Nutrient Timing Revisited Is There A Post Exercise Anabolic Window
Nutrient Timing Revisited Is There A Post Exercise Anabolic Window
Nutrient Timing Revisited Is There A Post Exercise Anabolic Window
To cite this article: Alan Albert Aragon & Brad Jon Schoenfeld (2013) Nutrient timing revisited:
is there a post-exercise anabolic window?, Journal of the International Society of Sports
Nutrition, 10:1, 5, DOI: 10.1186/1550-2783-10-5
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 open to question in terms of applicability, but recent
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.
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
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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. A number of studies 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
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Aragon and Schoenfeld Journal of the International Society of Sports Nutrition 2013, 10:5
Table 1 Post-exercise nutrition and muscle hypertrophy
Study Subjects Supplementation Protein matched Measurement Training protocol Results
with Control? instrument
Esmarck 13 untrained 10 g milk/soy protein combo consumed Yes MRI and muscle Progressive resistance training Significant increase in muscle CSA
et al. [69] elderly males either immediately or 2 hours after exercise biopsy consisting of multiple sets of lat with immediate vs. delayed
pulldown, leg press and knee supplementation
extension performed 3 days/wk
for 12 wk
Cribb and 23 young 1 g/kg of a supplement containing 40 g Yes DXA and muscle Progressive resistance training Significant increases in lean body
Hayes [70] recreational male whey isolate, 43 g glucose, and 7 g creatine biopsy consisting of exercises for the mass and muscle CSA of type II
bodybuilders monohydrate consumed either immediately major muscle groups performed fibers in immediate vs. delayed
before and after exercise or in the early 3 days/wk for 10 wks supplementation
morning and late evening
Willoughby 19 untrained 20 g protein or 20 g dextrose consumed No Hydrostatic Progressive resistance training Significant increase in total body
et al. [71] young males 1 hour before and after exercise weighing, muscle consisting of 3 sets of 6–8 repetitions mass, fat-free mass, and thigh
biopsy, surface for all the major muscles performed mass with protein vs. carb
measurements 4 days/wk for 10 wks supplementation
Hulmi 31 untrained 15 g whey isolate or placebo consumed No MRI, muscle Progressive, periodized total body Significant increase in CSA of the
et al. [72] young males immediately before and after exercise biopsy resistance training consisting of 2–5 vastus lateralis but not of the
sets of 5–20 repetitions performed other quadriceps muscles in
2 days/wk for 21 wks. supplemented group versus
placebo.
Verdijk 28 untrained 10 g casein hydrolysate or placebo No DXA, CT, and Progressive resistance training consisting No significant differences in
et al. [73] elderly males consumed immediately before and after muscle biopsy of multiple sets of leg press and knee muscle CSA between groups
exercise extension performed 3 days/wk for
12 wks
Hoffman 33 well-trained Supplement containing 42 g protein (milk/ Yes DXA Progressive resistance training consisting No significant differences in total
et al. [74] young males collagen blend) and 2 g carbohydrate of 3–4 sets of 6–10 repetitions of multiple body mass or lean body mass
consumed either immediately before and exercises for the entire body peformed 4 between groups.
after exercise or in the early morning and days/wk for 10 weeks.
late evening
Erskine 33 untrained 20 g high quality protein or placebo No MRI 4-6 sets of elbow flexion performed No significant differences in
et al. [75] young males consumed immediately before and after 3 days/wk for 12 weeks muscle CSA between groups
exercise
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received an oral protein/carbohydrate supplement im- untrained male subjects were randomly assigned to
mediately post-exercise while P2 received the same sup- either receive 20 g of protein or 20 grams dextrose
plement 2 hours following the exercise bout. Training administered 1 hour before and after resistance exercise.
was carried out 3 days a week for 12 weeks. At the end Training consisted of 3 sets of 6–8 repetitions at 85%–
of the study period, cross-sectional area (CSA) of the 90% intensity. Training was performed 4 times a week
quadriceps femoris and mean fiber area were signifi- over the course of 10 weeks. At the end of the study
cantly increased in the P0 group while no significant period, total body mass, fat-free mass, and thigh mass
increase was seen in P2. These results support the pres- was significantly greater in the protein-supplemented
ence of a post-exercise window and suggest that delaying group compared to the group that received dextrose.
post-workout nutrient intake may impede muscular Given that the group receiving the protein supplement
gains. consumed an additional 40 grams of protein on training
In contrast to these findings, Verdijk et al. [73] failed to days, it is difficult to discern whether results were due
detect any increases in skeletal muscle mass from con- to the increased protein intake or the timing of the
suming a post-exercise protein supplement in a similar supplement.
population of elderly men. Twenty-eight untrained sub- In a comprehensive study of well-trained subjects,
jects were randomly assigned to receive either a protein or Hoffman et al. [74] randomly assigned 33 well-trained
placebo supplement consumed immediately before and males to receive a protein supplement either in the
immediately following the exercise session. Subjects per- morning and evening (n = 13) or immediately before and
formed multiple sets of leg press and knee extension 3 immediately after resistance exercise (n = 13). Seven par-
days per week, with the intensity of exercise progressively ticipants served as unsupplemented controls. Workouts
increased over the course of the 12 week training period. consisted of 3–4 sets of 6–10 repetitions of multiple
No significant differences in muscle strength or hyper- exercises for the entire body. Training was carried out
trophy were noted between groups at the end of the study on 4 day-a-week split routine with intensity progres-
period indicating that post exercise nutrient timing strat- sively increased over the course of the study period.
egies do not enhance training-related adaptation. It should After 10 weeks, no significant differences were noted be-
be noted that, as opposed to the study by Esmark et al. tween groups with respect to body mass and lean body
[69] this study only investigated adaptive responses of sup- mass. The study was limited by its use of DXA to assess
plementation on the thigh musculature; it therefore is not body composition, which lacks the sensitivity to detect
clear based on these results whether the upper body might small changes in muscle mass compared to other im-
respond differently to post-exercise supplementation than aging modalities such as MRI and CT [76].
the lower body. Hulmi et al. [72] randomized 31 young untrained male
In an elegant single-blinded design, Cribb and Hayes subjects into 1 of 3 groups: protein supplement (n = 11),
[70] found a significant benefit to post-exercise protein non-caloric placebo (n = 10) or control (n = 10). High-
consumption in 23 recreational male bodybuilders. Sub- intensity resistance training was carried out over 21
jects were randomly divided into either a PRE-POST weeks. Supplementation was provided before and after
group that consumed a supplement containing protein, exercise. At the end of the study period, muscle CSA
carbohydrate and creatine immediately before and after was significantly greater in the protein-supplemented
training or a MOR-EVE group that consumed the same group compared to placebo or control. A strength of the
supplement in the morning and evening at least 5 hours study was its long-term training period, providing sup-
outside the workout. Both groups performed regimented port for the beneficial effects of nutrient timing on
resistance training that progressively increased intensity chronic hypertrophic gains. Again, however, it is unclear
from 70% 1RM to 95% 1RM over the course of 10 weeks. whether enhanced results associated with protein sup-
Results showed that the PRE-POST group achieved a sig- plementation were due to timing or increased protein
nificantly greater increase in lean body mass and increased consumption.
type II fiber area compared to MOR-EVE. Findings sup- Most recently, Erskine et al. [75] failed to show a
port the benefits of nutrient timing on training-induced hypertrophic benefit from post-workout nutrient timing.
muscular adaptations. The study was limited by the Subjects were 33 untrained young males, pair-matched
addition of creatine monohydrate to the supplement, for habitual protein intake and strength response to a 3-
which may have facilitated increased uptake following week pre-study resistance training program. After a 6-
training. Moreover, the fact that the supplement was taken week washout period where no training was performed,
both pre- and post-workout confounds whether an ana- subjects were then randomly assigned to receive either a
bolic window mediated results. protein supplement or a placebo immediately before and
Willoughby et al. [71] also found that nutrient timing after resistance exercise. Training consisted of 6– 8 sets
resulted in positive muscular adaptations. Nineteen of elbow flexion carried out 3 days a week for 12 weeks.
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No significant differences were found in muscle volume On the other hand, there are others who might train
or anatomical cross-sectional area between groups. before lunch or after work, where the previous meal was
finished 4–6 hours prior to commencing exercise. This
Discussion lag in nutrient consumption can be considered signifi-
Despite claims that immediate post-exercise nutritional cant enough to warrant post-exercise intervention if
intake is essential to maximize hypertrophic gains, muscle retention or growth is the primary goal. Layman
evidence-based support for such an “anabolic window of [77] estimated that the anabolic effect of a meal lasts 5-6
opportunity” is far from definitive. The hypothesis is based hours based on the rate of postprandial amino acid me-
largely on the pre-supposition that training is carried out tabolism. However, infusion-based studies in rats [78,79]
in a fasted state. During fasted exercise, a concomitant in- and humans [80,81] indicate that the postprandial rise
crease in muscle protein breakdown causes the pre- in MPS from ingesting amino acids or a protein-rich
exercise net negative amino acid balance to persist in the meal is more transient, returning to baseline within 3
post-exercise period despite training-induced increases in hours despite sustained elevations in amino acid avail-
muscle protein synthesis [36]. Thus, in the case of resist- ability. It thus has been hypothesized that a “muscle full”
ance training after an overnight fast, it would make sense status can be reached where MPS becomes refractory,
to provide immediate nutritional intervention--ideally in and circulating amino acids are shunted toward oxida-
the form of a combination of protein and carbohydrate-- tion or fates other than MPS. In light of these findings,
for the purposes of promoting muscle protein synthesis when training is initiated more than ~3–4 hours after
and reducing proteolysis, thereby switching a net catabolic the preceding meal, the classical recommendation to
state into an anabolic one. Over a chronic period, this tac- consume protein (at least 25 g) as soon as possible
tic could conceivably lead cumulatively to an increased seems warranted in order to reverse the catabolic state,
rate of gains in muscle mass. which in turn could expedite muscular recovery and
This inevitably begs the question of how pre-exercise growth. However, as illustrated previously, minor pre-
nutrition might influence the urgency or effectiveness of exercise nutritional interventions can be undertaken if a
post-exercise nutrition, since not everyone engages in significant delay in the post-exercise meal is anticipated.
fasted training. In practice, it is common for those with An interesting area of speculation is the generalizability
the primary goal of increasing muscular size and/or of these recommendations across training statuses and
strength to make a concerted effort to consume a pre- age groups. Burd et al. [82] reported that an acute bout of
exercise meal within 1-2 hours prior to the bout in at- resistance training in untrained subjects stimulates both
tempt to maximize training performance. Depending on mitochondrial and myofibrillar protein synthesis, whereas
its size and composition, this meal can conceivably func- in trained subjects, protein synthesis becomes more pre-
tion as both a pre- and an immediate post-exercise meal, ferential toward the myofibrillar component. This suggests
since the time course of its digestion/absorption can per- a less global response in advanced trainees that potentially
sist well into the recovery period. Tipton et al. [63] warrants closer attention to protein timing and type (e.g.,
observed that a relatively small dose of EAA (6 g) taken high-leucine sources such as dairy proteins) in order to
immediately pre-exercise was able to elevate blood and optimize rates of muscular adaptation. In addition to
muscle amino acid levels by roughly 130%, and these training status, age can influence training adaptations. Eld-
levels remained elevated for 2 hours after the exercise erly subjects exhibit what has been termed “anabolic re-
bout. Although this finding was subsequently challenged sistance,” characterized by a lower receptivity to amino
by Fujita et al. [64], other research by Tipton et al. [65] acids and resistance training [83]. The mechanisms under-
showed that the ingestion of 20 g whey taken immedi- lying this phenomenon are not clear, but there is evidence
ately pre-exercise elevated muscular uptake of amino that in younger adults, the acute anabolic response to pro-
acids to 4.4 times pre-exercise resting levels during exer- tein feeding appears to plateau at a lower dose than in eld-
cise, and did not return to baseline levels until 3 hours erly subjects. Illustrating this point, Moore et al. [84]
post-exercise. These data indicate that even minimal-to- found that 20 g whole egg protein maximally stimulated
moderate pre-exercise EAA or high-quality protein post-exercise MPS, while 40 g increased leucine oxidation
taken immediately before resistance training is capable without any further increase in MPS in young men. In
of sustaining amino acid delivery into the post-exercise contrast, Yang et al. [85] found that elderly subjects dis-
period. Given this scenario, immediate post-exercise played greater increases in MPS when consuming a post-
protein dosing for the aim of mitigating catabolism exercise dose of 40 g whey protein compared to 20 g.
seems redundant. The next scheduled protein-rich These findings suggest that older subjects require higher
meal (whether it occurs immediately or 1–2 hours individual protein doses for the purpose of optimizing the
post-exercise) is likely sufficient for maximizing recov- anabolic response to training. Further research is needed
ery and anabolism. to better assess post-workout nutrient timing response
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1
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doi:10.1186/1550-2783-10-5
Cite this article as: Aragon and Schoenfeld: Nutrient timing revisited: is
there a post-exercise anabolic window?. Journal of the International
Society of Sports Nutrition 2013 10:5.