To be in motion, muscles need energy that comes from food, which is why it’s so important to have a balanced alimentation. Variety and diversity raise the muscular efficiency and therefore the athlete’s performances. Muscles, which allow every move of our body, are like a factory transforming chemical energy into mechanical energy. Most of our body cells are able to use as a glucose substrate (carbohydrates) fatty acids (lipids), amino acids (proteins) in various proportions.
Note: Some cells, like nervous cells and red blood cells can only use glucose as a source of energy. They are named glucose-dependant cells.
Energy managing is crucial to the athlete as it directly influences its capacity to perform an effort. The most important parameter is to maintain a good level of glucose in the liver and in the muscles. It represents the main stock of glucose in our body. When these stocks are empty, our brain detects the information and order our body to stop the intense effort (this is the central fatigue, the will). Levels of glycogen can be maintained and improved through the use of lipids during the effort.
Note: There is also a peripheral fatigue directly linked to the muscular damages due to contraction. We can feel this fatigue in our muscles and it can stop the effort, even if the will to perform is still there.
Reminder about the energetic micronutrients of sport
Carbohydrates are sugars and derivates of sugars (including maltodextrins). Glucose is a carbohydrate from which our body pumps almost all its energy. Starch is an organic carbohydrate. Food that is rich in starch (pasta, rice, potatoes, quinoa, leguminous plants…) is called starchy food. Glycogen is the animal equivalent of starch. It represents the way our body stocks glucose in the liver (hepatic glycogen) and in the muscles (muscular glycogen).
Lipids are fats. There are two unsaturated fatty acids that our organism is unable to synthesise and therefore, that have to be provided by food : essential fatty acids = linoleic acid and linolenic acid. In a general manner, lipids represent a great source of energy for the organism as they interact with growth, reproduction, skin metabolism…
Proteins are part of a protein category in which you can also find amino acids. Our body is capable of synthesizing most of amino acids (there are about 20), but 8 of them have to be provided through alimentation : they are the essential amino acids. From those amino acids, our body builds its own protein cells that form the architecture of the living material (muscles, bones, viscera…). When in necessity, the body is capable to use its own proteins to cover its energetic needs (it becomes anthropophagous!)
Reminder abour the energetic metabolism during the effort
Physical efforts from seconds to several hours imply our muscles to consume substrates. Carbohydrate substrates represent the majority of those them. The quantity of consumed carbohydrates depends on two factors : intensity and duration of the effort. On one hand, intense activity activates the carbohydrates metabolism, whereas on the other hand, moderate efforts are mostly sustained by the aerobic process, which engages sugar, fat and protein stocks.
The balanced combination of the different energy sources depends on the duration of the effort. Given an equal intensity, the longer the effort, the richer in lipids the combination will be. Then the muscle deteriorates the energy stocks : the liver glycogen and especially the fatty acids from the adipose tissue. Our organism acts as a glycogen economiser, even better when it is well trained. Among all muscular cells, glycogen (hepatic and muscular stocks) is converted in glucose.
The depletion of muscular glycogen happens after more or less 90 minutes of exercise at 75% of maximum VO2 (Maximum consumption of oxygen), and after 4 hours at 55%. There is a link between the depletion of glycogen and the decrease of our ability to produce a high level intensity effort. The depletion of muscular glycogen does not imply the end of muscular action, but considerably lowers the maximum ability of work. Alongside, muscular triglycerides are degraded and provide the mitochondrial production of energy.
Note: The oxidation of fatty acids coming from those triglycerides might represent up to 50% of the total oxidation of lipids during the effort.
Likewise, amino acids’ metabolism is affected by the effort and particularly by the long lasting ones (more than 2 hours) because of the oxidation of the branch chained amino acids (BCAA). How much energy is produced by the oxidation of the amino acids depends on the type of exercise, its intensity, its duration, the level of training and the quality of ingested food. However, in standards conditions, the energy produced from the oxidation of amino acids is small, varying from 3 to 10% according to studies. Even if it is quantitatively small, the use of nitrogenous substrate during the effort can have two consequences : the lyse of contractile proteins (the skeletal muscles represent the most furnished amino acids stock) and the appearance of fatigue.
The hepatic glucose (liver) gradually raise during the muscular solicitation and during an intense exercise. Two metabolic ways generate the production of hepatic glucose: the glycogenolysis (from the glycogen stocks) and the gluconeogenesis (from lactate, analine and glycerol). Their involvement in the production of energy depends on the intensity and the duration of the effort. At the beginning of the exercise, the glycogenolysis is predominant whereas the gluconeogenesis becomes major during a long lasting effort.
Regarding the adipose tissues, triglycerides are deteriorated into fatty acids and glycerol. Produced fatty acids are then carried into the blood circulation by the albumin, up to the muscular cells in order to allow mitochondrial energy production.
Why having optimised glycogen stocks?
There is a real interest in having high levels of glycogen stocks as they allow us to perform a high speed exercise during a longer time. This is called capacity. Factors influencing positively glycogen stocks are:
- The initial stock of glycogen: a muscular fibre that is « empty in glycogen » is going to pump more « sugar », it is the Overcompensation phenomenon.
- The level of training: the more trained is the athlete, the better is its ability to stock (better sensibility to insulin)
- Alimentation: eating carbohydrates (8 à 10g/kg/day) during the first hours following the effort enhances the resynthesis of glycogen.
Good glycogen stocks are closely linked to a good hydration : 2 litres minimum! 1g of glycogen is stocked with 3g of water!
Total stock of glycogen is basically around 500g which represents with the associated water around 2kg for the lower. It can raise up to 1,5kg, meaning 6kg with the associated water.
It is then easy to lose gross weight on the scale the first days of a drastic diet when you stop eating carbohydrates (the famous cabbage soup during one week). But the only result will be the initiating of an unavoidable failure as soon as you restart eating normally. Associated with less ingested salt (including sodium), the water loss is increased and the weight loss raised quantitatively but certainly not qualitatively! (refer to my article about : Body scales: biased item).
Why eat specifically to produce glycogen before the effort?
Who doesn’t know the famous « marathon wall » (feeling amorphous and numb!) that many of us encountered at least one time in our lives (and that can be weakened by an energy drink). Physiologically, the « marathon wall » is the consequence of a strong decrease of muscular glycogen stocks (energy purposed sugar stocks). In the 1960’s sport diet searchers tried to find solutions to start competitions with optimised glycogen stocks (hepatic but mostly muscular). The result is that the more our stocks are full at first, the further the arrival of the wall will be!
Different trial diets have been tested. The first one was about giving a lot of carbohydrates to athletes the seven days before a stamina challenge. Then, was created the two steps food combining diet, named » The Scandinavian food combining diet ». The first phase (3 to 4 days) was about a low carbohydrates diet associated with intense exercises which aim was to diminish to its lower rate the level of glycogen. This step was followed by the second phase (3 to 4 days) during which the athlete’s alimentation was enriched with carbohydrates, associated or not with small intensity exercises.
Recently, this way of treating dieting has been slightly modified (Modified food combining diet) : it consists mostly in raising the carbohydrates intake during the last 3 days before the competition, which is more convenient to organise on the field. It is showed that the glycogenic overcompensation is what’s best for a rich carbohydrates diet (10 to 12g/kg/day) the 3 days before the physical challenge.
Glycogen during the effort
Body consumes energy during a physical effort. The energy is mostly brought by glycogen (either hepatic or muscular). Glycogen stocks are created thanks to the daily food intakes (breakfast, lunch, eventually snacks, diner). It is said that, considering a balanced alimentation, the food can provide the sufficient energy for a 1h to 1h30 mid-intense exercise. Passed this time, the energy stock is deeply weakened, the performance gets lower… except if you bring an exogenous, outside energy contribution such as an sports drink (refer to homemade sports drink), energy bar or energy gel or even any other food such as banana, dried fruits, compotes…
Interests of glycogen stocks after the effort
Carbohydrates intake after a physical effort is very important as it directly takes part into the resynthesis of glycogen stocks. In practice, a glucose and fructose input is recommended : Glucose helps resynthetising muscular glycogen whereas fructose is more about the resynthesis of the hepatic glycogen. Those glycogen stocks, well rebuilt, will help having a faster recovery in order to increase the effort efficiency along the season.
Sports Dietitian Nutritionist