Why Consume Carbs Around Training?

November 26, 2018

Carbohydrate consumption before and during a training session can improve training session performance. Post-training session consumption can improve liver and muscle glycogen resynthesis rates and magnitudes.

I tend to recommend ~50-70% of daily carbohydrate intake around training sessions (pre-, intra-, and post-training), and modestly different levels on rest days given less glycogen reduction from training.


There are at least a few good reasons.

Before providing those, let’s quickly review muscle metabolism as this will help bolster some justifiable reasons for carbohydrate consumption around training that follow.

The process of a neural activation signal from the nervous system resulting in muscle contraction has been referred to as excitation-contraction coupling. This involves an action potential originating in the motor cortex of the brain travelling down the spinal cord and peripheral motor nerve to the neuromuscular junction (i.e., where the motor nerve innervates the muscle membrane).

Once the neural signal arrives at the muscle, and particularly in the case of a sustained signal like during resistance exercise, a series of energy-expensive steps follow. Adenosine Triphosphate (ATP) is the energy currency of cells. ATP is made up of an adenine molecule, a ribose backbone, and three phosphate groups. When the high-energy phosphate bonds are broken, energy is released (i.e., ATP hydrolysis). This energy can be utilized for cellular processes pertinent to muscle contraction.

Since ATP is stored in a very limited amount at rest (e.g., only enough for a few seconds of muscle activity), ATP resynthesis depends on various enzyme-catalyzed reactions that utilize substrates stored in muscle cells or transported into the cell from the blood. An enzyme is a protein that catalyzes a chemical reaction. According to the laws of thermodynamics, reactions between substrates would eventually occur to achieve equilibrium, but, enzymes serve cells by enhancing the rate that reactions occur tremendously. Stated differently, enzymes allow reactions to proceed more quickly by joining substrates together to form a product, or dismantling chemical bonds to form separate products. In the case of our discussion, we are interested in forming ATP for effective muscle cell function during training from available substrate. To be clear, the lack of availability of substrate (e.g., glycogen) can negatively affect the process of resynthesizing ATP and result in impaired muscle function during exercise (e.g., fatigue).

Enzyme-catalyzed reactions in muscle that proceed at various rates to resynthesize ATP have been clustered together and referred to as “bioenergetic systems” for simplicity. The three primary energy systems in skeletal muscle cells can be referred to as the: a) ATP-PCr system, b) glycolytic system, and c) oxidative system. Three critical sites of ATP use in skeletal muscle cells relevant to muscle performance during training are: 1.) Na+-K+ ATPase, 2.) Ca2+ATPase, and 3.) Myosin ATPase. These three sites functioning properly depends on the availability of ATP to effectively couple the neural signal from the nervous system to skeletal muscle contraction. Briefly, Na+-K+ ATPase functions to regulate the electrical charge inside of the muscle cell for effective excitation-contraction coupling. Ca2+ATPase functions to regulate calcium release and storage for contraction. Myosin ATPase is a contractile protein directly involved with force production. Each energy system can supply ATP using available substrate at different rates. The ATP-PCr system refers to near-immediate reactions at the onset of exercise and primarily contributes to ATP resynthesis for up to the first 20-30sof exercise utilizing muscle phosphocreatine stores (PCr). The ATP-PCr system primarily involves the cleavage of a phosphate from phosphocreatine (PCr) and supply to an ADP molecule which is now missing a phosphate due to hydrolysis at various sites. I plan to write a future post on creatine supplementation that will relate to this and provide some insight into how creatine supplementation works and why it can be so effective.

The glycolytic system refers to a series of reactions that results in ATP resynthesis primarily after the first 20-30s of sustained exercise and utilizes glucose mobilized from stored muscle glycogen or from the blood to resynthesize ATP. The glycolytic system is primarily active after the first 20-30s of sustained contraction at a high intensity or power output (e.g., >~70% VO2 max). However, in the case of interval training or resistance training where bouts of work are broken up into different durations of contractile activity, the glycolytic system begins to play a more critical role during training sessions. The oxidative system refers to a series of reactions in the mitochondria that result in ATP resynthesis and primarily contributes to ATP production after the first few minutes of sustained contraction at submaximal work rates, but is also critical for between set- and between-session recovery. I also intend to write a future article on the interplay between systems and how it’s important to not view these in isolation as they are each active to lesser or greater extents all the time. Also, they largely depend on one another for proper function and shouldn’t be viewed as mutually exclusive (e.g.., PCr Shuttle). For those of you wanting extra reading on glycogen specifically, please see the recent magnificent review from Murray and Rosenbloom. Also, my favorite resource on exercise bioenergetics is the Exercise Physiology textbook written by Scott Powers and Edward Howley.

To stay focused on the point of the current article and start to bridge the gap, one must not simply consider what’s happening within the set bioenergetically, but also between sets. That is, ATP-costly processes also occur once a working set is terminated that can reduce glycogen stores or utilize blood glucose and not only within the set. This key point is why some have mistakenly leveled arguments against the importance of carbs during or around training since resistance training sets typically only last 10-30 seconds, thinking that the ATP-PCr is the only important system. However, it’s not only during sets that ATP is being utilized but also between sets. Effective resynthesis, therefore, should be considered between sets as well.

This is why strategically drinking carbs during the session can be of benefit, but this practice depends on a host of factors I’m not going to discuss in detail in this post (e.g., diet focus, phase of training, training volume, etc.). In short, defending blood glucose and muscle glycogen is a worthwhile consideration for any athlete looking to maximize performance both in training and competition. With a brief survey of muscle metabolism out of the way, below are some key reasons why carbohydrate consumption around training is worth consideration.

(Note that the points below are more relevant for higher-volume routines, but most forms of resistance training can benefit from proper peri-training carbohydrate timing strategies like this. Also, I must mention the often forgotten fuel lactate. Lactate can be utilized as a fuel sourceby the brain, heart, and adjacent muscle for oxidation and gluconeogenesis in the liver so this is meant to be more of a general survey but is quite nuanced and contextual. Specific strategies should be considered for different athletes and in different contexts, and this is why having a qualified professional program or consult for specific situations can be very helpful. There are reasons why intentionally training with low glycogen storesand manipulating intakes should be considered for different athletes, so this is meant to apply more generally to strength athletes or power athletes interested in maximizing force production and/or muscle growth.)

Practical reasons to consume carbs before, during, and after training:

1.) To elevate pre-training levels of blood glucose to provide fuel for the nervous system and muscle glycolytic activity.

Consequently, this increases glucose availability for muscle cell carbohydrate metabolism during the training session.

This can improve muscle’s ability to generate ATP (i.e., energy currency of the cell required for muscle contraction) during training and thereby improve the likelihood of effective force production.

The central nervous system loves glucose. Ensuring blood levels are elevated and defended can improve the likelihood the nervous system will function at desired capacity.

A recent study concluded: “The findings of this study suggest that pre-exercise meals with varying quantity and quality of carbohydrates (CHO) can have an effect on central nervous system fatigue (CF), where greater CHO oxidation and insulin response found in both high CHO and high glycemic index lead to attenuation of CF.”

2.) To defend blood glucose levels during training.

(similar reasons to above)

3.) Provide glucose for liver and muscle  glycogen resynthesis post-training when these tissues are sensitive.

A series of physiological responses occur in response to resistance training that prime the body to partition consumed carbohydrates into the liver and muscle tissue after training (e.g., hormonal, molecular).

One of the most powerful is that of GLUT4 translocation (i.e., moves from the intracellular environment to the membrane) in muscle cells.

This is a protein that pulls glucose into muscle cells and improves glycogen resynthesis post-training.

It turns out that GLUT4 protein expression increases at least for a few hoursafter training, with expression being dependent upon the volume and intensity of the training session.

In general, the more glycogen-depleting the session, the more likely we’ll observe higher GLUT4 expression.

This improves the likelihood that consumed carbohydrate for 3-6 hours after training will be drawn into muscle cells and will be less likely to fuel fat cell expansion.

Here are some potential strategies to consider:

1.) Budget in to your total carbohydrate intake for the day a pre-training, low-fat, high-glycemic index carbohydrate serving of between 30-60 grams. Gatorade, cereal, and other fast-digesting sources work nicely for this. Consuming this ~15-60 minutes prior to training, with 15 minutes relating more to liquid meals, and ~30-60 minutes or more to solid foods that still digest quite quickly. The key with timing is ensuring glucose levels are elevated and defended when training begins.

2.) Easily half but potentially more of your total CHO intake should typically be placed after training, depending upon the nature of training (i.e., how much glycogen use occurs). ~1g/kg/hr is ~as much glucose that can be absorbed in this timeframe. Depending upon the appropriate amount for you to be consuming daily based on your goals, placing much of it in the 6 hour post-workout period is a good idea while noting this top-end absorption likelihood per hour.

This means that if I have ~300g allotted for the day, and I weigh 100kg, my intake might look something like this.

Meal 1: 50g (pre-training)

Meal 2: 100g (post-training)

Meal 3 (a few hours later): 100g

Meal 4: 50g (dinner)

Hope you found this helpful!

Feel free to reach out with any questions or clarifications.