13/01/24
7
The popularity of endurance and ultraendurance events continues to grow. Athletes participating in these events must sustain sustained work paces for a prolonged period, resulting in high rates of sweating and energy expenditure.
Fatigue during these competitions does not arise only from a single factor, but is the result of a multifactorial phenomenon that includes: dehydration, hyperthermia, carbohydrate depletion, central nervous system fatigue and hypoglycemia. To delay the onset of fatigue and optimize performance, it is desirable to compensate for losses of fluids and electrolytes and carbohydrates.
In recent years there have been significant changes in the understanding of the role of carbohydrates during endurance physical activity, allowing for more specific and personalized advice regarding carbohydrate intake during endurance exercise. The new proposed guidelines take duration (and intensity) into account and the advice is not limited to the quantity of carbohydrates, but also to the type of them.
The carbohydrate source may be a liquid, semi-solid or solid and recommendations may need to be adjusted downwards when the absolute intensity of physical activity is low and therefore carbohydrate oxidation rates are similarly low.
It wasn’t until the early 1900s that carbohydrates were discovered to be an important fuel for physical activity.
In the 1960s it became clear that muscle glycogen played a significant role and in the 1980s the first studies demonstrated that ingesting carbohydrates during exercise improved exercise performance. Not much progress was made over the next 20 years until around 2004, which marked the beginning of an era with a series of important discoveries regarding carbohydrate intake during physical activity. As these discoveries and their effects on sports nutrition have become available over time, recommendations for athletes have also evolved during this time. Studies have shown that relatively small amounts of carbohydrates (20 g/h) were sufficient to observe a performance benefit. In other studies it was believed that a minimum of 22g of carbohydrate per hour was needed to observe a performance benefit.
The most recent guidelines from the American College of Sports Medicine (ACSM) state that a carbohydrate intake of 30-60 g/h is recommended during physical activity. This is a relatively large window and is independent of the type of activity, the duration of the activity or the level of the athlete.
First, it is essential to know what the goal of a training session is before we can recommend amounts and types of carbohydrates. If optimal performance is the goal, the following recommendations are appropriate.
Starting from the assumptions that:
- not all carbohydrates are the same, some are used more quickly than others and these are the ones we need for optimal performance
- carbohydrate intake even in small quantities can improve performance during prolonged physical exercise (> 2 hours)
- Even a mouth rinse with a carbohydrate-containing solution can improve short-term, high-intensity performances such as a 40km time trial
Let’s see some more detailed instructions.
The basis on which to start is that to determine the recommended amount of carbohydrates, it is essential to know the duration of the session.
Exercise <30min
If the session is less than 30 minutes you do not need to consume any carbohydrates. There is little to no evidence that carbohydrate intake will be beneficial in this situation.
Exercise for 45 to 75 minutes
When the session is a little longer, performance will definitely benefit from carbohydrate intake or a carbohydrate mouth rinse. What is best depends on the practicalities of ingesting carbohydrates. Sometimes it is easier to simply rinse and sometimes it will be just as easy to take the carbohydrate solution. The types of carbohydrates don’t seem to matter much in these circumstances.
Prolonged exercise
For a session lasting 1-2 hours, some carbohydrates have been shown to improve performance and 30 grams per hour is probably sufficient. Again the types of carbohydrates don’t seem to matter much in these circumstances.
Exercise for more than 2 hours
As the duration increases it is recommended to increase the intake up to 60-90 g / h. There seems to be a dose-response relationship in the intake of this energy source; in any case, such high intakes are recommended as long as this does not cause gastrointestinal disorders.
If your intake is not more than 60 g/h, any rapidly metabolized carbohydrate will work for you (glucose, sucrose, maltodextrins and some forms of starch). When the intake is higher than 60g/h, it is advisable to use carbohydrate mixtures that use different transporters.
Carbohydrates ingested during exercise can in fact be oxidized at a rate of no more than 1 g/min (60 g/h), regardless of the type of carbohydrate. So, even when ingesting large amounts of glucose, for example, 100 g/h, no more than 60 g/h will be used.
The rationale for this situation is that glucose uses the sodium-dependent transporter (SGLT1) for absorption, which becomes saturated at a carbohydrate intake of approximately 60 g/h. When glucose was instead ingested together with another carbohydrate (e.g.: fructose) that uses a different transporter, oxidation rates were well above 1 g/min (about 1.3 g/min). In some studies, however, using different sources of carbohydrates, it was possible to record an increase of up to 75% in the oxidation rate compared to the “canonical” 60 grams/hour.
The advice in all cases is to never put into practice whatever strategy you intend to use directly in a race. The key word must be: practice, practice, more practice. Your intestines will adapt and the chances of gastrointestinal disorders will be reduced. This advice is even more crucial when you consume large amounts of carbohydrates and carbohydrates from different sources.
However, it should always be kept in mind that the incidence of gastrointestinal problems increases with exercise time (see for example competitions such as Ironman) and could increase in hotter and more humid climate and competition conditions.
Finally, there seems to be a correlation between gastrointestinal symptoms and a general background of general gastrointestinal suffering already existing in some athletes, therefore suggesting an individual predisposition to gastrointestinal discomfort.
The effect of intensity
Carbohydrate needs may be different at different exercise intensities. When exercise intensity is low and total carbohydrate oxidation rates are low, carbohydrate intake recommendations may need to be adjusted downward.
As exercise intensity increases, active muscle mass becomes increasingly dependent on carbohydrates as an energy source. Both increased muscle glycogenolysis and increased plasma glucose oxidation will contribute to increased energy demands. It is therefore reasonable to expect that exogenous carbohydrate oxidation will increase with increasing exercise intensity.
Effect of body weight
Guidelines for carbohydrate intake during exercise are expressed in grams per hour and these figures are not correlated to body weight (BW). The rationale for this is clear as there appears to be no correlation between BW and exogenous carbohydrate oxidation. The reason for this lack of correlation is probably that the limiting factor is carbohydrate absorption, and absorption is largely independent of BW. It is likely, however, that the absorptive capacity of the intestine is modified by the carbohydrate content of the diet, as it has been shown in animal studies that intestinal transporters can be up-regulated with increased carbohydrate intake. These results clearly show that there is no rationale for making carbohydrate recommendations for athletes based on per kilogram of body weight.
Effect of athletic condition
A question that often arises is whether the results of the studies (often conducted on advanced level individuals) can also be valid for medium or low level athletes.
In one study, categories of high and medium/low level athletes were compared. All the men trained at around 60% of their Vo2max though while the elite athletes exercised at a significantly higher absolute intensity.
A source of glucose was ingested at regular intervals and the average intake was approximately 1.1 g/min. Total carbohydrate oxidation was similar in both groups, but fat oxidation and energy expenditure were, obviously, higher in elite athletes.
It is therefore interesting to observe that, even though the more trained individuals exercised at a higher absolute power, the oxidation of exogenous carbohydrates was not different between the two groups.
It should be noted, however, that less trained subjects had higher Vo2max values than the sedentary population, so the guidelines can be applied for athletes of different levels, but not necessarily to the sedentary population.
Reference:
- Tsintzas K, Williams C. Human muscle glycogen metabolism during exercise: effect of carbohydrate supplementation. 1998
- Asker Jeukendrup A Step Towards Personalized Sports Nutrition: Carbohydrate Intake During Exercise 2014
- Asker E Jeukendrup Carbohydrate intake during exercise and performance. 2004
- Pfeiffer B, Stellingwerff T, Hodgson AB, et al. Nutritional intake and gastrointestinal problems during competitive endurance events. Med Sci Sports Exerc. 2012
- Jeukendrup AE, Mensink M, Saris WH, et al. Exogenous glucose oxidation during exercise in endurance-trained and untrained subjects. J Appl Physiol. 1997
