The role of carbohydrates in sports
Carbohydrates represent the only substrate among the macronutrients capable of generating ATP ( adenosine triphosphate) in an anaerobic way, in the absence of oxygen.
This phenomenon is evident in the execution of maximal exercises that require the release of energy above the levels provided by the aerobic metabolism. Per activities with an intensity greater than 70% of VO 2 max (maximum oxygen consumption), carbohydrates represent the main energy substrate. The athlete's performance is therefore strongly influenced by the levels of muscle and liver glycogen with which he faces the competition.
During activity, with the depletion of glycogen stores, the ability to prolong Energy exercise is significantly reduced. Similarly, even in a prolonged exercise such as the marathon , athletes often suffer from fatigue associated with the progressive depletion of hepatic and muscle glycogen.
That said, it should be borne in mind that muscle glycogen stores are limited and largely depend on the athlete's nutritional status, body composition and degree of training. Having a greater quantity of glycogen available is therefore also useful in all those sports of duration in which there is a strong depletion of muscle reserves (for example, cycling and marathon) where they are carried out with high intensity. With the same oxygen consumed, glucose (deriving from glycogen) is in fact able to produce a higher amount of energy than fatty acids.
Unfortunately, while the fat reserves are considerable (we are talking about kg), the glycogen reserves are limited (about 400/500 g). A depletion of glycogen reserves therefore inevitably results in a decrease in performance ! The body's glucose deposits, that is:
- glycogen: 300-400 grams in skeletal muscles and 80-100 grams in the liver;
- glucose: 2-6 grams in the blood.
They are therefore of fundamental importance also in aerobic metabolism. A sufficient intake of carbohydrates (along with optimal hydration) is certainly the most important factor in relation to pre-workout or pre-competition nutritional preparation.
During intense training or competition, the intensity and volume of activity are progressively reduced in parallel with the depletion of glycogen reserves . The athlete's ability to concentrate also tends to decline. All this is associated with:
- onset of fatigue,
- increased catabolism of muscle tissue,
- reduction of immune defenses.
The reduction of body carbohydrate reserves is associated with the appearance of a sense of fatigue and consequently with a lower intensity of work up to the impossibility of continuing it (exhaustion). As the duration of exercise increases, the energy contribution provided by muscle glycogen is progressively reduced and that of blood glucose and lipids (triglycerides and free fatty acids) proportionally increases. When muscle glycogen and blood glucose become insufficient to guarantee more or less 40% of the total energy demand, fatigue begins to arise.
Nutrition for athletes: how to manage it
Per background or long-lasting activities - but also for activities conducted at high intensity - a balanced diet therefore becomes such an important parameter that it can affect the final result . In fact, for such events, it is essential to be in the best condition from an energy point of view, that is to optimize the stores of muscle glycogen, the main fuel of the body.
The supercompensation of glycogen
In the international scientific literature, the concept that the supercompensation of pre-exercise muscle glycogen ( Fueling up ) can improve performance, especially in activities lasting more than 90 minutes, has found consensus . Muscle glycogen loading is related to a strategy that leads to an unusual and high glycogen storage in the muscles. Starting from the first studies in the late 60s, numerous authors (Scandinavian school in primis), following experimentation, were able to affirm that a diet with a good intake of carbohydrates in the days preceding the sporting activity allows to optimize even jobs lactacid type. In these cases, however, the pre-competition carbohydrate meal was not considered useful, unless it was a high intensity competition but not more than 90% of the VO 2 max and lasting more than 30/40 minutes.
In any case, the experiments conducted made it possible to affirm that increasing the intake of carbohydrates in the performance week is able to improve long-term performance .
Planning a high-carbohydrate diet in the days leading up to performance can prove useful in medium and long-term commitments and repeated short-term trials. Increasing the carbohydrate content is intended to saturate and increase body glycogen reserves , conditioning muscle performance and delaying the onset of fatigue during exercise.
Carbohydrates in the pre-competition diet
To achieve this result, various strategies have been used, including providing 70% of the calories introduced daily in the form of carbohydrates . It started from a full week of carbohydrate loading up to, recently, the only day before the performance. Speaking of complex carbohydrates, such as starch , certainly not simple sugars.
In parallel to the carbohydrate load, it was considered necessary to provide moderate / low intensity workouts for a period of time shorter than the usual training volumes. Among the various proposals, an appropriate diet provided for a distribution in macronutrients equal to:
- 70% carbohydrates,
- 20% fat
- 10% protein.
Studies of the time reported the following data, later denied by more recent research: for activities lasting more than 90 minutes, carrying out the glycemic load would bring an estimated benefit of up to a 3% increase in performance . By increasing the intake of carbohydrates in the 3 days preceding the start of the competition or intense workout, the glycogen stores can be increased by 50-100%, increasing endurance capacity by up to 20%.
However, recent studies confirm that, in addition to the energetic aspect, the pre-competitive fueling up can represent a countermeasure to mitigate the immunological alterations that arise in the post-workout phase in the heaviest sports disciplines and in ultra-endurance. These alterations have been shown to delay muscle recovery, increasing the incidence of infectious-inflammatory syndromes in athletes. Consuming lots of carbs before strenuous activity must be accompanied by mild muscle activity or rest. Per athletes who are already introducing high levels of carbohydrates, reducing the training load in the 2-3 days prior to the competition may be sufficient to increase body glycogen levels.
In endurance or ultra-endurance sports, the indications are to significantly increase the intake of carbohydrates in the 3-4 days before the competition until reaching 8-10 up to 12 g / kg (70 -80% of the total energy ) in parallel with the reduction of the intensity and volume of training.
As already mentioned, the glycogen supercompensation strategies proposed in the last decades are many but in this article I would like to describe mainly two:
- The classical method of Bergstrom et al. , combines exercise with diet and consists in initially fatiguing, through exercise, the muscles that will then have to be loaded. Subsequently, the athlete undergoes the hyperglucidic diet for a few days (3-4), thus significantly increasing the glycogen stores (considerations of the time). Again, no physical exercise should be performed during the carbohydrate diet. It lasts about a week, consists of two phases and has been designed to prepare a single race;
- Phase 1 (= depletion) During the first 4 days you follow an intense training program (90 min. Of submaximal activity) and a low carbohydrate diet (10%).
- PHASE 2 (= CARBOHYDRATE LOAD) In the next 3 days you follow a moderate training program and a diet rich in carbohydrates (80-90%) with normal protein intake.
- Costill's method is a variation of the previous method, but without the initial depletion phase. The lack of this phase makes it less stressful , therefore preferred by athletes. Exercise also in this case has the function of exhausting the pre-existing glycogen stores, in order to create the stimulus for the subsequent supercompensation.
Exercise must be decreased starting one week before the test (every 2 days the quantities of exercise are halved until you reach complete rest the day before the race), while the carbohydrate intake in the diet must be 60 -70%.
Classical supercompensation has a limited applicability and is not without negative aspects, which is why today it is not seen with the same expectations as in the past. One gram of glycogen binds about 2.7 g of water, resulting in a consistent body weight gain. Furthermore, phase 1, so low in carbohydrates, can decrease lean mass due to excessive protein catabolism for energy purposes.
Many of these effects were reduced with the Costill method but not eliminated. Very often, in order to obtain the phenomenon of glycogen supercompensation, the quantity of carbohydrates ingested was exaggerated, not having clear the concept of the individual capacity to store a limited quantity of glycogen. Once these reserves are saturated, in fact, excess glucose will inevitably be transformed into fat .
To roughly calculate the maximum level of glycogen that can be stored in your body, just apply the following formula:
- body weight x 25 and divide the result by 4 (kcal of 1 gram of carbohydrates)
Per example, a 70 kg man of normal weight can store a maximum of 25 x 70 = 1750 kcal which corresponds approximately to 430 grams of carbohydrates / glycogen. It should be remembered that the "load" of carbohydrates is associated with weight gain linked to water retention (about 2.7 - 3 g / g glycogen), a factor that is often not well tolerated by the athlete.
A practical advice to reduce digestive problems, avoid unpleasant sensations of heaviness and maximize glycogen stores, is to divide the carbohydrate intake with small snacks to be distributed 1-2 hours apart throughout the day with high-grade foods. intake of carbohydrates, low in fat and fiber. Another possibility is to take sugars in liquid form (for example sports drinks) mainly during and immediately after workouts.
Before the actual endurance competition, and compatibly with the time required for digestion, a light meal based on complex carbohydrates should be provided , consumed 3/4 hours before the start of the competition. This type of pre-competition meal is effective in improving athletic performance by increasing the availability of glucose. It was considered fundamental in the choice of foods to use the concept of "glycemic index", that is the expression of the speed with which blood sugar increases in the two hours following the intake of 100g of a food compared to the ingestion of an equal quantity of another food taken as a reference (usually glucose).
It was (by many still now) considered a serious mistake to take high glycemic index carbohydrates before a race, because this would have caused a rapid increase in blood insulin levels.(insulin hinders the release of free fatty acids from adipose tissue, i.e. it opposes the consumption of fat to provide energy, with the consequent lack of availability for the energy metabolism of the muscles in addition to the possible feared reflex hypoglycemia). There are now numerous experimental evidences that consider this phenomenon not so relevant and in any case absolutely dependent both on the quantity of carbohydrates taken but above all on the time that passes between ingestion of carbohydrates and performance. If there are more than 2 hours and the meal was not abundant, the glycemic index is not so important indeed, I would say rather irrelevant.
Today it is considered much more important to respect the following characteristics of the meal immediately before the race :
- simple and light,
- easily digestible,
- little bulky,
- low in insoluble raw Fibre (responsible for rapid gastrointestinal emptying),
- consumed no less than 3-4 hours earlier.
Finally, before the competition, to obviate the possible risk of hypoglycemia (it could also be caused by hyperactivity of the adrenergic system, typical in some "anxious" athletes) and to the increase in glycogenolysis (degradation of glycogen) in the muscles in the first 30 -60 minutes of competition, it is certainly advisable to take a water, carbohydrate and suitably saline "waiting ration" . This ration can be consumed up to 30-40 minutes before the start of the competition, in order to dampen the psychic stress that precedes the competition, prevent the onset of hypoglycemia and thus favor the maintenance of constant levels of glucose in the blood. fundamental requirement especially in the early stages of the race.
In this way the liquid and saline component will prevent and correct the dehydration that begins before the athletic commitment, while the sugars will help to provide ready-to-use energy and save muscle glycogen, so laboriously accumulated in the days and hours preceding the race.
In addition to pre-exercise or pre-competition meals, the intake of carbohydrates in a water-saline solution is also recommended during exercise for all those activities lasting more than 60 minutes. During exercise, it should be remembered that, contrary to what happens at rest, carbohydrate intake causes little increase in blood insulin levels. The oxidation capacity of exogenous carbohydrates during exercise does not depend on the ability of the muscles to extract carbohydrates from the blood, but is limited by their absorption in the intestine. From this point of view it should be remembered that under stress the blood supply to the intestine is considerably reduced and with it the ability to assimilate nutrients.
Given these considerations, the choice of carbohydrates during exercise falls on forms that can be immediately assimilated in the iso / hypotonic liquid or gel form . With these precautions, the carbohydrate intake during exercise should be 30 - 60 g / h up to a maximum of 90g for activities longer than 2.5 hours.
The ingestion of carbohydrates during exercise significantly improves performance, removes the onset of fatigue and the decline in immune defenses.
As we have said, it should be emphasized that in consideration of the frequent gastrointestinal disorders during intense and prolonged physical activities, it is recommended that athletes "train" the intestine to assimilate carbohydrates during exercise by doing the necessary tests in the sessions of training by frequently simulating racing conditions.
Once the pre-exercise mechanisms have been defined and during this effort, all that remains is to analyze the post-exercise phase. Glycogen recovery after particularly long competitions such as marathons can take several days.
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