Nutrient timing revisited

Do you rush to gobble down a protein shake or munch on a protein bar immediately after your strenuous strength workout? Many people still believe that the post-workout ‘anabolic window’ is one of the most important factors that will make or break your muscular gains in the gym.

However, while there are undeniably times when you will make better use of particular nutrients, as usual, there is no one-size-fits-all approach that you completely ‘must’ follow to achieve such great results. We are going to explore the science and thinking around the ‘anabolic window’ and how you can use nutrient timing to maximize the results you get from your workouts.

What is the ‘anabolic window?’

Over the past few decades, beside the ongoing development and interest in exercise and sports science, there has been increasing academic focus on nutrient timing, sparking numerous research studies and reviews. The term ‘anabolic window’ as known as “muscle protein synthesis window” focus on timing specific nutrients, most often carbohydrates and protein around an exercise or training session.

The purpose of this nutritional approach is to maximize exercise-induced adaptations and accelerate the recovery of damaged tissue during the workout. It has been suggested that specific nutrient timing strategies provide several positive outcomes concerning improvements in body composition and may even be more important than overall calorie intake.

This ‘anabolic window’ has been defined as a limited period post-training to optimize strength training related muscular adaptations. We know that intense resistance training depletes stored fuel in the form of muscle glycogen and amino acids. It also results in damage to muscle fibers. Therefore, it makes sense that consuming a particular ratio of nutrients in the post-workout window helps repair damaged tissue, restore energy reserves, and improve body composition and performance.

What is the ‘nutrient timing?’

Nutrient timing is a popular nutritional strategy that involves the consumption of a mixture of nutrients, primarily carbohydrate and protein, in and around a workout session. Some have claimed that this nutritional approach can produce impressive 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 super compensated 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.

Introduction

Over the past 20 years, nutrient timing has been the subject of numerous research studies and reviews. The basis of nutrient timing involves the consumption of combinations of nutrients–primarily protein and carbohydrate–in and around an exercise session. The strategy is designed to maximize exercise-induced muscular adaptations and facilitate repair of damaged tissue.

Some have claimed that such timing strategies can produce dramatic improvements in body composition, particularly with respect to increases in fat-free mass. 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 often considered the most critical part of nutrient timing. An intense resistance training workout results in the depletion of a significant proportion of stored fuels (including glycogen and amino acids) as well as causing damage to muscle fibers.

Theoretically, consuming the proper ratio of nutrients during this time not only initiates the rebuilding of damaged tissue and restoration of energy reserves, but it does so in a super compensated 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 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 on anabolism.

Therefore, the purpose of this article will be twofold: 1) to review the existing studies on the effects of nutrient timing with respect to post-exercise muscular adaptations, and; 2) to draw relevant conclusions that allow evidence-based nutritional recommendations to be made for maximizing the anabolic response to exercise.

The Science Behind Post-Workout Nutrient Timing

Glycogen repletion

A primary goal of traditional post-workout nutrient timing recommendations is to replenish glycogen stores. Glycogen is considered essential to optimal strength training performance, with as much as 80% of ATP production during such training derived from glycolysis. MacDougall et al. demonstrated that a single set of elbow flexion at 80% of 1 repetition maximum (RM) performed to muscular failure caused a 12% reduction in mixed-muscle glycogen concentration, while three sets at this intensity resulted in a 24% decrease.

Similarly, Robergs et al. reported that 3 sets of 12 RM performed to muscular failure resulted in a 26.1% reduction of glycogen stores in the vastus lateralis while six sets at this intensity led to a 38% decrease, primarily resulting from glycogen depletion in type II fibers compared to type I fibers. It therefore stands to reason that typical high volume bodybuilding-style workouts involving multiple exercises and sets for the same muscle group would deplete the majority of local glycogen stores.

In addition, there is evidence that glycogen serves to mediate intracellular signaling. This appears to be due, at least in part, to its negative regulatory effects on AMP-activated protein kinase (AMPK). Muscle anabolism and catabolism are regulated by a complex cascade of signaling pathways. Several pathways that have been identified as particularly important to muscle anabolism include mammalian target of rapamycin (mTOR), mitogen-activated protein kinase (MAPK), and various calcium- (Ca2+) dependent pathways.

AMPK is a cellular energy sensor that serves to enhance energy availability. As such, it blunts energy-consuming processes including the activation of mTORC1 mediated by insulin and mechanical tension, as well as heightening catabolic processes such as glycolysis, beta-oxidation, and protein degradation. mTOR is considered a master network in the regulation of skeletal muscle growth, and its inhibition has a decidedly negative effect on anabolic processes. Glycogen has been shown to inhibit purified AMPK in cell-free assays, and low glycogen levels are associated with an enhanced AMPK activity in humans in vivo.

Creer et al. demonstrated that changes in the phosphorylation of protein kinase B (Akt) are dependent on pre-exercise muscle glycogen content. After performing 3 sets of 10 repetitions of knee extensions with a load equating to 70% of 1 repetition maximum, early phase post-exercise Akt phosphorylation was increased only in the glycogen-loaded muscle, with no effect seen in the glycogen-depleted contralateral muscle.

Glycogen inhibition also has been shown to blunt S6K activation, impair translation, and reduce the amount of mRNA of genes responsible for regulating muscle hypertrophy . In contrast to these findings, a recent study by Camera et al. found that high-intensity resistance training with low muscle glycogen levels did not impair anabolic signaling or muscle protein synthesis (MPS) during the early (4 h) postexercise recovery period. The discrepancy between studies is not clear at this time.

Glycogen availability also has been shown to mediate muscle protein breakdown. Lemon and Mullin found that nitrogen losses more than doubled following a bout of exercise in a glycogen-depleted versus glycogen-loaded state. Other researchers have displayed a similar inverse relationship between glycogen levels and proteolysis. Considering the totality of evidence, maintaining a high intramuscular glycogen content at the onset of training appears beneficial to desired resistance training outcomes.

Studies show a supercompensation of glycogen stores when carbohydrate is consumed immediately post-exercise, and delaying consumption by just 2 hours attenuates the rate of muscle glycogen re-synthesis by as much as 50%. Exercise enhances insulin-stimulated glucose uptake following a workout with a strong correlation noted between the amount of uptake and the magnitude of glycogen utilization.

This is in part due to an increase in the translocation of GLUT4 during glycogen depletion thereby facilitating entry of glucose into the cell. In addition, there is an exercise-induced increase in the activity of glycogen synthase—the principle enzyme involved in promoting glycogen storage. The combination of these factors facilitates the rapid uptake of glucose following an exercise bout, allowing glycogen to be replenished at an accelerated rate.

There is evidence that adding protein to a post-workout carbohydrate meal can enhance glycogen re-synthesis. Berardi et al. demonstrated that consuming a protein-carbohydrate supplement in the 2-hour period following a 60-minute cycling bout resulted in significantly greater glycogen resynthesis compared to ingesting a calorie-equated carbohydrate solution alone.

Similarly, Ivy et al. found that consumption of a combination of protein and carbohydrate after a 2+ hour bout of cycling and sprinting increased muscle glycogen content significantly more than either a carbohydrate-only supplement of equal carbohydrate or caloric equivalency.

The synergistic effects of protein-carbohydrate have been attributed to a more pronounced insulin response, although it should be noted that not all studies support these findings. Jentjens et al. found that given ample carbohydrate dosing (1.2 g/kg/hr), the addition of a protein and amino acid mixture (0.4 g/kg/hr) did not increase glycogen synthesis during a 3-hour post-depletion recovery period.

Despite a sound theoretical basis, the practical significance of expeditiously repleting glycogen stores remains dubious. Without question, expediting glycogen resynthesis is important for a narrow subset of endurance sports where the duration between glycogen-depleting events is limited to less than approximately 8 hours. Similar benefits could potentially be obtained by those who perform two-a-day split resistance training bouts (i.e. morning and evening) provided the same muscles will be worked during the respective sessions. However, for goals that are not specifically focused on the performance of multiple exercise bouts in the same day, the urgency of glycogen resynthesis is greatly diminished. High-intensity resistance training with moderate volume (6-9 sets per muscle group) has only been shown to reduce glycogen stores by 36-39%.

Certain athletes are prone to performing significantly more volume than this (i.e., competitive bodybuilders), but increased volume typically accompanies decreased frequency. For example, training a muscle group with 16-20 sets in a single session is done roughly once per week, whereas routines with 8-10 sets are done twice per week. In scenarios of higher volume and frequency of resistance training, incomplete resynthesis of pre-training glycogen levels would not be a concern aside from the far-fetched scenario where exhaustive training bouts of the same muscles occur after recovery intervals shorter than 24 hours.

However, even in the event of complete glycogen depletion, replenishment to pre-training levels occurs well-within this timeframe, regardless of a significantly delayed post-exercise carbohydrate intake. For example, Parkin et al compared the immediate post-exercise ingestion of 5 high-glycemic carbohydrate meals with a 2-hour wait before beginning the recovery feedings. No significant between-group differences were seen in glycogen levels at 8 hours and 24 hours post-exercise. In further support of this point, Fox et al. saw no significant reduction in glycogen content 24 hours after depletion despite adding 165 g fat collectively to the post-exercise recovery meals and thus removing any potential advantage of high-glycemic conditions.

Protein breakdown

Another purported benefit of post-workout nutrient timing is an attenuation of muscle protein breakdown. This is primarily achieved by spiking insulin levels, as opposed to increasing amino acid availability. Studies show that muscle protein breakdown is only slightly elevated immediately post-exercise and then rapidly rises thereafter.

In the fasted state, muscle protein breakdown is significantly heightened at 195 minutes following resistance exercise, resulting in a net negative protein balance. These values are increased as much as 50% at the 3 hour mark, and elevated proteolysis can persist for up to 24 hours of the post-workout period.

Although insulin has known anabolic properties, its primary impact post-exercise is believed to be anti-catabolic. The mechanisms by which insulin reduces proteolysis are not well understood at this time. It has been theorized that insulin-mediated phosphorylation of PI3K/Akt inhibits transcriptional activity of the proteolytic Forkhead family of transcription factors, resulting in their sequestration in the sarcoplasm away from their target genes.

Down-regulation of other aspects of the ubiquitin-proteasome pathway are also believed to play a role in the process. Given that muscle hypertrophy represents the difference between myofibrillar protein synthesis and proteolysis, a decrease in protein breakdown would conceivably enhance accretion of contractile proteins and thus facilitate greater hypertrophy.

Despite claims that immediate post-exercise nutritional intake is essential to maximize hypertrophic gains, evidence-based support for such an “anabolic window of opportunity” is far from definitive. The hypothesis is based largely on the pre-supposition that training is carried out in a fasted state. During fasted exercise, a concomitant increase in muscle protein breakdown causes the pre-exercise net negative amino acid balance to persist in the post-exercise period despite training-induced increases in muscle protein synthesis.

Muscle Hypertrophy

Various studies have sought to determine whether there is an advantage to muscle hypertrophy from post-workout nutrient timing. While several analyses have found slight advantages from post-workout consumption of protein, it’s not clear in many cases whether this is as a result of timing or simply increased protein intake.

Part of the problem with overestimating the importance of post-exercise nutrition intake is that it largely assumes that training always occurs in a fasted state. When you train in a fasted state, there is an increase in muscle protein breakdown that results in a net negative balance of amino acids, the building blocks for muscle growth that provide the parts for recovery. So, if you train first thing in the morning before eating, it makes sense to time your first meal in this post-workout window, ideally containing some carbohydrates and protein. Over time, this could result in increased gains.

However, there are plenty of people who don’t like training fasted. Let’s say you eat one to two hours before training. Depending on the size of the meal, it’s likely that this would count as both a pre- and post-meal given the time required to digest and absorb it. Further evidence also shows that consuming even minimal amounts of essential amino acids beforehand is sufficient to last through the training window and into the post-exercise period.

This indicates that anything consumed post-workout will do little to prevent catabolism (muscle breakdown). If this period were further extended, say if you train 4-6 hours after your last meal, post-exercise intake of carbohydrates and protein may be beneficial if muscle growth is your goal.

Of course, in all three scenarios, a final caveat is training age, which reflects an individual’s training history and potential for continued improvement. For the majority of people when they first start training, the stimulus is so new that nearly anything they do will produce gains. However, over time, this scope decreases, and as a result, protein timing and quality may become more important in advanced trainees.

Maximizing the window of opportunity

So, with these three considerations in mind, what recommendations can we give regarding post-workout nutrition?

1. A post-workout shake probably isn’t a bad idea for most people.

A post-workout protein shake provides the amino acids required to prevent catabolism or muscle protein breakdown. While you probably won’t need one if you ate immediately before training, if you train fasted in the morning or after several hours without eating, an immediate protein hit post-workout is likely beneficial from a muscle growth perspective. However, remember that total daily protein intake is the catalyst for hypertrophy rather than specific timing.

2. You are most insulin sensitive in the post-workout window.

Scientific research shows us that exercise, specifically muscle contraction activates the transport and uptake of glucose, a process that triggers an increase in insulin sensitivity. As a result, if improved body composition is your goal, you are likely to make the best use of any carbohydrates during this time. So, if you often train with around 4-5 hours since your last meal, a post-workout shake with a fast-acting form of carbohydrate, such as cyclic-dextrin could give you a slight edge.

3. A post-workout protein shake could be beneficial if you don’t have time for a solid meal.

In most instances, it’s better to make time for a solid meal that prioritizes high-nutrient density from single-ingredient sources within around one or two hours of training. However, the reality is that not everyone’s lifestyle will allow for this. If you’re often short on time post-workout, a high-quality protein shake will help maximize your returns.

4. A post-workout meal can help support performance if you exercise multiple times each day.

If you exercise multiple times a day, the post-workout window becomes more important. Let’s say you weight train in the morning, but you play sports in the evening. In this instance, you’re likely to improve your performance and recovery on those days by timing a meal containing some carbs and protein after each exercise bout.

Closing the windows

While a post-workout serving of protein and carbs certainly won’t hurt your gains, it’s probably not as important as most people believe either. From the perspective of glycogen depletion, muscle protein synthesis and hypertrophy, most of the benefits of post-workout nutrition seem to stem primarily from your last meal prior to training. Total daily protein intake is generally of greater importance than specific meal timing, so find a post-workout strategy that works for your lifestyle and personal preferences.

Practical Application

Though there is evidence that post-workout protein is important for muscle strength and growth, there is also evidence that pre-workout protein can eliminate the need for immediate post-exercise protein consumption. The findings suggest that protein ingestion can be based on other factors, such as preference, tolerance, convenience, and availability. The recommended amount of protein to consume surrounding training is 0.4-0.5 g/kg of lean body mass. For most individuals, 20-40 grams of protein is adequate. These needs can be met by consuming one serving of whey protein, four ounces of animal protein, or one cup of mixed plant proteins, such as rice and beans.

Overall, the evidence suggests that the anabolic window may not be as small as we once thought. The only time that post-workout nutrition is keenly important is when training is performed in the fasted state. If a pre-workout meal is consumed, post-workout nutrition does not need to occur as soon as you re-rack your weights. Though nutrient timing is important in some cases, total daily protein intake as well as usual protein intake are equally, if not more, important to muscular health. Nonetheless, there is little to no risk in consuming a protein shake immediately after a workout if daily protein intake is still in the acceptable range, so if you enjoy a shake post-workout, consume it, but for the taste and enjoyment of it rather than the perceived health benefits.

Key Takeaways

  • The term ‘anabolic window’ refers to timing specific nutrients – most often carbohydrates and protein  – around an exercise or training session.
  • Glycogen is the dominant substrate used during high-intensity exercise, but this is often only an issue for endurance sports or those training twice a day.
  • Insulin plays a role in shuttling nutrients to cells but, given that it takes time for food to digest and absorb, the foods you eat immediately around your workout probably don’t make that much difference.
  • There may be a slight edge to post-workout feeding from a hypertrophy perspective if you train fasted or several hours after your last meal.
  • A post-workout shake or meal certainly won’t harm your gains, but it’s your overall nutritional intake that makes the biggest overall difference to your gains.

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