Endurance sports are generally characterized by prolonged exertion. Performance during these activities is determined, among other factors, by energy supply. While there are two different metabolic pathways available – aerobic and anaerobic – the aerobic pathway has a performance-determining influence in endurance sports. Energy for necessary muscle work can only be provided for a maximum of 90 seconds through the purely anaerobic pathway, without oxygen. Energy supply through the aerobic pathway, with oxygen, primarily occurs in endurance sports through the utilization of carbohydrates and fats and is theoretically unlimited in duration, although factors such as exercise intensity and training status have a limiting influence. Compared to fats, carbohydrate stores are relatively limited. It is difficult to provide general information about this, as it also varies from athlete to athlete. However, it can be assumed that activities relying predominantly on carbohydrate energy production can be sustained for a maximum of 60-90 minutes. In contrast, every person has enough fat reserves to potentially provide the necessary energy production for several days. For example, a person weighing 70kg with only 10% body fat has approximately 70,000 kcal of fat reserves. When comparing this to energy consumption – around 800kcal per hour at 25km/h on a bike – it becomes clear that the amount of fat reserves does not have a limiting influence on athletic performance.
These numbers illustrate the significant role of fat metabolism, especially in endurance sports such as triathlons or marathons. The scientific community has already investigated this significance multiple times. For example, it has been shown that the maximum capacity for fat oxidation during long-duration endurance activities positively influences performance and, ultimately, competition results.
Which energy source is used when?
In general, the metabolic processes of both energy sources occur simultaneously. This means that during aerobic exertion, both fats and carbohydrates are metabolized and utilized for energy production at any given time. Therefore, the body can be considered a combustion engine running on a mixed fuel of carbohydrates and fats. The fuel mixture primarily depends on the intensity of the exertion and the duration of the activity. However, there is always a smooth transition between energy sources. Scientists also refer to these transition rates as substrate rates of energy sources. Particularly during low-intensity activities, the proportion of fats increases while that of carbohydrates decreases. On the other hand, the substrate rate of carbohydrates increases with increasing intensity, and that of fats decreases. The extent to which substrate rates change also depends on the individual’s training status, as exercise duration and intensity need to be considered on an individual basis.
By supplying carbohydrates in liquid or solid form, the carbohydrate stores can be replenished during exertion, thereby maintaining high exercise intensity for a longer period. To achieve maximum success in long-duration endurance activities, this strategy is an essential component for optimizing performance. However, there is a catch. While carbohydrates can be supplied in equal amounts as they are consumed, the body cannot utilize them in the same quantity. Therefore, depleting the carbohydrate stores is merely delayed but not prevented. The goal of every athlete is to achieve the highest possible performance, which is closely associated with exertion at high intensity and duration. For this reason, it is advisable to optimize fat metabolism to spare the carbohydrate stores and be able to rely on them for a longer period.
Forms of Fat
Fat is generally available in two forms for energy production. On one hand, intramuscular fats (IMTG), also known as fat droplets, are stored in the muscle fibers. These fats are directly linked to the mitochondria (the powerhouse of the muscle cell). When the number of fat droplets in the muscle fibers increases, a greater amount can be utilized for energy production. On the other hand, fat is transported from subcutaneous adipose tissue to the muscle cell through the blood and used for energy production. This process can also be improved through targeted training. Therefore, both forms can be optimized through specific endurance training, particularly in terms of quantity and transport rate. Moreover, a well-functioning fat metabolism is an important factor for overall health. A good fat metabolism influences insulin sensitivity as well as weight reduction.
Fat Metabolism Training
As mentioned earlier, fat metabolism depends on exercise intensity. Fat oxidation increases initially with increasing exercise intensity before decreasing at higher intensities when carbohydrates are predominantly utilized.
Therefore, in the context of specific fat metabolism training, it is important to determine the intensity at which this metabolism reaches its maximum, also referred to as the intensity of maximum fat oxidation rate. To precisely determine this, a performance diagnostics test (indirect calorimetry) is required to draw conclusions about substrate rates. It should be noted that the value of the highest oxidation rate can vary significantly among individuals. Additionally, this value changes with exercise duration, making it impractical to provide universally applicable values. For most recreational athletes, it is sufficient to know that moderate intensities sustained over several hours (low-intensity training) are most effective for training fat metabolism.
Influence of Nutrition
Diet already plays a certain role in controlling fat oxidation. Our bodies have a preference for carbohydrates and, to some extent, prioritize them over fats for energy production. Fortunately, the body also recognizes when carbohydrate stores have been partially depleted and subsequently increases the utilization of fats. Athletes can take advantage of these facts. For an individual training session aimed at optimizing fat burning, it is advisable not to consume additional carbohydrates 2-3 hours before the session, allowing for increased reliance on fats. Over a longer period of time, it also makes sense to adopt a low-carbohydrate diet during training phases with the primary goal of improving fat oxidation in order to promote the described processes. Another proven method is fasted training, especially in combination with partial depletion of carbohydrate stores the day before, followed by an intense session and subsequent consumption of low-carbohydrate food. The described mechanism also applies here. Through the intense session the day before, which predominantly utilizes carbohydrates for energy production, the availability of carbohydrates decreases. By not replenishing carbohydrates afterwards, in the subsequent session (at moderate intensity), more fats are utilized to spare the carbohydrate stores, and as a result, the body continuously learns to rely more on fats as an energy source.
But be careful! Fasted training should not be excessively applied as it poses a risk to the immune system. Additionally, it can lead to severe hypoglycemia in athletes and cause a catabolic effect by utilizing protein structures. Therefore, such training sessions should not last longer than 90 minutes to avoid subjecting the body to excessive stress. There are already approaches that completely exclude carbohydrates to remove the choice between fats and carbohydrates from the body. However, it is advised against this because “fats burn in the fire of carbohydrates,” which means that carbohydrates (or pyruvate in another form) are necessary for fat burning, and a certain amount needs to be present for efficient energy production.
The key to an improved fat metabolism, therefore, lies in proper training (low intensity) coupled with the right nutritional strategy. It should be noted that optimizing fat oxidation is just one factor contributing to enhanced performance and better results in the next competition. Consequently, this type of training should be seen as an essential component among many others to improve performance and utilized accordingly.
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