It has long been known that carbohydrate is an essential fuel source for exercise, in particular exercise at high intensities where anaerobic energy processes dominate and during prolonged aerobic exercise.
Carbohydrates are stored in the body in the form of glycogen. Our muscles have an average or typical capacity to store around 400 grams of glycogen, while our liver stores around 80 to 100 grams. Of course, the greater a person’s muscle mass the greater capacity to store glycogen. Concerning glucose, we have a small amount, of around 5 grams circulating in the blood.
The importance of carbohydrates for exercise performance
The importance of carbohydrate availability
Metabolic regulation
The concept of carb coding and fueling for the work required
With a total storage capacity of an average of 500 grams, you may think of this as being quite high but when we compare this value to fat stores, fat greatly outnumbers glycogen in terms of total available energy. Fat is more energy-dense meaning that each gram stored yields 9 calories, in comparison, each gram of carbohydrate only yields 4 calories. An 80kg male with 15% body fat has 12,000 grams of fat (12kg), or 108,000 calories. The 2,000 calories of carbohydrates are nothing in reality.
The above does not mean we should not consume carbohydrates, rather it highlights the importance of fueling your energy requirements in terms of exercise and recovery. There are regulatory factors that impact fuel use during exercise, with many impacted upon by carbohydrate availability and the composition of our diet around exercise.
Early studies and the advancement of muscle biopsies by Scandinavian researchers confirmed that our stores of carbohydrates deplete following exercise, and that the consumption of a high carbohydrate diet restored these levels to baseline or close to baseline. They also highlighted that pre-exercise intakes of carbohydrates increase time to fatigue.
Carbs are king when it comes to attaining and maintaining high levels of performance.
Carbohydrate availability refers to the amount of readily available carbohydrates for use during a given physical task. If you complete an exhaustive bout of exercise, you will deplete both muscle and liver glycogen stores by a given amount depending on the duration and intensity of exercise. If these stores are not replenished through diet, then a state of low carbohydrate availability will occur.
Additionally, if you follow a habitual (or structured) low carbohydrate diet, a similar state of low carbohydrate availability would gradually present as carbohydrates are not just selectively used during exercise. Even on the day-to-day you will utilize a given amount of carbohydrate, however, the extent of this will vary between individuals. A good example of carbohydrate use outside of exercise is when we are sleeping. Upon waking our liver glycogen stores are depleted by close to 80% as our brain uses up these stores during the restorative process called sleep!
Carbohydrate availability can be improved by, as you may have guessed, increasing the intake of carbohydrates in our diet. Our muscle and liver stores will then increase as a result. This would be termed as our endogenous stores. Before exercise, we can consume a carbohydrate rich meal as a means of metabolic regulation to preferentially utilize a greater percentage of carbohydrates during said exercise. This consumption of carbohydrates blunts the lipolytic pathways and thus favors carbohydrate oxidation.
In conjunction, while we exercise, carbohydrate availability can be maintained through exogenous consumption of carbohydrates i.e. through the consumption of energy gels etc. This mainly concerns those who compete in exercise lasting >60 minutes.
Concerning diet composition, if you follow a diet that contains adequate carbohydrates to support your fueling needs you will benefit from an increased capacity or affinity for glycogen storage and thus exercise capacity. An interesting physiological response to a glycogen-depleting bout of exercise is that of a super-compensation effect whereby there is an enhanced capacity for the resynthesis of muscle glycogen stores the extent of which is dependent on carbohydrate availability. Therefore, the consumption of carbohydrates in the two hour window post exercise is essential. Waiting longer than this will mean a slower rate of synthesis.
Several potential sites of control can regulate the interaction between carbohydrate and fat utilization during exercise and the extent or ratios of use of either fuel source. They aren’t like light switches that you can switch between but rather dimmer switches where one can dominate, or they can be pretty even.
These sites of control include the availability of intra-muscular and extra-muscular substrates, the actions of key hormones such as insulin, the abundance of transport proteins along with the activity of key enzymes involved in the metabolic pathways. From a physiological perspective, key factors such as exercise intensity, duration, nutritional status, and training status all play a role in the regulation of fuel utilization during exercise. High intensity exercise that is anaerobically dominant will use carbohydrates as fuel – think sprinting, high-speed running, and weight lifting.
Diet is of course a major impacting factor on the fuel used during exercise as it impacts substrate availability and the action of certain hormones involved.
The effects of diet are both acute and chronic. While on the other hand exercise is a key component to the physiological adaptation that occurs and the level of performance that can be attained. Both diet and exercise are not mutually exclusive things, and the composition of the diet can impact the adaptations that occur.
With the knowledge that diet composition impacts on the adaptive response to training and the performance level, diet can then be manipulated in favor of the training adaptation desired for the level of performance that is being targeted. This is nutritional periodization of sorts that changes daily, weekly, or monthly according to the training cycles.
A traffic light system has been developed to help athletes better manage their diet to fuel for the work required, to support high performance, immune health and the maintenance or attainment of a particular body composition as it serves as a means to alter energy intake, easily. Training days or sessions with high carbohydrate demands would be categorized as “green days” or “green meals” meaning the meals on that high-demand day or meals surrounding (pre, during and post) those high demand sessions would include high amounts of carbohydrate for the work required and to support recovery.
Training days or sessions with moderate carbohydrate demands would be categorized as “amber days” or “amber meals” meaning the meals on that moderate demand day or the meals surrounding those moderate demand sessions would include lower amounts (albeit adequate amounts) of carbohydrates for the work required and to support recovery. Training days or sessions with low carbohydrate demands, such as rest days, would be categorized as “red days” or “red meals” meaning the meals on that low-demand day or the meals surrounding those low-demand sessions (slow/light regenerative type exercise) would include low amounts of carbohydrate. These may also serve a purpose concerning improved fat oxidation rates and the related physiological adaptations that occur from training in a state of low carbohydrate availability. Caution must be warranted with the use of training in such a state over the long term and at intensities that are too high.
Understanding the energetic demands and physical demands/constraints of the exercise you're doing is essential.
Carbs are king, there is no other way of putting it. If you want to excel you need to ensure you are fueling appropriately for high-performance attainment and maintenance.