis with only a minor influ-                  EAA-carbohydrate solution resulted in a significantly
                  ence from reduced muscle protein breakdown. These                          greater and more sustained MPS response compared to
                  results were seen regardless of the extent of circulating                  the immediate post-exercise ingestion, although the val-
                  insulin levels. Thus, it remains questionable as to what,                  idity of these findings have been disputed based on
                  if any, positive effects are realized with respect to muscle               flawed methodology [36]. Notably, Fujita et al [64] saw
                  growth from spiking insulin after resistance training.                     opposite results using a similar design, except the EAA-
                                                                                             carbohydrate was ingested 1 hour prior to exercise com-
                  Protein synthesis                                                          pared to ingestion immediately pre-exercise in Tipton
                  Perhaps the most touted benefit of post-workout nutri-                     et al. [63]. Adding yet more incongruity to the evidence,
                  ent timing is that it potentiates increases in MPS. Resist-                Tipton et al. [65] found no significant difference in net
                  ance training alone has been shown to promote a                            MPS between the ingestion of 20 g whey immediately
                  twofold increase in protein synthesis following exercise,                  pre- versus the same solution consumed 1 hour post-
                  which is counterbalanced by the accelerated rate of pro-                   exercise. Collectively, the available data lack any consist-
                  teolysis [36]. It appears that the stimulatory effects of                  ent indication of an ideal post-exercise timing scheme
                  hyperaminoacidemia on muscle protein synthesis, espe-                      for maximizing MPS.
                  cially from essential amino acids, are potentiated by pre-                   It also should be noted that measures of MPS assessed
                  vious exercise [35,50]. There is some evidence that                        following an acute bout of resistance exercise do not al-
                  carbohydrate has an additive effect on enhancing post-                     ways occur in parallel with chronic upregulation of
                  exercise muscle protein synthesis when combined with                       causative myogenic signals [66] and are not necessarily
                  amino acid ingestion [51], but others have failed to find                  predictive     of   long-term hypertrophic responses to
                  such a benefit [52,53].                                                    regimented resistance training [67]. Moreover, the post-
                    Several studies have investigated whether an “anabolic                   exercise rise in MPS in untrained subjects is not recapi-
                  window” exists in the immediate post-exercise period                       tulated in the trained state [68], further confounding
                  with respect to protein synthesis. For maximizing MPS,                     practical relevance. Thus, the utility of acute studies is
                  the evidence supports the superiority of post-exercise                     limited to providing clues and generating hypotheses
                  free amino acids and/or protein (in various permutations                   regarding hypertrophic adaptations; any attempt to ex-
                  with or without carbohydrate) compared to solely carbo-                    trapolate findings from such data to changes in lean
                  hydrate or non-caloric placebo [50,51,54-59]. However,                     body mass is speculative, at best.
                  despite the common recommendation to consume pro-
                  tein as soon as possible post-exercise [60,61], evidence-                  Muscle hypertrophy
                  based support for this practice is currently lacking.                      Anumberofstudies have directly investigated the long-
                  Levenhagen et al. [62] demonstrated a clear benefit to                     term hypertrophic effects of post-exercise protein con-
                  consuming nutrients as soon as possible after exercise as                  sumption. The results of these trials are curiously
                  opposed to delaying consumption. Employing a within-                       conflicting, seemingly because of varied study design
                  subject design,10 volunteers (5 men, 5 women) con-                         and methodology. Moreover, a majority of studies
                  sumed an oral supplement containing 10 g protein, 8 g                      employed both pre- and post-workout supplementation,
                  carbohydrate and 3 g fat either immediately following or                   making it impossible to tease out the impact of consum-
                  three hours post-exercise. Protein synthesis of the legs                   ing nutrients after exercise. These confounding issues
                  and whole body was increased threefold when the sup-                       highlight the difficulty in attempting to draw relevant
                  plement was ingested immediately after exercise, as                        conclusions as to the validity of an “anabolic window.”
                  compared to just 12% when consumption was delayed.                         What follows is an overview of the current research on
                  A limitation of the study was that training involved                       the topic. Only those studies that specifically evaluated
                  moderate intensity, long duration aerobic exercise. Thus,                  immediate (≤ 1 hour) post-workout nutrient provision
                  the increased fractional synthetic rate was likely due to                  are discussed (see Table 1 for a summary of data).
                  greater mitochondrial and/or sarcoplasmic protein frac-                      Esmarck et al. [69] provided the first experimental evi-
                  tions, as opposed to synthesis of contractile elements                     dence that consuming protein immediately after training
                  [36]. In contrast to the timing effects shown by Levenha-                  enhanced muscular growth compared to delayed protein
                  gen et al. [62], previous work by Rasmussen et al. [56]                    intake. Thirteen untrained elderly male volunteers were
                  showed no significant difference in leg net amino acid                     matched in pairs based on body composition and daily
                  balance between 6 g essential amino acids (EAA) co-                        protein intake and divided into two groups: P0 or P2.
                  ingested with 35 g carbohydrate taken 1 hour versus 3                      Subjects performed a progressive resistance training pro-
                  hours post-exercise. Compounding the unreliability of                      gram of multiple sets for the upper and lower body. P0