There are various types of carbohydrates and each type in our body is treated differently. For example, both glucose and bran are carbohydrates, but are found at different ends of the energy spectrum. Glucose in the bloodstream rapidly stimulates the rapid and intense secretion of insulin, while the energy from the bran never leads to blood flow because they are indegistable and tend to diminish the insulin response by reducing the tempo by which other energy sources enter the bloodstream. Differences between carbohydrates indicate that athletes need to carefully consider which types of carbohydrates are best in different circumstances. Glucose is the primary source of energy for muscular activity and the higher the intensity of exercise, the greater the reliance on glucose as energy fuel. When all the glucose is spent, the athlete stops with intense exercise. Therefore, one of the main goals of sports nutrition is to prevent glucose consumption. Keeping the amount of carbohydrates at a satisfactory level is very problematic, carbs, in relation to proteins or fats, people have limited capacity to store carbohydrates. Adequate carbohydrate intake becomes even more significant at extremely high levels of exercise intensity, as energy metabolism relies heavily on carbohydrates as a source of energy for muscles. Despite years of research confirming the importance of carbohydrate availability to maintain endurance and mental function, many athletes continue to believe that proteins are the most important nutrients for achieving success in sports. Although all essential nutrients are important, intake of an adequate amount of carbohydrates at the right time optimizes limited carbohydrate reserves, facilitates better transport of carbohydrates to the brain and improves the parameters of muscular and general sustainability. For comparison, excessive protein intake, which often happens among amateur and professional athletes, has little effect on improving sports results.

Types of carbohydrates

Not all carbohydrates have the same form, function and health impact. The basic unit of all carbohydrates is monosaccharides, single-molecular carbohydrates. Monosaccharides usually have 6 carbon atoms and, although only slightly vary in the numerical relationship between oxygen and hydrogen, and subtle variations are often the cause of important metabolic differences. The basic metabolic unit of human cells is monosaccharide glucose; Other monosaccharides are transformed into glucose by various biochemical processes in human cells. The number of monosaccharides linked to larger molecular units is the main basis for the classification of carbohydrate types.

Each of the three major monosaccharides (glucose, fructose and galactose) has different characteristics of solubility, sweetness and reactivity with the food environment in which it is found. With the exception of fructose, which is increasingly widely represented in various types of processed foods such as high-fructose (corn) sweetener, most monosaccharides enter the food ingredients as a product of disaccharide disintegration, biomolecular carbohydrates (consisting of two linked monosaccharides).

There are three major disaccharides – saccharose, maltose and lactose, each containing a different combination of monosaccharides (Table 1). Monosaccharides and disaccharides are commonly called simple carbohydrates (sugars), while polysaccharides are often called complex carbohydrates. Carbohydrates that are not subject to digestion processes in the gastrointestinal system are complex carbohydrates and are often called dietary fibers. Sugars (disaccharides and monosaccharides) have different sweetness characteristics, whereas fructose is the sweetest, followed by saccharose, glucose and lactose (which is the least sweet). However, sugars differ in the sense that they create in the mouth and solubility (for example, fructose is less soluble than sucrose), all of which affects food manufacturers when choosing which sugars to use in the preparation of foods. Athletes now have a range of potions (drinks) available which they can choose the most optimal one, each containing a different dimension of monosaccharides and disaccharides, whereby manufacturers try to achieve the best combination of taste, bowel tolerance, optimum gastric emptying speed, electrolyte recovery, and energy delivery required by active muscles.

Carbohydrate metabolism

People can deposit about 350g (1400cal) carbohydrates in the form of muscle glycogen, an additional 90g (360kcal) in the liver, and a small amount of blood circulating glucose (about 5g or 20kcal). The higher the muscle mass, the greater the potential of muscle glycogen is, but the potential need for energy fuel is also growing.

The human organism has several mechanisms to maintain blood glucose levels (glycemia) within a relatively small range (3-5mmol / L), primarily with the help of insulin and glucagon. Insulin and glucagon are pancreatic hormones that work synergistically, which helps control glycemic processes. Excess insulin production may lead to hypoglycaemia (low blood glucose concentration), conditions that could cause excessive production and fat accumulation; insufficient insulin production causes hyperglycemia (high blood glucose levels) and the occurrence of diabetes.

Insulin secrete the beta cells of the pancreas, while the glucagon secretes the alpha-cells of the pancreas. Stimulus for insulin secretion is a high blood glucose level – the higher the glucose level, the stronger the insulin response is. However, the pancreas constantly secretes a small amount of insulin, even when the blood glucose level is within normal limits, which allows a stable flow of glucose to the brain and muscle cells. Insulin lowers blood glucose levels by affecting cell membranes of muscle cells and fat tissue cells. allowing the blood glucose to enter the cells. The process of transition of glucose from the blood into the inside of the cell explains the effect that insulin has on lowering blood glucose levels; also provides cells with the necessary source of energy.

When blood glucose is at a low level, as is the case between meals and during exercise, the glucagon is light. The lower glycemic value, the production of glucagon is higher. Glucagon causes glycogen catabolism in the liver, causing the release of glucose molecules into the circulation. Glucagon can also stimulate gluconeogenesis (the production of glucose from amino acids, fatty acids and other compounds other than carbohydrates). The amino acid alanine, for example, is formed from the protein and in the liver is transformed into glucose.

About 60% of glucose released from the liver, in order to maintain glycemic control within the optimum values, originates from glycogen reserves in the liver, and the rest comes from glucose synthesized from lactate, pyruvic acid (pyruvate), glycerol and amino acid. The amount of glucose released from the liver during exercise is dependent on the intensity of the activity, when exercising with greater intensity causes a faster pace of release of glucose from the liver. At the same time, the presence of low insulin concentrations, increased levels of adrenaline and glucagon in circulation during prolonged activity stimulate the release of glucose from the liver.

In addition to insulin and glucagon, another two hormones are affected by blood glucose levels. Adrenaline is a stress hormone that causes an extremely rapid splitting of glycogen in the liver to rapidly increase glycemic activity. Cortisol, which comes from the adrenal gland, is also a stress hormone that promotes protein catabolism. The protein splitting ensures that certain glucose amino acids are available for gluconeogenesis, resulting in an increase in blood glucose levels. Both adrenaline and cortisol are released as a result of exercise-related stress and the concentration of both hormones is affected by carbohydrate intake. Controlled formation of adrenaline helps maintaining the level of glycogen in the liver, and controlled cortisol secretion helps preserve proteins in the muscles. This is a strong argument that supports the consumption of carbohydrates during exercise.

Glucose is circulating in the blood obtained mainly from dietary carbohydrates, among which starch is the main source. Complex carbohydrates (starch) are dissolved in the gastrointestinal system on monosaccharides (glucose, fructose and galactose) to absorb into the blood. Some Individuals do not have enough amount of lactase enzyme to treat milk sugar (lactose) to its constituent monosaccharides (glucose and galactose), which prevents decomposition of milk. This condition is known as lactose intolerance and leads to bloating, stomach pain, diarrhea, and dehydration.

Carbohydrates that are introduced through food to the degraded form of glucose, are deposited in the liver and muscles in the form of glycogen, but only to a certain extent. The maximum capacity of the liver to deposit glycogen amounts is about 87 to 100 grams (348 to 400 kcal), while in muscles it can be deposited approximately 350 grams (1400 kcal) or more when it comes to more muscular people. If cells supplement the amount of glucose after the glycogen reserves have been filled, glucose is transformed and its excess is deposited in the form of fat (in muscle cells and fat tissue cells). Glycogen in the liver is primarily responsible for stabilizing blood glucose levels. On the other hand, glycogen deposited in muscles is mainly used as a source of energy for active muscles, during aerobic and anaerobic activities. The blood glucose level is not easy to maintain when the complete amount of glycogen from the liver is consumed , even when the muscle reserves are full.

Glucose in the blood is the primary source of fuel for the central nervous system (CNS). If the blood glucose level is low then the central nervous system function is affected, which is accompanied by increased irritability and reduced concentration. For thletes, a low level of glucose in the blood can cause the occurrence of mental (central) fatigue, which is associated with muscle fatigue. As glycogen reserves from the liver and blood glucose are easily consumed, even during short-term activities, carbohydrate intake during the activity is a key factor for maintaining both muscular and mental functions.


Adenosine triphosphate (ATP) is a high energy cellular compound. The human organism has limited reserves of directly available ATP, so it must be rapidly generated during exercise. As the exercise is more intense, ATP must regenerate faster. In hibernation and in low-intensity activities, ATP can be the product of iron-based oxidation of carbohydrates and fats. However, as the intensity of exercise increases, athletes need an ATP production level that can not be completely replaced by aerobic energy ways. (The table summarizes the energy metabolic systems.)

System Characteristics Duration
Phosphocreatine (PCr) system Anaerobic production of ATP by decomposition of phosphocreatine. It is used for short-term activities of maximum intensity.
Anaerobic glycolysis (lactic acid system) Anaerobic production of ATP by glycogen splitting; By-product of this system is lactic acid. It is used for submaximal intensity activities that exceed the ability of an athlete to enter sufficient oxygen content; ATP of this system can not be manufactured longer than 2min.
Aerobic glycolysis Aerobic production of large amounts of ATP by glycogen separation. It is used for high-intensity activities that require large amounts of ATP, but in which an athlete can enter sufficient oxygen content.
Oxidative phosphorylation Aerobic ATP production by decomposition of carbohydrates and fats. The benefit is for low-intensity and long duration activities that can produce a significant level of ATP, but without creating by-products that limit the system.

Glycolysis is a process in which a large amount of ATP can be produced by digestion of glycogen to glucose; it is processed in the presence of oxygen (aerobic glycolysis) or without oxygen (anaerobic glycolysis). Aerobic glycolysis has the capacity to produce a higher amount of ATP than anaerobic glycolysis and, unlike anaerobic glycolysis, ATP is produced without the production of laconic acid. For this reason, anaerobic glycolysis is also called the lactic acid system. In activities where exercise intensity exceeds the ability to enter sufficient oxygen content to meet the need for aerobic digestion of glucose, anaerobic glycolysis becomes the main energy system for the production of ATP. However, high-intensity anaerobic activity limits itself because the accumulation of lactic acid allows the activity to last up to 1.5 to 2 minutes. Accordingly, high intensity sports are typically organized to leave a chance for recovery. For example, the exercise at the sports gymnastics session lasts 1.5min, after which the gymnast is resting and recovering to prepare for the next high-intensity discipline; In hockey, a player’s change is often made (the hockey player almost never clings longer than 2 minutes in continuity) to enable muscle recovery.

It is best that lactic acid produced in anaerobic glycolysis is considered to be a form of reserve energy, waiting to be metabolized when a sufficient amount of oxygen is allowed again. When the exercise intensity decreases and when an athlete enters enough oxygen for aerobic metabolic processes, then lactic acid is converted into pyruvic acid and a benefit for aerobic production of ATP.


Gluconeogenesis is the process of glucose production from non-carbohydrate substances. As we have seen, blood glucose is crucial for the function of the central nervous system, it helps in the metabolism of fat and provides fuel to active muscle cells. However, due to its limited storage capacity in the form of glycogen, a certain amount of glucose can become available by making glucose from non-carbohydrate substances. There are three systems that allow gluconeogenesis:

Triglycerides are the main form of fat reserves in the human body; is composed of three fatty acids linked to the glycerol molecule. By freeing up the triglyceride, free glycerol molecules are formed (a compound containing three carbon atoms), and by joining two glycerol molecules in the liver, one molecule of glucose (a compound containing 6 carbon atoms) is formed.

By decomposing muscle proteins, a series of free amino acids have a role to play. One of these amino acids, alanine, liver can convert to produce glucose.

In anaerobic glycolysis, lactic acid is produced. Lactic acid salts (lactates) can be converted to pyruvic acid (pyruvate) during aerobic production of ATP. On the other hand, two lactic acid molecules can be combined in the liver to form glucose. Conversion of lactate to glucose is called the Cori cycle – lactic acid is replaced by glucose in active muscles. If the blood glucose level is low, pyruvic acid (pyruvate) can be converted to lactic acid and glucose produced through the Cori cycle.

“Advanced Sports Nutrition”

Dan Benardot

Marko Popin

🇬🇧 Advanced student of medical science and nutrition expert, skyrunner and dog lover.

🇷🇸 Apsolvent medicine i ekspert za ishranu, skyrunner i ljubitelj pasa.

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