Muscle glycogen represents a source of glucose, therefore energy, that can be used by muscle during physical activity: it is an energy store where needed!
Furthermore a close relationship exists between the onset of fatigue and depletion of its muscle stores.
Glycogen as energy source
Carbohydrates and fatty acids represent the main energy source for muscle during exercise and their relative contribution varies depending on:
the intensity and duration of exercise;
the level of training.
If for fatty acids there are no problems regarding body stores so it is not for carbohydrates whose stores, present in glycogen form principally in the liver and the muscle, are modest, less than 5% of total body energy stores: in a non-fasting 70 kg adult male there are about 250 g of glycogen in the muscle and 100 g in the liver, for a total energy of about 1400 kcal. In athletes the amount could be higher, for example in the best marathoners, again considering an adult male as above, you can reach up to 475 g in total, muscle plus liver, which corresponds to about 1900 kcal.
In spite of this, glycogen contribution to the total energy needed to sustain muscular workload rises with the increase of exercise intensity, whereas we reduce that in the form of fatty acids.
Furthermore, in the absence of replenishment with exogenous carbohydrates, performance is determined by the endogenous stores of liver and skeletal muscle glycogen, of which relative consumption is different: an increase of intensity increases that of the second (muscle) while remain more or less constant in that of the first (liver).
Skeletal muscle glycogen and intese exercises
In fact, skeletal muscle glycogen represents the most important energy reserve for prolonged moderate-high intensity exercise, an importance that increase in the case of high-intensity interval exercise (common in training session undertaken by swimmers runners, rowers or in team-sport players) or in resistance exercise, therefore both endurance and resistance exercises. If for example we consider the marathon about 80% of utilized energy derives from carbohydrate oxidation, for the most part skeletal muscle glycogen.
Finally, the replenishment rate of glycogen stores in post-exercise is one of the most important factors in establishing necessary recovery time.
Muscle glycogen and fatigue
Fatigue and low glycogen levels are closely correlate but it is not clear which mechanisms are at the basis of this relationship; one hypothesis is that there exists a minimum glycogen concentration that is “protected” and is resistant to being used during exercise, perhaps to ensure an energy reserve in case of extreme necessity.
Because of the closely relationship between skeletal muscle glycogen depletion and fatigue, its replenish rate in the post-exercise is one of the most important factors in determining necessary recovery time.
Arienti G. “Le basi molecolari della nutrizione”. Seconda edizione. Piccin, 2003
Beelen M., Burke L.M., Gibala M.J., van Loon J.C. Nutritional strategies to promote postexercise recovery. Int J Sport Nutr Exerc Metab 2010:20(6);515-32 doi:10.1123/ijsnem.20.6.515
Cozzani I. and Dainese E. “Biochimica degli alimenti e della nutrizione”. Piccin Editore, 2006
Giampietro M. “L’alimentazione per l’esercizio fisico e lo sport”. Il Pensiero Scientifico Editore, 2005
It is now accepted by athletes, coaches and athletic trainers that proper diet is one of the cornerstones for achieving better athletic performance. Despite this widely spread assumption, many, even at the highest levels, still believe that an high protein intake is fundamental in the athlete’s diet. This opinion is not new and is deeply rooted in the imaginary of many people almost as if, eating meat, even of big and strong animals, we were able to gain their strength and vitality too.
The function of proteins as energy-supplier for working muscle was hypothesized for the first time by von Liebig in ‘800 and it is because of his studies if, even today, animal proteins, and therefore meats, are often believed having great importance in the energy balance in the athlete’s diet, despite nearly two centuries in which biochemistry and sports medicine have made enormous progress.
Really, by the end of ‘800 von Pettenkofer and Voit and, at the beginning of ‘900, Christensen and Hansen retrenched their importance for energy purposes, also for the muscle engaged in sport performance, instead bringing out the prominent role played by carbohydrates and lipids.
Of course we shouldn’t think that proteins are not useful for the athlete or sedentary people. The question we need to answer is how many proteins a competitive athlete, engaged in intense and daily workouts, often two daily sessions (for 3-6 hours), 7/7, for more than 10 months a year, needs per day. We can immediately say that, compared to the general population, and with the exception of some sports, (see below) the recommended amount of protein is greater.
Metabolic fate of proteins at rest and during exercise
In a healthy adult subject engaged in a non-competitive physical activity, the daily protein requirements is about 0.85 g/kg desirable body weight, as shown by WHO.
Proteins turnover in healthy adults, about 3-4 g/kg body weight/day (or 210-280 g for a 70 kg adult), is slower for the muscle than the other tissues and decreasing with age, and is related to the amount of amino acids in the diet and protein catabolism. At rest the anabolic process, especially of synthesis, uses about 75% amino acids while the remaining 25% undergoes oxidative process, that will lead to CO2 and urea release (for the removal of ammonia). During physical activity, as result of the decreased availability of sugars, i.e. muscle glycogen and blood glucose used for energy purposes, as well as the intervention of cortisol, the percentage of amino acids destinated to anabolic processes is reduced while it increases that of amino acids diverted to catabolic processes, that is, it occurs an increase in the destruction of tissue proteins. At the end of physical activity, for about two hours, anabolic processes remain low whereupon it occurs their sharp increase that brings them to values higher than basal ones, so, training induces an increase in protein synthesis even in the absence of an increase in proteins intake.
What determines the daily protein requirements?
There are many factors to be taken into account in the calculation of the daily protein requirements.
The age of the subject (if, for example, he/she is in the age of development).
Gender: female athletes may require higher levels as their energy intake is lower.
An adequate carbohydrate intake reduces their consumption.
During physical activity, glucogenic amino acids may be used as energy source directly in the muscle, after their conversion to glucose in the liver through gluconeogenesis.
An adequate carbohydrate intake before and during prolonged exercise lowers the use of body proteins.
The energy intake of the diet.
A reduced energy intake increases protein requirements; conversely, the higher energy intake, the lower the amount of protein required to achieve nitrogen balance; usually there is a nitrogen retention of 1-2 mg per kcal introduced.
If the athlete is engaged in very hard competition/workouts, or if he requires an increase in muscle masses (e.g. strength sports) nitrogen balance must be positive; a negative balance indicates a loss of muscle mass.
The nitrogen balance is calculated as difference between the nitrogen taken with proteins (equal to: g. proteins/6.25) and the lost one (equal to: urinary urea in 24 hours, in g., x0.56]; in formula:
Nb (nitrogen balance) = (g. protein/6.25) – [urinary urea in 24 hours, in g., x0.56)]
The type of competition/workouts that the athlete is doing, either resistance or endurance, as well as the duration and intensity of the exercise itself.
Resistance training leads to an increase in protein turnover in muscle, stimulating protein synthesis to a greater extent than protein degradation; both processes are influenced by the recovery between a training and the next one as well as by the degree of training (more training less loss).
In the resistance and endurance performances the optimal protein requirements in younger people as for those who train less time are estimated at 1.3 to 1.5 g protein/kg body weight, while in adult athletes who train more time is slightly lower, about 1-1.2 g/Kg of body weight.
In subjects engaged in a hard physical activity, proteins are used not only for plastic purposes, which are incremented, but also for energy purposes being able to satisfy in some cases up to 10-15% of the total energy demand.
Indeed, intense aerobic performances, longer than 60 minutes, obtain about 3-5% of the consumed energy by the oxidation of protein substrates; if we add to this the proteins required for the repair of damaged tissue protein structures, it results a daily protein demand about 1.2 to 1.4 g/kg body weight.
If the effort is intense and longer than 90 minutes (as it may occur in road cycling, running, swimming, or cross-country skiing), also in relation to the amount of available glycogen in muscle and liver (see above), the amount of proteins used for energy purposes can get to satisfy, in the latter stages of a prolonged endurance exercise, 15% of the energy needs of the athlete.
The physical condition.
When needed, the desired weight.
Athletes attempting to lose weight or maintain a low weight may need more proteins.
From the above, protein requirements don’t exceed 1.5 g/kg body weight, also for an adult athlete engaged in intense and protracted workouts, while if you consider the amount of protein used for energy purposes, you do not go over 15% of the daily energy needs.
So, it’s clear that diets which supply higher amounts (sometimes much higher) of proteins aren’t of any use, stimulate the loss of calcium in bones and overload of work liver and kidney. Moreover, excess proteins don’t accumulate but are used to fat synthesis.
How to meet the increased protein requirements of athletes
A diet that provides 12 to 15% of its calories from protein will be quite sufficient to satisfy the needs of almost all of the athletes, also those engaged in exhausting workouts.
In fact, with the exception of some sports whose energy expenditure is low, close to that of sedentary subject (for example: shooting, or artistic and rhythmic gymnastics), athletes need a high amount of calories and, for some sports such as road cycling, swimming or cross-country skiing, it may be double/triple than that of a sedentary subject.
The increase in food intake is accompanied by a parallel increase in protein intake, because only a few foods such as honey, maltodextrin, fructose, sugar and vegetable oils are protein-free, or nearly protein-free.
Calculation of protein requirements of athletes
If you consider an energy demand of 3500 kcal/die, with a protein intake equal to 15% of total daily calories, you have:
3500 x 0.15 = 525 Kcal
As 1 gram of protein contains 4 calories, you obtain:
525/4 = 131 g of proteins
Dividing the number found by the highest protein requirements seen above (1.5 g/kg body weight/day), you obtain:
131/1.5 = 87 kg
that is, the energy needs of a 87 kg athlete engaged in intense workouts are satisfied.
Repeating the same calculations for a caloric intake of 5000 , you obtain 187 g of protein; dividing it by 1.5 the result is 125 kg, that is, the energy needs of a 125 kg athlete are satisfied.
These protein intakes can be met by a Mediterranean-type diet, without protein or amino acids supplements.
Giampietro M. L’alimentazione per l’esercizio fisico e lo sport. Il Pensiero Scientifico Editore. Prima edizione 2005
Protein and amino acid requirements in human nutrition. Report of a joint FAO/WHO/UNU expert consultation. 2002 (WHO technical report series ; no. 935).
Invert sugar (also known as inverted sugar) is sucrose partially or totally cleaved into fructose and glucose (also known dextrose) and, apart from the chemical process used (see below), the obtained solution has the same amount of the two monosaccharides.
Moreover, according to the product, not cleaved sucrose may also be present.
Invert sugar production
The breakdown of sucrose may happen in a reaction catalyzed by enzymes, such as:
sucrase, active at our own intestinal level;
invertase, an enzyme secreted by honeybees into the honey and used industrially to obtain invert sugar.
Another process applies acid action, as it happens partly in our own stomach and as it happened in the old times, and still happens, at home-made and industrial level. Sulfuric and hydrochloric acids was used, heating the solution with caution for some time; in fact the reaction is as fast as the solution is acid, regardless of the type of acid used, and as higher the temperature is. The acidity is then reduced or neutralized with alkaline substances, as soda or sodium bicarbonate.
A chemical process as described occurs when acid foods are prepared; i.e. in the preparation of jams and marmalades, where both conditions of acidity, naturally, and high temperatures, by heating, are present. The situation is analogous when fruit juices are sweetened with sucrose.
The reaction develops at room temperature as well, obviously more slowly.
What is the practical outcome of that?
It means that, during storage, also sweets and acid foods, even those just seen, go towards a slow reaction of inversion of contained/residue sucrose, with consequent modification of the sweetness, since invert sugar at low temperatures is sweeter (due to the presence of fructose), and assumption of a different taste profile.
Properties and uses
It is principally utilized in confectionery and ice-cream industries thanks to some peculiar characteristics.
It has an higher affinity for water (hydrophilicity) than sucrose (see fructose) therefore it keeps food more humid: e.g. cakes made with invert sugar dry up less easily.
It avoids or slows down crystal formation (dextrose and fructose form less crystals than sucrose), property useful in confectionery industries for icings and coverage.
It has a lower freezing point.
It increases, just a bit, the sweetness of the product in which it has been added, as it is sweeter than an equal amount of sucrose (the sweetness of fructose depends on the temperature in which it is present).
It may take part to Maillard reaction (sucrose can’t do it) thus contributing to the color and taste of several bakery products.
It should be noted that honey, lacking in sucrose, has almost the same composition in fructose and glucose of the 100% invert sugar (fructose is slightly more abundant than glucose). So, diluted honey, better if not much aromatic, may replace industrial invert sugar.
Belitz .H.-D., Grosch W., Schieberle P. “Food Chemistry” 4th ed. Springer, 2009
Bender D.A. “Benders’ Dictionary of Nutrition and Food Technology”. 8th Edition. Woodhead Publishing. Oxford, 2006