Fuel use during exercise
Fueling optimally during exercise can help athletes to increase exercise capacity, improve exercise performance and skill execution, and delay fatigue. It is important for athletes to understand how they can meet fueling requirements, which differ depending on exercise duration and intensity. The information below will explain key considerations for fueling during exercise.
and 
are both important fuels during exercise, with the contribution of carbohydrate to energy metabolism increasing as exercise intensity increases (see below). There are limited stores of endogenous carbohydrate (i.e., glycogen) within the body, and these stores reduce during exercise when they are used for energy. Glycogen stores will become reduced at a quicker rate during higher intensity exercise, which limits the rate of carbohydrate oxidation. Consuming carbohydrate (i.e., exogenous carbohydrate) provides additional fuel to be used during exercise, which helps to maintain carbohydrate oxidation rates, and in turn delay fatigue and maintain performance.
Potential benefits of optimal fueling during exercise
Sex differences in
carbohydrate oxidation rates
Females vs. males
Research has found that females have lower rates of carbohydrate oxidation and higher rates of fat oxidation during fasted endurance exercise compared to males.
These sex differences in substrate metabolism might be due to many factors such as higher maximal oxygen uptake, greater muscle mass, and lower estrogen levels in males, although the exact mechanisms remain unclear. However, when carbohydrate is consumed during exercise, these sex differences in substrate metabolism are not seen. In other words, both males and females oxidize ingested carbohydrate at the same rate.
Therefore, currently there are no sex-specific carbohydrate recommendations for during exercise.
Carbohydrate intake recommendations during exercise
What are the carbohydrate recommendations
for exercise?
Exercise duration:
< 30 minutes
No carbohydrate necessary
During exercise that is < 30 minutes, muscle glycogen is not a limiting factor for performance. It is therefore not necessary for athletes to consume carbohydrate during exercise lasting < 30 minutes, especially if they are optimally fueled beforehand, or have time to refuel post exercise.
Exercise duration:
30-75 minutes
Small amount of carbohydrate
OR carbohydrate mouth rinse
During exercise lasting 30-75 minutes, athletes may benefit from either a small amount of carbohydrate intake, or a carbohydrate mouth rinse, especially if the exercise is high intensity.
Carbohydrate mouth rinsing involves athletes swilling a carbohydrate containing solution in their mouth (e.g., a sports drink) for 5-10 seconds, which they then spit out. The exact mechanisms remain unclear, but it is likely that the carbohydrate (energy) is detected by receptors in the oral cavity, which activates certain brain regions, causing improved motor drive and/or motivation. During shorter duration exercise bouts, carbohydrate mouth rinsing has been shown to produce very similar performance improvements as those seen when the carbohydrate is actually consumed. The choice between swallowing the carbohydrate solution, or expectorating the solution, depends on the practicalities of the sport the athlete is competing in, as well as individual preferences.
Exercise duration:
1-2 hours
30-60 grams of carbohydrate per hour
There is extensive research to show that during exercise lasting 1-2 hours, consuming 30-60 g/h can improve performance. If the exercise is longer in duration (e.g., close to 2 hours) and/or high intensity, then athletes should aim for the upper end of the recommendation. When athletes are consuming carbohydrate at a rate of < 60 g/h, single sources of rapidly oxidized carbohydrate (e.g., glucose or sucrose) can be ingested.
Exercise duration:
2-3 hours
60-90 grams of carbohydrate per hour
Carbohydrate intake becomes even more important when exercise duration extends beyond 2 hours, in order to prevent a decrease in performance. It is recommended that athletes consume carbohydrate at rates > 60 g/h, and up to 90 g/h for exercise durations > 2.5 hours. This should be in the form of multiple transportable carbohydrates (see next section).
Multiple transportable carbohydrates
Carbohydrates are taken up and absorbed by the body using a variety of transporters located in the intestines. However, exogenous carbohydrate oxidation is limited by the rate at which carbohydrates can be absorbed, with intestinal transporters becoming saturated at carbohydrate intakes of ~60 g/h.
Consuming multiple transportable carbohydrates (i.e., carbohydrates which use different intestinal transporters), like glucose and fructose, can enhance carbohydrate absorption and utilization compared to a single carbohydrate source. For example, glucose and fructose use different intestinal transporters: glucose uses sodium-glucose cotransporter 1 (SGLT1), which is independent of the transporter that fructose uses, glucose transporter 5 (GLUT5). Therefore, co-ingesting carbohydrates that use different transporters results in higher total carbohydrate oxidation rates, which is beneficial for prolonged or intense endurance exercise (i.e., when carbohydrate intakes of > 60 g/h are required). In addition, ingesting multiple carbohydrate sources has been shown to increase fluid intake in the intestine in comparison to ingesting a single carbohydrate source.
Therefore, when carbohydrate is consumed at a rates of up to 60 g/h, single sources of carbohydrate can be utilized (e.g., glucose, sucrose).
Absorption of glucose and fructose
In practical terms, it is recommended that athletes consume a 2:1 ratio of glucose:fructose when consuming carbohydrate at rates > 60 g/h. This can be achieved by consuming sports drinks or gels which contain a combination of glucose and fructose. Overall, consuming multiple transportable carbohydrates is an effective strategy for maximizing carbohydrate utilization for endurance activities.
Sources of carbohydrate
Carbohydrate can be consumed during exercise in a variety of forms, with no differences in carbohydrate oxidation or performance outcomes seen between different forms. If a solid food option is chosen, then it should ideally be low in fat, protein and fibre to avoid any gastrointestinal issues.
Gastrointestinal complaints
There are three main causes of gastrointestinal symptoms during exercise which are:
It has been shown that ~30-50% of athletes experience gastrointestinal complaints during exercise. Early research suggested that prevalence of exercise-associated gastrointestinal symptoms was greater in female athletes vs. male athletes. However, more recent investigations have not seen any differences between genders. Gastrointestinal complaints can be classified into upper and lower abdominal symptoms, click the circles below to find out more:
Physiological
(reduced blood flow to the gut)
Mechanical
(repetitive gut jostling e.g. running)
Nutritional
(see below)
Nutritional causes can include:
Consuming high fiber foods too close to exercise may lead to bloating, gas and abdominal cramps as the fiber is poorly digested and absorbs water in the gut.
High fructose ingestion from sports drinks, gels and chews can result in abdominal cramps, bloating and diarrhea.
The osmolality and carbohydrate concentration of some carbohydrate-containing drinks can delay gastric emptying and draw fluid into the gut, potentially contributing to reflux and nausea.
Athletes can reduce the risk of experiencing gastrointestinal discomfort during exercise, caused by consuming fluids/food, by ‘training their gut’
(see next section, as well as 
).
Training the gut
The gastrointestinal tract is crucial for delivering ingested carbohydrate and fluids to the body during exercise. It has been demonstrated that the gastrointestinal system is highly adaptable and that both gastric emptying (the rate at which food and fluids empty from the stomach) and stomach comfort can be ‘trained’ over time through various methods.
Training the gut can indirectly improve performance by effectively absorbing and metabolizing carbohydrates, and also reducing any gastrointestinal discomfort experienced by athletes. The diagram below shows various methods which can be used to train the gut, along with their physiological effects.
Considerations
- Athletes should be encouraged to practice their competition nutrition strategy prior to a competition, especially if the exercise duration is prolonged, so that they can become familiar with higher carbohydrate intakes.
- Athletes who have a low consumption of carbohydrate e.g., those following a low-carbohydrate, high fat or ketogenic diet, or those who are reducing calorie intake to lose weight, will have a reduced capacity to absorb carbohydrates during exercise.
- Encourage athletes to regularly practice consuming carbohydrate during exercise, including high carbohydrate intakes during prolonged training sessions.
References
