The overall storage of CHO in the body is rather small.
- Highest concentration of glycogen is found in the liver, this organ is not the large site of glycogen storage. There is a higher concentration of glycogen in liver- typically ranging from 4% after an overnight fast and up to 8% after meals, compared to muscle.
- Glycogen in muscle typically has a concentration of < 1.5% and requires deliberate CHO loading to approach and exceed 3%.
|Glycogen storage- adult-375-475g of CHO
MUSCLE-325G * 4=1300 KCAL +
LIVER –90-110G * 4= 360-440 KCAL
CIRCULATING GLUCOSE=20*4=80 KCAL
- However, the average amount of glycogen stored in muscle (300-400g) is > in liver (80-90g) due to > overall mass of skeletal muscle.
The large mass of skeletal muscle, and its relatively high glycogen concentration, results in skeletal muscle having the most stored glycogen.
- Liver glycogen helps glucose homeostasis. In post exercise period (when muscle glycogen is depleted) first the muscle stores will be built up and then liver stores.
- During rest period the uptake of glucose by muscle is influenced by insulin causing high rate of glycogen synthesis. Muscle may also use lactate as a precursor for glycogen formation (esp after intense activity).
- Liver glycogen is restored rapidly; at least 48 hours are required to restore muscle glycogen.
PRINCIPLE OF CHO LOADING
In sporting vernacular, maximizing glycogen content by dietary manipulation is referred to as “carbohydrate loading.” The so-called classical regimen of glycogen loading resulted from investigations in the late 1960’s by Scandinavian scientists. This involved intense session of intense exercise to exhaustion by two days of low-carbohydrate diet (<10%), to “starve” the muscle of carbohydrate. This was followed by three days of high-carbohydrate diet (>90%) and the rest. The event would be performed on day seven of the regimen after this regimen; muscle glycogen levels approached 220 mmol/kg wet-weights (expressed as glucose residues), more than double the athletes resting level.
The content of CHO in the diet markedly influences the storage of glycogen in the body. When CHO rich diet is consumed over couple of days proceeded by a prolonged exercise period and some days of no significant CHO intake or of starvation, a doubling or tripling of the CHO stores of the body is observed.
- Endurance trained muscle contains more glycogen synthase (both D AND I forms) and these muscles can store an above normal glycogen content in 10-15 hours, if adequately supplied with CHOs. This compares with a requirement of 24-48 hours for similar storage to occur in untrained muscle.
- Trained muscle has more glycogen synthase and thus more glycogen can be stored. The glycogen stored in the human skeletal muscle appears to be evenly distributed within the muscle fiber, but FT fibers contain slightly more glycogen than do ST fibers. It is note worthy that glycogen storage is associated with the binding of water. One gram glycogen= 3 g water.
CARBOHYDRATE LOADING- A WAY TO INCREASE GLYCOGEN RESERVES
Research has shown that a particular combination of diet and exercise can result in significant packing of muscle glycogen. This procedure is termed CHO loading and is commonly in vogue among endurance athletes. The end result of this specific dietary modification is an even greater increase in muscle glycogen than would occur with a carbohydrate rich diet.
Because glycogen super compensation occurs only in those specific muscles exercised, the person must engage the muscles involved in his or her sport. In preparation for marathon, a 15 or 20 mile run is usually necessary, whereas for swimming and bicycling, moderate intense sub maximal exercise, also for 90 minutes is required.
- The combination of diet and exercise to produce glycogen packing or super compensation should be of considerable interest to the endurance athlete, especially the marathon runner and long distance swimmer, whose success depends in part in the magnitude of the body’s reserves.
- However, for those who are not endurance athletes, normal levels of muscle glycogen are more than adequate to provide the energy to sustain exercise. Normal levels of glycogen can be assured by ingesting50-60% (500-600g of CHO in 2000-2400 kcal/day diet) of the daily calorie as carbohydrates.
- Many endurance athletes consume a spaghetti and rice diets to achieve a high level of CHO intake. A week before the actual competition, they use the 3 step exercise and dietary modification program to assure the desired glycogen super compensation.
TWO STAGE DIETARY PLAN FOR INCREASING MUSCLE GLYCOGEN STORAGE
BEGIN ONE WEEK PRIOR TO THE EVENT
STAGE I: DEPLETION
DAY 1: Exhausting exercise performed to deplete muscle glycogen in specific muscles.
DAY 2, 3, 4: LOW carbohydrate only 60-100 g CHO (UP TO 10% enCHO on day 3 and 20% en on day 4)(high % en from protein and fat in the daily diet).
STAGE II: CARBOHYDRATE LOADING
DAY 5, 6, 7: HIGH CHO food intake. (250-550 g of CHO) 55-60% EN from CHO (>90%).
Muscle glycogen levels approached 220 mmol/kg wet-weight (expressed as glucose residues), more than double the athletes resting level.
COMPETITION DAY:follow high CHO PRE EVENT MEAL.
USE OF CARBOHYDRATE LOADING IS CONTROVERSIAL
Use of carbohydrate loading is controversial because it causes physiological side effects that may physically or psychologically impair performance of some athletes.
In the first phase
- Heavy workout without carbohydrate repletion leaves the athlete feeling weak, sore, dizzy and tired.
- A severe chronic CHO overload, interspersed with periods of high lipid and/ or high protein intake can increase blood cholesterol and urea nitrogen levels, type 2 diabetes, heart and renal disease or certain muscle enzyme deficiency.
- It may be noted that repeated bouts of CHO loading interspersed with periods of high fat and protein intake could pose problems to people susceptible to adult diabetes’, heart or kidney disease.
- High lipid intake often causes gastric distress and poor recovery from the exercise depletion sequence of the overloading procedure.
- Low CHO diet causes ketosis, vitamin and mineral deficiencies, as this is not a balanced diet.
- Glycogen depleted state reduces one’s capability to train hard- leading to detraining effect during loading sequence.
- Moods are altered, low CHO diet sets stage for lean tissue loss as muscle protein used as substrate to maintain blood glucose levels in glycogen –depleted state.
- During the subsequent loading phase, the athlete might feel bloated, heavy and uncomfortable. 3 g water is retained per gram of glycogen stored. The extra-retained water would account for the typical 2-7 lbs weight gain, encountered in this technique, adding to energy cost. While the athletes might be feeling fatter and sluggish, he or she is also asked to reduce activity. The regular training pattern is disrupted. Because of the dietary manipulations involved in this type of CHO loading, younger athletes, individual with diabetes, persons at high risk of cardiovascular diseases or survivors of a myocardial infarction should completely refrain from this regimen. If CHO loading is to be used it should be by well-trained athletes; maximum 2-3 times yearly.
- Athletes who have practiced CHO loading have reported
- Muscle cramps
- Abnormal heartbeat patterns, angina-like pain and ECG abnormalities.
- Interferes with maximum performance rather than stimulating it.
CHO loading alters supply and utilization of fat as sources of energy- decreases glycerol and FFA concentrations, while increasing concentrations of ketone bodies, insulin and growth hormone levels.
Glycogen loading is beneficial for 1-2 hours, continuous strenuous exercise. However the athlete should be informed about its disadvantages.
MODIFIED LOADING PROCEDURE
More recent theories, toward CHO loading advocate a more prudent, non-exhausting approach. It is believed that CHO deprivation of the depletion phase is not necessary as it has numerous detriments. Endurance training itself stimulates increase glycogen synthase activity. Normal glycogen depletion via exercise is a powerful enough stimulus to this activity. Enforced dietary CHO depletion is unnecessary.
Athletes should consume a high CHO diet along with their regular conditioning program to obtain maximal muscle glycogen stores safely. Studies confirm the important role that diet plays in optimizing initial glycogen content and subsequent exercise performance.
3 DIETARY REGIMENS ARE IDENTIFIED THAT PRODUCE ELEVATED GLYCOGEN STORES
The modified version, “tapering down” of exercise during 6 day prior to event that is 90, 40, 40, 20, 20 minutes exercise and then rest on 6yh day. The exercise is performed at 73% VO2 MAX. The daily CHO intake increased to 350-550g (70% kcal) during last 72 hour before competition.
- Change from a mixed to a CHO-rich diet for 3-4 d prior to exercise or competition.
- Deplete muscle glycogen stores with near – exhaustive exercise and follow this with a CHO rich diet for 3-4 days.
- Deplete muscle glycogen stores with near exhaustive exercise and follow this with CHO- poor diet for 3 days, a 2ndnear exhaustive exercise bout, and finally a CHO rich diet for 3 days.
All 3 regimens begin with a mixed diet and suggest exhaustive exercise not to be performed during CHO rich diet. It is important that exercise used to deplete muscle glycogen before the start of the CHO-rich diet taxes the same muscle groups needed in the upcoming exercise. In active/ untrained subject regimen 3 produces highest muscle glycogen and 1 lowest. Super compensation of muscle glycogen improves performance in trained races of 30 km and longer. It did not improve performance in short races (<21km).
The less stringent modified loading procedure eliminates potential negative outcomes of the classic glycogen loading sequence. The modified dietary protocol stimulates increase in glycogen synthase
- Without requiring dramatic glycogen depletion with exercise demanded by the classic loading procedure and
- Increase glycogen storage to nearly the same level achieved with the classic protocol.
- The 6-day protocol does not require prior exercise to exhaustion. Rather the athlete trains at about 75% of vo2 max (85% hr max) for 1.5 hour and then on successive days, gradually reduces (taper exercise duration).
- During first 3 days, CHO represent 50% of total kilocalories, and 3 days before competition, CHO content increased to 70% of total energy intake.
The preferred method of glycogen super compensation starts seven days before an event. During the next week, one should consume a large amount (70%of total calories) of foods rich in complex carbohydrates (starches). Up to 600g of carbohydrates is considered to be a high-carbohydrate diet. In practical terms, 600g of carbohydrates is equivalent to two loafs of bread, three cups of sugar, 15 medium baked potatoes, or 12 cups of rice. Every second day exercise amount is cut to half, until no exercise is performed the day before the event. Carbohydrate consumption is continued throughout the pre-event period, culminating in a pre-exercise meal. A high carbohydrate (<300g), pre exercise meal should be consumed 3-4 hours before the event. Pre exercise carbohydrates should be low-fibre, low-fat, and rich in complex carbohydrates (starches) instead of sugar to avoid an insulin response with resulting hypoglycemia during exercise.
The timing of the final meal before exercise is crucial because fasting results in the reduction of the labile glycogen stores of liver. Whereas carbohydrate meals consumed too close in time to the event may cause hyperinsulinemia. In the latter situation a rapid reduction in the plasma glucose ensues, and work capacity is significantly impaired. In addition, elevated plasma insulin inhibits liver glucose output and the normal rise of plasma free fatty acids. Under such conditions, excessive muscle glycogen degeneration occurs, resulting in early fatigue.
The form of carbohydrate ingested is also an important consideration in optimizing endurance performance, the principle factor being the glycemic index (GI) of the food. The GI refers to the rate at which the dietary carbohydrate is digested and metabolized so as to stimulate a concomitant increase in blood glucose. High GI carbohydrate includes the sugars glucose, fructose, and sucrose, while complex carbohydrates such as pasta and rice exhibit a low GI. Potato starch is considered to have relatively high GI, although not as high as the simple sugars. Generally, low to moderate GI carbohydrate loading prior to the performance is preferred to high GI carbohydrate intake. This is because of the hyperinsulinemic effect of the high GI food, which results in rapid reduction in blood glucose, suppressed release of fatty acids from the store, and inhibition of hepatic glycogenolysis.
Glycogen super compensation is designed for optimal performance during a single endurance event, such as a triathlon, marathon, ultra marathon, long cycling road races, or competitive sports events. Any event lasing more than 90 min and leading to exhaustion signals a need for glycogen super compensation. However, glycogen super compensation should not be performed more than two or three times in a week.
Glycogen super compensation (a carbohydrate loading) is recommended for the conditions listed in the table given below. In general, long-term endurance events lasting more than 90 minor repetitive events occurring in single or multiple days (such as cycling road races) are primary conditions for maintaining a high-carbohydrate diet.
|More benefits||Less benefits|
|Soccer||Runs <10 km|
|Ultra marathon||Hockey games|
|Ultra-endurance events||Football games|
|Cross- country skiing||Baseball games|
|Long distance swimming||Most rowing events|
|Long distance canoe or kayak||Most track and field events|
|Rock climbing||Walking and hiking|
|Mountain climbing||Downhill ski runs|
CARBOHYDRATE UTILIZATION IN EXERCISE
CHO, as glycogen, is the major fuel stored within skeletal muscle. It occupies a key role in the energy production and its depletion may be responsible for exhaustion.
- At rest and at low exercise intensities fats provide 80-90% energy, carbohydrate 5-18% and protein 2-5%.
- As exercise intensity increases, the contribution of CHO to the total metabolism also increases, until CHO may be sole energy source during exercise intensities that approach maximal oxygen uptake.
- During heavy exercise muscle relies heavily on its glycogen stores because of the immediate proximity of the store to the metabolic pathways and the fact that it can be metabolized aerobically and anaerobically.
KINDS OF WORKLOAD AND CHO UTILIZATION
(Beats Per Minute)
A light workload is defined as that which demands 30 to 40% of an individual’s V0₂max with a heart rate of about 110bpm. The R-value in the early phase of this exercise is on the order of 0.80 to 0.85, and it gradually declines to just below 0.80 after several hours of exercise. Sedentary persons will not become exhausted at this exercise intensity. Muscle glycogen utilization is low, averaging less than 10 mmol glucose units/Kg/hr. During the first couple of hours all of the glycogen depletion occurs in the slow-twitched fibers, overall depletion of the fast-twitch fibers is very minor. No increase in blood lactate occurs.
Moderately heavy workloads demand 60 to 70 % of the V0₂max and elicit heart rate of 160 to 170 bpm. The R-value will be close to 0.90 early in the exercise & does not drop during exercise. Fatigue will be experienced, & after 2 to 3 hours of exercise even well-motivated subjects become exhausted. Muscle glycogen content is reduced in a triphasic manner at this workload, being faster during the first 20 to 30 minutes of the exercise than later in the exercise. Thereafter a steady high rate of muscle glycogen utilization is observed, until muscle glycogen stores are depleted. Exercise usually can continue sometime after this point is reached. Early in the exercise, glycogen depletion occurs in the slow-twitch fibers, but some few fast-twitch fibers display depletion of glycogen when a major portion of the slow-twitch fiber types lose glycogen at a high rate. Of interest is the fact that blood lactate concentration is the highest after 5 to 10 minutes of exercise when the fast-twitch fiber involvement is small or nonexistent. Blood lactate level is declining & approaching the pre-exercise concentration later in the exercise when fast-twitch fibers are involved to a greater extent. At this exercise intensity, the arterial blood glucose level remains fairly stable. A decline in blood glucose concentration may be observed more frequently than at the lighter workloads, and this coincides with a fall in liver glucose output. The inability of the liver to balance peripheral glucose utilization tissues probable occurs in the late stages of the exercise when glycogen-empty muscle fibers still contribute to the force development but have to rely to a great extent on extra-cellular fuels. At this workload exhaustion coincides with depletion of glycogen stores in both liver & muscle. Attempts to enhance the utilization of blood substrates have only minor effects.
At a high exercise intensity demanding approximately 90% of the Vo₂ max, the heart rate approaches its maximal level & the subject may become exhausted anytime between 5 & 60 minutes of exercise. The R-value is found to be around 0.95 throughout the exercise period. Muscle glycogen depletion is pronounced, but at exhaustion, substantial amounts of glycogen are still present in the muscle. This glycogen is located in the slow-twitch fibers, with both fast-twitch fiber types being most commonly glycogen depleted. This does not indicate that slow-twitch fibers are not recruited in the exercise, but rather demonstrates differences in metabolic potential between the various fiber types. More complete use of the energy stored in glycogen, that is, more oxidation of the pyruvate formed, can take place in the mitochondria-rich slow-twitch fibers, blood lactate concentration peaks at 5 to 10 mmol /L – after 5 to 10 minutes of exercise and is fairly stable thereafter. In contrast to the effects of exercise at lower work levels, in high-intensity exercise arterial glucose concentration may increase slightly, and insulin concentration will be markedly reduced. Glucose uptake by skeletal muscle per unit time is probably greater that at the moderate workloads, but the total amount taken up by the muscle is less, due to the shorter work time.
The normal amount of glycogen stored in the liver is then sufficient to supply extra-cellular fluid & the peripheral tissues with the glucose for which there is a demand. Thus, CHO is available in both the muscle & the liver when exhaustion occurs, but a majority of the fast-twitch fibers are glycogen depleted.
At exercise close to or above V0₂ max it is very difficult to quantitate exactly the contribution of various substrates to the energy metabolism. RQ values are unreliable due to the rapidly developing metabolic acidosis & unloading of CO₂ from tissues & blood. All indications favor exclusive use of CHO as fuel at these high work rates. A very high rate of muscle glycogen utilization is observed in the active muscle. In such cases, Glu-6-P would also accumulate in the muscle fibers, inhibiting hexokinase activity. These fibers also experience the greatest they have the lowest hexokinase accumulation of high concentration of lactate in muscle & blood at maximal work is a further sign of a large dependence upon muscle glycogen as a substrate during heavy exercise.
The basic mechanism regulating CHO utilization is also at a play in static contractions. The highest rate of glycogen breakdown is observed in very intense static contractions, but exhaustion occurs within such a short time that the total amount of lactate accumulated in the muscle is quite small. The highest concentration of lactate is found when the contractions are in the range of 40 to 50% of MVC or above. In these intense contractions it is likely that muscle glycogen is the sole substrate, as blood flow is very low or nonexistent. At lower contraction tensions, & when such static contractions are performed intermittently, an inter-play is likely between the extra-muscular & intra-muscular supply of CHO, as in dynamic exercise.
GLUCONEOGENESIS AS AN ENERGY SOURCE DURING EXERCISE
Attempts have been made to determine the amount of glucose synthesized via gluconeogenesis in the liver during exercise in humans. This could account for about 8% of the total energy consumed per hour during such exercise. Thus, at present it appears that gluconeogenesis may be important only in the state of starvation, but its marginal role in prolonged exercise should not be neglected.
LACTATE AS A FUEL FOR MUSCULAR WORK
It is an old often verified observation that the initial elevation in lactate concentration in blood at the onset of moderately severe, prolonged exercise is followed by a gradual decline toward resting values. Another indication of use of lactate as a substrate for oxidative metabolism in contracting muscles is the finding of a markedly larger post-exercise clearance of lactate during exercise compared with the resting state.
On the basis of these tracer studies of lactate metabolism in exercise, it has been suggested that lactate is produced at equal rates in trained & untrained muscle, the difference in lactate concentration in blood being attributable to a larger clearance rate of lactate from the blood in the trained state.
CARBOHYDRATE METABOLISM AND MUSCULAR-FATIGUE
The pH of muscle at rest is about 7.0. The production of lactate during exercise results in a reduction in pH of the muscle. It has been reported that PFK (phosphofructokinase) is very sensitive to pH. In fact, it has been further reported that at pH 6.9, PFK activity is almost nil. On the basis of available findings, this appears to be unlikely in vivo. Of importance is the fact that the lactate concentration of muscle during heavy exercise has been reported to be around 6.5.
Since it appears that a lowering of muscle pH does not have a severe an effect on the regulation of PFK as has been proposed it is possible that a decrease in pH might exert an effect on the contractile properties of muscle.
In more pronounced exercise, lactate does not accumulate & the pH is not markedly reduced. The importance of local glycogen stores to muscle metabolism during exercise may have several reasons, such as
(a) Their intracellular localization & the lack of need to transport CHO from external sites of its use,
(b) The slightly higher energy yield per unit of 0₂ consumed,
(c) The anaerobic phase of the Embden-Meyerhof pathway, and
(d) Relative ease for translocation within the cell. Although these points are true & may be important for maximal performance. It is our opinion that the significance of muscle glycogen concentrations stems from the inability of glucose & fatty acids to cross the cell membrane rapidly enough to provide adequate acetate units for mitochondrial respiration.
EFFECT OF PRE-EXERCISE INJECTION OF GLUCOSE
The importance of depression in blood insulin concentration during exercise has been demonstrated by pre-exercise injection of glucose level during exercise. Glucose ingestion DURING exercise does not produce an insulin surge, in spite of the fact that glucose is taken up into the blood stream from the gut. The reason for this is that during exercise, insulin release is inhibited by the sympathetic nervous system. Ingestion of fructose appears to elevate insulin concentration even less. Oral intake of sugar during exercise has not greatly altered the rate by which muscle glycogen stores are depleted, probably because the total amount made available to the body is rather small. The ingested sucrose is used by working muscles.
Glucose infused directly into the blood stream has maintained blood glucose levels & reduced the rate of glycogen depletion in muscle. A prolongation of exercise capacity occurred in humans after oral ingestion of CHO, & it occurred without any modification of the contribution of fat & CHO in the total metabolism.
Liver takes up FRUCTOSE more rapidly than Glucose. Several factors must be considered when examining the possible role of FRUCTOSE ingestion in sparing muscle glycogen during exercise. An important factor is the rate of GI absorption, which is lower than glucose. However, the rate of glycogen synthesis is the same for both-glucose & fructose.
During exercise the insulin concentration in blood is normally depressed & if glucose were ingested after the onset of exercise, any adverse effect from a rise in insulin would be large negated. Thus, at present time the evidence that fructose ingestion PRIOR to exercise will significantly conserve muscle glucose stores is not sufficiently strong to recommend its consumption.