Fats and carbohydrates are the primary source of energy for endurance activities. Fat oxidation can be sufficient for most energy demands in low-to-moderate intensity endurance training. However, compared to carbohydrates, the maximum rate of energy production from fat oxidation is quite low. Therefore, carbohydrates become the primary fuel source during high-intensity endurance training.

The energy yield (ATP/s) of carbohydrate breakdown allows for a much higher power output. Consequently, maintaining performance often depends on the availability of carbohydrates for muscle energy production. Fat oxidation can be compared to a diesel engine, while carbohydrate oxidation can be compared to a turbocharged gasoline engine.

The key to success: promoting carbohydrate oxidation and avoiding stomach problems

The fat reserves for energy production are almost unlimited. The opposite is true for carbohydrate stores, which are stored in the form of liver and muscle glycogen. During training or competition, it is important to add carbohydrates in order not to deplete glycogen stores and to maintain high availability of glucose in the blood.

The oxidation of substrates (fats or carbohydrates) requires oxygen. The use of oxygen in the muscles during exercise is limited by the percentage (%) of VO2max (maximum rate of oxygen uptake) that can be used without a persistent accumulation of lactate (anaerobic threshold). An interesting fact is that this limited amount of available oxygen results in higher energy production (ATP/s) when used to oxidize carbohydrates.

The rate of carbohydrate uptake (g/h) should never exceed the rate of glucose absorption in the digestive tract. An excess of carbohydrates can cause gastrointestinal problems (diarrhea, nausea, vomiting), which leads to a drop in performance. In this context, a lot of research has been done in recent decades to find the optimal intake and composition of dietary supplements with carbohydrates (sports drinks, energy gels and bars). The main purpose of these studies was to increase carbohydrate oxidation during prolonged exercise (> 1 h). Extensive research has resulted in practical and scientifically validated recommendations, which are summarized in the table below (Burke et al., 2010; Jeukendrup, 2014).


Figure: Guideline values for carbohydrate intake depend on the duration and intensity of the training/competition. The longer the training/competition lasts, the more carbohydrates we need. For a workout/competition that lasts less than 1 hour, carbohydrate intake has no effect on performance.

What is the optimal ratio of glucose to fructose?

Glucose intake is limited to 60 g per hour. The use of sports drinks containing only glucose is limited to endurance activities that require no more than 60g of carbohydrates per hour. For longer endurance activities, it is necessary to consume at least 90 g of carbohydrates per hour. To allow for a higher rate of absorption, sports drinks contain a mixture of two types of carbohydrates, namely glucose/maltodextrin and fructose. The addition of fructose enables the intake of at least 30 g of additional carbohydrates per hour (Fuchs et al., 2019). Sports drinks containing glucose and fructose in a 2:1 ratio enable the intake of at least 90 g of carbohydrates per hour.

A 2011 study by O'Brian and Rowlands compared a 2:1 mixture of glucose and fructose to a 1:0.8 mixture. During the more than 2.5-hour bike test, the cyclists consumed 110 g of carbohydrates per hour. The test showed that during this intake, the rate of carbohydrate oxidation increases slightly and performance improves. Meanwhile, several other studies have reported (Rowlands et al., 2015) that a lower glucose-fructose ratio (2:1) in 2.5-hour endurance activities may contribute more to performance improvement than a higher intake (+90 g per hour) . ), since the incidence of gastrointestinal symptoms increases with increasing intake.

Experts mostly unanimously recommend an intake of 90 g of carbohydrates per hour for endurance activities of 2.5 hours or more. At the same time, it should be emphasized that higher intakes can further improve performance, at least in some athletes (Urdampilleta et al., 2020; Viribay et al., 2020).

Should high fructose become the "Gold Standard" in sports drinks?

Since 2004 (Jentjens et al., 2004) dozens of studies and tests have demonstrated the effectiveness of 2:1 sports drinks in improving performance. The opposite is true for the 1:1 ratio (glucose-fructose), where scientific data on the effectiveness of the above matrix is lacking. Further testing is required to investigate the effects of the hydration ratio in practice under specific conditions, e.g. B. during prolonged activity, stage events, heat stress, etc., where the occurrence of gastrointestinal symptoms is increased.

Despite the recommendations (90 g carbohydrates per hour), athletes often eat fewer carbohydrates during long-term endurance exercise (≥2.5 h) (Cox et al., 2010, Pfeiffer et al., 2012). In this case, supplements with a 2:1 ratio do the job perfectly. The same applies to endurance activities that last less than 2.5 hours.

We often hear that the 2:1 ratio with a higher carbohydrate intake (+90g) puts a strain on the digestive system. Proper nutrition training is key to avoiding unwanted stomach problems during competition. Reports from practice show that athletes tolerate sports drinks in a ratio of 2:1 quite well even with extremely high carbohydrate intake (90-120 g). However, it is worth emphasizing that the athletes previously “trained the digestion” and gradually introduced high carbohydrate intake.

Literature

The author of the article is Prof. DR. Peter Hespel from the Athletic Performance Center at the University of Leuven in Belgium. The article was created in collaboration with Bakala Academy and 6d Sports Nutrition.

Burke et al., 2010 (LINK)

Cox et al., 2010 (LINK)

Fuchs et al., 2019 (LINK)

Jeukendrup, 2014 (LINK)

Jeukendrup, 2017 (LINK)

O’Brien & Rowlands, 2011 (LINK)

Pfeiffer et al., 2012 (LINK)

Prado de Oliveira et al., 2014 (LINK)

Rowlands et al., 2015 (LINK)

Urdampilleta et al., 2020 (LINK)

Viribay et al., 2020 (LINK)