Energy supply

NO CARBS, NO GLORY! is not just our company name, but our core message for everyone who exercises and is interested in improving their performance. Without adequate energy supply, no muscle work or corresponding performance is possible. 

Exercise means muscle work, which requires fuel. This is called ATP (adenosine triphosphate), which is a so-called “energy-rich phosphate” that enables muscle contraction through its hydrolysis. Since only a very small amount of ATP is stored in the muscle cell, this chemical energy must be constantly generated in the muscle metabolism so that it can be converted into mechanical energy. 

Which energy sources are available for ATP production? 

1. As already mentioned, only a very small amount of ATP is stored in the muscles (approx. 2 kcal). In addition, there is a second “energy-rich phosphate,” phosphocreatine, which can immediately regenerate ATP from ADP through its hydrolysis, but is also only available in small amounts (4–8 kcal). Although energy-rich phosphates provide immediately available chemical energy that enables immediate peak physical performance, they can only do so for a few seconds. It follows that there must be energy sources with a greater capacity for ATP production.
2. The actual energy carriers are the nutrients carbohydrates and fats. Carbohydrates are stored in the muscles and, to a lesser extent, in the liver (max. 100 grams) as glycogen (the storage form of glucose). Depending on training status and nutrition, up to 500 grams of glycogen can be stored in the muscle cells (corresponding to approx. 2000 kcal). This energy source enables intense endurance exercise for up to about one and a half hours. The vast majority of energy stores are provided by fats, which are stored not only under the skin (subcutaneous fat) but also in the abdominal cavity around the internal organs (visceral or abdominal fat). In lean people, the energy contained in the fat deposits—some fat is even stored in the muscles themselves—is 30 to 50 times greater than the energy stored in the form of glycogen (even greater in overweight individuals). This makes endurance performance possible for hours or even days, albeit at a lower intensity. Even at rest, we burn almost exclusively fat or fatty acids in our muscles.
3. If the body cannot obtain sufficient energy from carbohydrates and fats, it can also use the macronutrient protein as an energy source. This exceptional situation, which we as athletes want to avoid at all costs, is known as catabolic protein metabolism.

Carbohydrates and fats are therefore our preferred energy-providing nutrients. It should be noted on this occasion that the breakdown of carbohydrates yields energy more quickly than the burning of fatty acids. Carbohydrates are therefore THE decisive energy provider. 

Whether carbohydrates or fats are used as energy sources and thus ATP producers depends on the intensity and duration of the physical exercise. At low intensity, the body initially relies on fats and less on carbohydrates. Ultimately, the body wants to conserve its carbohydrate stores in the muscles. However, some carbohydrates are always consumed, as fats are always burned in the fire of carbohydrates. 

This means that the combustion processes of carbohydrates and fats occur simultaneously in aerobic energy supply (with oxygen). Depending on the intensity, one type of combustion will predominate. In anaerobic energy supply (without oxygen), the body relies solely on carbohydrate combustion.  

This means:

• During low-intensity exercise, energy supply is primarily provided by fats. We recommend a significantly reduced intake of carbohydrates during these easy training sessions.
• During moderate-intensity exercise (when breathing becomes noticeably heavier), the body increasingly relies on carbohydrates, and fat burning takes a back seat. At high intensities, energy supply is ultimately provided solely by carbohydrates.
• At high intensities, energy supply is provided solely by carbohydrates.

Energy supply

anaerobic

1. alactacid

ATP

Phosphocreatine

2. lactacid

Glycolysis

aerobic

3. Glycolysis

4. Beta-oxidation (fat burning)

Fig. 1: 4 mechanisms of energy supply

If we now delve a little deeper into the topic, we can distinguish between 4 mechanisms of energy production (Fig. 1):

1. Aerobic energy production from carbohydrates (aerobic glycolysis) in the mitochondria of the cell—complete combustion of glucose (grape sugar)
2. and fats (beta-oxidation) in the mitochondria of the cell—burning of fatty acids after fat breakdown: This mechanism of ATP production is relevant in endurance sports where maximal oxygen uptake (VO2max) is crucial. If physical exercise of a large muscle group lasts longer than 90 seconds, aerobic (=oxidative) energy production begins to play the decisive role. Carbohydrates and fats are always used as energy providers, with a fluid transition in the proportion of energy supply depending on the exercise intensity, which depends primarily on training status. During very intense aerobic demands (e.g., a 5000m run), almost exclusively carbohydrates (in the form of glycogen or glucose) are used, while during more extensive, longer-duration exercise (e.g., road cycling), more fatty acids are burned. During intense endurance exercise, glucose is partially burned incompletely, so anaerobic glycolysis also contributes to a certain extent to the otherwise aerobic energy supply. In this case, however, lactate formation (anaerobic) and lactate breakdown (aerobic) must balance each other out to avoid over-acidification. This corresponds to the individually possible maximum intensity that can be maintained over a longer period, the so-called “power output” at the so-called anaerobic threshold (more precisely: the aerobic-anaerobic threshold or continuous power output limit), which is the decisive criterion in endurance sports. The anaerobic threshold is often stated as 4 mmol/l of lactate, but this is only an average value, which is why it should be determined individually in competitive sports (e.g., for marathon runners or road cyclists, the continuous power output limit is usually significantly below 4 mmol/l, while for untrained individuals it is usually above). If the exercise intensity is set too high (above the anaerobic threshold), increasing muscle over-acidification, accompanied by a corresponding accumulation of protons and thus also lactate (lactate production exceeds lactate elimination), will force premature termination of the exercise. Lactate determination is important in performance diagnostics and training control in competitive endurance sports, but not in recreational sports. Depending on training status, muscle glycogen stores are largely depleted after 60 to 90 minutes of intense continuous exercise. If the endurance exercise continues, muscle metabolism now relies on increased fat burning, although this energy supply requires more oxygen and occurs only half as fast as oxidative glucose combustion. This means that a reduction in exercise intensity (e.g., running speed) is generally necessary (the notorious “man with the hammer” e.g., in a marathon when one has not paced oneself well or has “forgotten” to regularly consume carbohydrates). It is even worse if insufficient or “forgotten” carbohydrate intake also depletes the liver glycogen stores, because this leads to a drop in blood sugar levels. This can lead to a “hypo” (=hypoglycemia), which is known and equally feared in cycling circles as a “hunger crash,” because it causes a “weakness attack”.
3. Anaerobic energy production from energy-rich phosphates, phosphocreatine, and ATP—anaerobic-alactacid energy supply: This energy source is very small and only lasts for a short time, namely 6 to 10 (max. 15) seconds. It is crucial for maximal strength, explosive power, and speed (examples: 100m sprint, weightlifting, shot put, high jump, etc.). The energy-rich phosphates consumed during this process are also very quickly replenished (depending on training status, after a few seconds to a few minutes). For several years now, high-dose creatine supplementation has been common in strength and sprint sports to increase the muscle phosphocreatine stores and thus improve performance. 
4. Anaerobic energy production from the incomplete breakdown of glucose with the formation of lactate (lactic acid)—anaerobic-lactacid energy supply: This mechanism, which is crucial for strength endurance and speed endurance, provides the necessary energy for very intense, maximally possible performance between 15 and 45 (max. 60) seconds. During this process, glucose derived from muscle glycogen is incompletely broken down, producing lactate (“lactic acid,” more precisely: the anion of lactic acid), which accumulates in the stressed muscles. This leads to metabolic acidosis (“over-acidification”), which is not only painful but ultimately performance-limiting, because in the acidic environment (the limit is at a pH of 7), muscle contraction is inhibited by enzyme inhibition—you are “blue,” as it is called in technical jargon. A classic example of this is the 400m run (where athletes become noticeably slower in the final meters due to extreme over-acidification), as well as the 500m speed skating, the 1000m track cycling time trial, but also a prolonged final sprint in long-distance running. 400m sprinters achieve the highest lactate levels of all due to their large anaerobic capacity and acid tolerance (up to 30 mmol/l).

Table 1 Mechanisms of energy supply by exercise duration and – intensity

Literature:

Raschka, Christoph et al.: 2012 Sport and Nutrition DOI: 10.1055/b-0034-41797 

Jeukendrup, A. E. (2011). Nutrition in endurance sports: Marathon, triathlon and road cycling. Food, Nutrition and Sports Performance III, 29, 91-99.