Carbohydrate fueling during ultra-endurance races

Ultra-endurance races like the ones organized by Transiberica® require a submaximal aerobic exercise (≤ 70 % del VO_2max) during 3-4 days. In order to respond to the energy demand required for such exercise, muscles obtain fuel from carbohydrates and fats.

Carbohydrates are mainly stored in the liver and muscles as a long glucose chain called glycogen. The liver can store between 80-100 g of glycogen and muscle can contain approximately 400 g. Altogether, the body can normally store up to 500 g of glycogen. As each gram of glucose produces 4 kcals, 2,000 kcal can be obtained from this carbohydrate storage. This amount of energy allows to sustain the submaximal aerobic exercise needed for these races a maximum of 2 hours and 50 minutes. By only using the energy from the stored carbohydrates we wouldn’t be able to reach the first checkpoint of the race!

Luckily, fat store capacity is much bigger, and each gram of fat produces 9 Kcal against the 4 kcal produced by carbohydrates. Fats are mainly stored in the adipose tissue and in a smaller amount in the muscle. In the case of well-trained male cyclists, they can store an average of 12 kg of fat in the adipose tissue. With such amount of fat, they can produce up to 108,000 Kcal. This amount of energy will allow additionally 153 hours of exercise. Enough energy to complete Basajaun 3 times!

Carbohydrates and fats are used simultaneously during exercise but with relative contribution of each one depending mainly on the duration and intensity of the exercise. Environment factors, such as high temperatures where the consumption of glycogen increases, also alters the usage ratios of carbs and fats. In subjects who are well-trained in endurance, the maximum oxidation rate for fats is around their 75 % of VO_2max. At that intensity then, the organism main source of energy are fats. During a 5 hours and endurance race-pace (70 % VO_2max), fat oxidation accounts for the 46 % of the energy used^1,2. The following graphs show the different usage ratios of the different energy sources depending on the duration of the exercise and the intensity of the exercise express in maximum Watt percentage^3,4:

 

Aerobic breakdown of carbohydrates for energy occurs more rapidly than the generation of energy from the breakdown of fats. When glycogen storage is depleted, there is a considerable reduction on the ability to produce power during exercise, for example, the necessary to perform a sprint or the

capacity to rhythm changes as the organism needs more time to produce and transport the energy needed to the active muscle.

During prolonged aerobic exercise athletes often suffer from liver and muscle glycogen depletion related fatigue. Carbohydrate intake during exercise prevents the depletion of this storages by using the recently ingested carbohydrates first. It has been seen that carbohydrate feeding during exercise improves performance in endurance events and delays fatigue by several mechanisms: maintenance of plasmatic glucose levels, maintenance of high carbohydrate oxidation rate and maintenance of muscular glycogen, as well as production of other effects on the central nervous systems that elicit a smaller effort perception⁵.

 

Our organism is capable of absorbing up to 60 g of ingested glucose or glucose polymers, such as maltodextrin, per hour. Glucose absorption from the small intestine to blood vessels for its transport to muscles is mediated by a specific transported named SGLT1. These transporters work as doors that only allow glucose entrance. They have a capacity of 1 g of glucose per minute. Intakes greater than 60 g of glucose per hour saturate these doors and glucose is trapped in the small intestine. This situation can cause intestinal discomfort that could impair performance.

 

There are several strategies that allow greater intakes of carbohydrates:

Strategy 1: Gut training. The gut can be trained by the consumption of carbohydrates during exercise. This training will maximize the number and the performance of the SCGT1 transporters^,6,7.

Strategy 2: Intake of several transportable carbohydrates. Intakes of glucose and fructose mixes enhances carbohydrate absorption as fructose is absorbed through GLUT5 transported and not SCGT1^8,9. Recently published studies has shown that the combination of glucose/maltodextrine:fructose in a ratio of 1:0,5 can increase carbohydrate absorption up to 120 g per hour¹⁰. Recently, other studies found out that by increasing the amount of fructose to ratios of 1:0,8 improves the efficiency of carbs utilization, reducing the amount used from the organim storages. Moreover, this ratio potentially reduces the symptoms of stomach bloating and nausea provoked by energetic gel ingestion ⁸.

 

Recommendations for carbohydrate intakes for exercises of different durations are show on the following table ^10,12. Recommendations normally end for exercises longer than 3 hours, where the recommended amount of carbs are 90 g per hour ¹². But, ¿what happens for events which durations are much greater than 3 hours like the events organized by Transiberica®? A recently published article has shown not only that the ingestion of 120 g of carbs is possible by energy gels of carb mixture of glucose and fructose on a 2:1 ratio, but also that this amount of carbs improves performance and decreased muscle damage ¹⁰.

As it is stated at the previous table, carbohydrate intake must be obtained by a combination of food, energetic gels, sport drinks, energy bars and energy chews. Moreover, they must be distributed in small portions along the entire event in prevention of gastrointestinal discomfort and hydration issues. Some examples of foods that can be consumed in one setting and contain 30 g of carbohydrates are:

 

Accordingly, the carbohydrate ingestion objective for these events is 120 g per hour. The fastest cyclist on previous editions toured 760 km with very little rest in about 46 hours. Such effort requires the approximately intake of 5,520 g of carbohydrates which can be obtained by the consumption of 184 energy gels or 21 kg of rice. Race nutrition must be trained because the organism does not respond to fuel the same way at rest or during extraneous exercise where energy is mostly use on active muscles and not at the digestive system. Moreover, it is necessary to test individual tolerance to different sport foods (gels, bars, chews…) as some will be more suitable than others for us. The amount of carbohydrates on each intake must also be increased progressively with time so that our intestine can adapt to the absorption of bigger carbohydrates quantities ^6,7. Finally, it is important to remember that sport foods do not provide with all our nutrient requirements and, for long events like these it is necessary to eat nutrient-rich foods.