Carbohydrate intake during training and racing
How come that pro’s can consume more than 90g of carbohydrate per hour while 20 years ago the recommendation was only 60g/hour?
Everybody how is into endurance sports sure have already read somewhere the role and importance of carbohydrates on exercise performance.
During physical activity, the muscles (mainly) use carbohydrates and fats as energy source. Depending on some factors such as training status, intensity and duration, the relative contribution of the fuel sources changes. At low to medium intensity the muscles (ideally) use predominantly fat, however at high intensity carbohydrates become the main fuel source.
Importantly, in contrast to fat, carbohydrates can only be stored in a limited amount in the body (mainly in the muscles and liver) in the form of glycogen (complex carb). When energy is needed, stored glycogen will break down into glucose (simple carb), which will then go through many steps to create energy. Glycogen stores can sustain around 90 min of intense exercise. Therefore, if exercise duration is longer than 90 min and we want to keep intensity high, we need to consume carbohydrates during workouts.
Since the 80’s, it is known the exogenous carbohydrate ingestion during exercise improves physical performance. Various studies have investigated the type and amount of carbohydrates and feeding regimes. Until the early 2000, it was generally accepted that carbohydrate oxidation is limited by 60 g per hour (1g/min)
However in 2004, a groundbreaking study demonstrated that by using multiple sources of carbohydrates, carbohydrate oxidation can reach up to 1.75g/min (105g/hour).
What did the researchers find out?
Glucose, a rapidly digested monosaccharide (simple carb) is absorbed through a transporter called sodium dependent glucose transporter 1 (SGLT1) in the small intestine. This transporter saturates at a rate of ≈ 60g/hour of glucose ingestion during exercise.
However, fructose, which is a slower oxidised simple carbohydrate, uses a different transporter called GLUT5 (Ferraris & Diamond, 1997).
So when the researchers added fructose on top of glucose, they could significantly increase carbohydrate oxidation.
So does it mean that you can just go and eat 90-100g of carbohydrate per hour during your next training session? Unfortunately not.
The role of our GI (gastrointestinal) tract is crucial during prolonged exercise. When the GI system functions improperly, not only the transport of nutrients and fluid is less efficient but also a range of GI symptoms (e.g. bloating, cramping absorption, diarrhea, vomiting) can occur.
For this reason, we want to make sure that our GI tract can absorb carbohydrates well.
Our GI tract is highly adaptable. Through specific “diets” we can increase or decrease the capacity of the absorption of certain nutrients. In sports nutrition literature we call it: training the gut. For example, following a high carbohydrate diet results in an upregulation of SGLT1 transporters. But also, the opposite is true as well: being on a ketogenic diet (high in fat, low in carbohydrate), decreases the activation of glucose transporters.
Therefore it is crucial to practice race specific nutrition before the race so the gut can adapt to it. If you are new to consuming carbohydrates during training, make sure you progress slowly. Start with one gel first, see how your body reacts. If it goes well, add an extra gel/sports drink during your next training session. As a side note, consuming carbohydrates at lower exercise intensity is more manageable than at high intensity, so make sure you practise your race nutrition on the intensity you will race in.
By training the gut, the ability to consume and absorb carb will increase. Proper fuelling is a trainable skill that is relatively easy to do and has a huge pay-off.
In summary:
-Carbohydrates are important energy source during high intensity exercise
-Carbohydrates stores are limited in the body, therefore if the high intensity exercise is longer than 90 min and, we need to consume exogenous carbohydrates.
-Glucose is absorbed at a maximum rate of 60g/hour. To increase carbohydrate absorption (as well carbohydrate oxidation), fructose is added to sports nutrition products as fructose is absorbed through a different transporter than glucose.
-GI system plays an important role as absorption of nutrients takes place here.
-Training the gut helps to maximise carbohydrate absorption.
In an upcoming article, we will break down the carbohydrate recommendations during exercise!