NUTRITION

Carb Intake: Understanding Glycogen and CHO
by Eric Goulet

Maintaining a high volume, high intensity and frequency of training, as we can see in highly motivated ultra-endurance triathletes competing for races such as the Ironman, requires a regimented intake of carbohydrates (CHO) on consecutive days, weeks and months. In truth, without an adequate ingestion of a certain amount each day, athletes will see a very rapid decrease in their ability to perform well in training and, consequently, competition. In fact, it is well known by exercise scientists that glycogen resynthesis (GR), and glycogen depletion, are one of the most important factors governing an athlete's training schedule. Therefore, athletes who understand best how to keep their glycogen reserves near their maximum capacity by manipulating their dietary intake, training stimulus and rest, are the ones most likely to be healthier and faster.

Moreover, athletes who are able and willing to use their post-training time adequately by consuming a certain quantity and type of CHO during a very definite time window are those that will show better and faster glycogen resynthesis after exercise. The faster the repletion of glycogen reserves, the faster the recovery and the sooner it will be possible to embark on more high volume and intense training. It is now clear from scientific literature that without understanding the time constaints involved in beginning the resynthesis process, in alliance with specific amounts and type of CHO, athletes are missing a great opportunity to further increase, even above pre-exercise level (in certain conditions), their glycogen stores. This text will consequently focus on the effects of post-exercise food ingestion on the replenishment of glycogen stores, in concert with what can increase or decrease its rate of resynthesis.

TIME REQUIRED FOR FULL RESTORATION OF GLYCOGEN RESERVES
Most studies have shown that 24 hours are normally sufficient to completely restore glycogen reserves after exhaustive exercise was practiced (5,11). In this case, for hard training athletes, completely means a restoration of approximately 80 m mol/g wet weight muscle/day (5). Not so bad, but in a taper situation where you lessen your overall volume and frequency of training and increasing your CHO ingestion, muscle glycogen concentration may exceed 210 m mol/g wet weight muscle (13). This, in part, explains why you feel so good and fast after a taper. To be sure that you will be able to make the most out of the 24 hours available or the hours separating your training sessions, two things are important to respect.

Firstly, you will have to eat a definite quantity of CHO immediately after and then at each subsequent 2 hours following exercise. Secondly, you will have to keep your intake of CHO between 525 to 650g/day (5). This quantity represents for a 70-kg person between 7,5 to 9 grams/CHO/day. Costill et al. in an excellent study done in 1981 demonstrated the foundation of these numbers (5). They examined the effects produced by the ingestion of different quantities of CHO on the rate of GR after exercise. Using an exercise protocol where athletes were completely glycogen depleted (liver and muscles), they progressively increased the amount of CHO eaten until athlete's stores were completely replenished. They came to the conclusion that complete restoration of glycogen was achieved in 24 hours when between 525 to 650 grams of CHO/day were consumed.

TIMING OF INTAKE AFTER TRAINING
It is well documented that the first 6 hours following training are the most important as far as glycogen resynthesis is concerned (2,9,11). This period then will be followed by a progressive decline in the rate of glycogen resynthesis over the next 18 hours. Studies have shown that the pattern of muscle glycogen resynthesis after exercise is a byphasic response (10). Following exercise and with adequate amount of CHO consumed, muscle glycogen is rapidly resynthesis within the next 24 hours to near pre-exercise level. Then, muscle glycogen will slowly increase to above-normal levels over the next few days—providing that enough CHO is consumed and rest accorded. If at least 0.70 to 1 gram of CHO/kg bodyweight is ingested immediately and every two hours after exercise, athletes can expect a rate of glycogen resynthesis of approximately 5 to 8 m mol/g wet weight muscle/hour, which is ideal (1,2,8,9,11). This total measure represents for a 70-kg person between 50 and 70 grams of CHO or 200 and 280 calories, respectively. And this is most of the time met by all endurance athletes. The ingestion of more these amounts won't necessarily augment the rate of GR because of the constraints imposed by gastric emptying and glucose transport into the cell (7). Inversely, delaying carbohydrate ingestion by two hours can reduce the rate of resynthesis to only 3 m mol/g wet weight muscle/hour (9), which is far from the ideal if tomorrow's training is supposed to be a hard one. The increase activity of glycogen synthase in the I-form in conjunction with an enlarged insulin sensitivity and an increased permeability of muscle cell membrane to glucose are what can explain the greater rate of resynthesis in the first 6 hours following exercise (7,10).

TYPE OF CARBOHYDRATE INTAKE
Glucose and sucrose ingestion following exercise have been shown to be better than fructose ingestion alone for promoting the rate of glycogen resynthesis (1). In fact, Blom et al. had athletes that were given, post-exercise, 0.35, 0.70 or 1.40g/kg bodyweight of either fructose, glucose or sucrose solutions at hours 0, 2 and 4. The results showed a 100% higher rate of muscle glycogen resynthesis after six hours of recovery for the group consuming glucose and sucrose only at doses of 0.70 g/kg bodyweight if compared to fructose ingestion alone. These higher rates of resynthesis can be explained by the fact that fructose metabolism takes place in the liver whereas the majority of glucose and sucrose bypass the liver to be stored directly into the muscle cells (10). As a result, glucose and sucrose can be considered the ideal macronutrients for the replenishment of muscle glycogen whereas fructose is unarguably the ideal source for liver glycogen restoration.

But, it is obvious that most of athletes won't only ingest, post-exercise, glucose, sucrose or fructose alone; that is, they will usually take a blend of different foods. Consequently, for a high rate of glycogen resynthesis, macronutrients should have a high glycemic index value (HGI) (table 1) which produces higher insulin and glucose levels (4,12). Athletes in strenuous training, accordingly to a study done by Burke et al., achieved a higher rate of GR during the 24 hours following an exercise after the ingestion of HGI food like cornflakes, potatoes and wholemeal bread if compared with low glycemic food (4). Also of importance is that athletes should stay away from high fat food in the next 6 hours following exercise.

In fact, consuming high fat foods can reduce the rate of gastric emptying, which in return can slow down the absorption rate of CHO. Keizer et al. in a study done in 1986 found that there were no significant differences in the rate of GR between solid and liquid food (11). They compared the absorption rate of a commercial recovery drink with the ingestion of whole wheat bread with jam or bananas and honey. There was no difference in GR in the next hours following exercise. Usually coaches recommend a liquid meal following exercise rather than solid food because it allows faster rehydration, is less filling and easier to digest.

TABLE 1. High Glycemic Value Foods
Maltodextrin (150)
Rice, Instant, Boiled 6 min. (128)
Baked Potatoes (121)
Corn Flakes (119)
Pretzels (116)
Tapioca, boiled with milk (115)
White Bread/Bagel (100)
Sucrose (92)
Power BarTM (82)
Banana (77)

CARBOHYDRATE-PROTEIN COMBINATION
Zawadzki et al. conducted a study where they compared, after exercise, the rate of GR using CHO drink, CHO + protein drink (PRO) and protein drink alone. Athletes received the drinks immediately after exercise and again 2 hours later. After four hours of recovery, glycogen concentration did not differ significantly between CHO and CHO + PRO treatments. However, the rate of storage was 38% faster for the CHO + PRO treatment. The increase clearance of glucose by the muscles due to an increased plasma insulin response and glycogen synthase activity explains these results. Of great importance also is that the rate of GR for CHO + PRO treatment was not exceptionally fast. In truth, similar rates have been seen using only standard CHO (1,8). In the light of that data, I would only suggest to add protein only when the CHO source is not very adequate (14).

EFFECT OF ECCENTRIC EXERCISE
Eccentric exercise like running, where a muscle lengthens as it develops tension, has the potential to retard the rate of GR because of the microscopic damages produced to muscle fibers (13). In fact, white blood cells (WBC) infiltration beginning 12 hours post-exercise will viscously compete with muscles for glucose; glucose being the substrate of choice used by WBC. But, because these lonely soldiers won't appear in the muscles before 12 hours post-exercise, GR won't be affected if compared with non-damaged muscles in the next 12 hours following exercise. It is therefore the slow phase of GR that can be affected. Sherman et al. found that 5 days following a marathon muscle glycogen was only 67% of pre-race levels even though subjects had consumed a high CHO diet. Another cause for slow GR can be explained by the damages produced to the sarcolemma membrane, which can interfere with muscle glucose transport.

PRACTICAL RECOMMENDATIONS
» In intense training, athletes are encouraged to consume between 6.5 to 10g/kg of CHO/day (5). An endurance athlete during hard training needs between 3500 and 7000 calories/day to sustain its performances (10)

»As soon as you have finished your training session, consume immediately 0.7 to 1.40 g/kg bodyweight of CHO in liquid form and then at hours 2, 4 and 6. Going over 1 to 1.4g/kg bodyweight/hour won't necessarily increase the rate of glycogen resynthesis. You may also have, 15 to 30 minutes after training, a recovery drink containing 100 grams of CHO (9). Next, after your stretching and shower are done, have a meal that contains high glycemic value CHO (4).

»For a faster rate of GR in the 6 hours following exercise, CHO sources should preferably come from sucrose, glucose or glucose polymers (1). Fructose alone should be added to a drink to help speed up liver glycogen resynthesis. During the day (s) following a hard training session—providing that you are resting, a starch-based diet is more effective than glucose (simple CHO) for promoting glycogen resynthesis (5)

»Food used in the next 24 hours after exercise should possess a high glycemic index value (4,12)

»When inadequate (quantity or quality) sources of CHO are used, you may add some protein because it will help the rate of GR (14)

» Stay away from high fat food in the hours following exercise

» Solid or liquid food induces a similar glycogen repletion rate (11). But, liquid ingestion after exercise facilitates digestion, speed up rehydration and usually do not disturb an athlete's appetite. It is therefore recommend over solid food

» Do not hesitate to take a day off per week when you feel unusual physical or mental fatigue

» Before an important race, taper for at least three days where you will progressively increase your CHO intake to 70% of your overall diet. Also and of equal importance, is that you must reduce your overall volume and frequency of training (10)

» After a race or a very demanding training session, do not overdo active recovery. Surely that it will help speed up the rate of lactate clearance, but the downside of it is that studies done on this subject have shown that active recovery may slow down the rate of GR (3,6)

» You are unable to eat immediately after a race or a hard training session, do not panic. In fact, a study done by Parkin et al. in 1997 showed that retarding by up to two hours, post-exercise, the ingestion of HGI food resulted in similar glycogen concentration at 8 and 24 hours during recovery if compared to immediate feeding of HGI food (12). The authors concluded that delaying ingestion of HGI CHO by up to two hours post-exercise does not affect glycogen storage if the initial meal is followed by further CHO intake.

CONCLUSION
Exhaustion or increased fatigue in exercise, most of the time, coincides with glycogen depletion or low glycogen levels. Thus, it means that, in order to maintain a high working capacity, glycogen resynthesis should be your number priority after completion of exercise—along with proper hydration and replacement of lost sodium. In truth, not being able to replenish glycogen stores fast enough won't only affect you physically but also mentally. Consequently, knowing a little about GR physiology, I hope, will help you achieve better life performances by using adequately all the knowledge mentioned in this text.

Summary of CHO intake for appropriate glycogen resynthesis
Goal: 4 to 7 hours of moderate intensity exercise daily; extremely prolonged exercise (7-10 hours) containing short intense bouts; very high energy requirements; insuring daily muscle glycogen recovery. (Tour de France cyclists, double and triple+—Ironman athletes)
CHO intake target: 10-12 g/kg bodyweight daily; 1.5 g/kg bodyweight of sucrose, glucose or glucose polymers soon after exercise and at each 2 hours subsequently; CHO eaten must preferably possess a high glycemic index value

Goal: Maximizing daily muscle glycogen to assure good performances day after day; assuring adequate glycogen reserve before a very prolonged competition
CHO intake target: 7-10 g/kg bodyweight daily; 0.7-1 g/kg bodyweight of sucrose, glucose or glucose polymers soon after exercise; starch-based CHO on easy days

Goal: Meeting fuel needs during less demanding training program—for example, 1 hour or less of moderate intensity exercise; prolonged exercise done at very low intensity
CHO intake target: 5-7 g/kg bodyweight daily; starch-based diet

Goal: Enhancing recovery between two training sessions
CHO intake target: 0.7-2 g/kg bodyweight of sucrose, glucose or glucose polymers soon after exercise and at each 2 hours subsequently; fructose must be added to restore liver glycogen; HGI value food must preferably not be eaten in the hour preceding the training bout

Goal: Enhancing liver glycogen resynthesis after a night of sleep

CHO intake target: 40-50 grams of fructose to be taken 3-4 hours before the training bout

REFERENCES
1. Blom et al. (1987). Effect of different post-exercise sugar diets on the rate of muscle glycogen synthesis. Medicine and Science in Sports and Exercise, 19, 491-496

2. Blom et al. (1986). Factors affecting changes in muscle glycogen concentration during and after prolonged exercise. Acta Physiologica Scandinavica, 128, 67-74

3. Bonen et al. (1985). Mild exercise impedes glycogen repletion in muscles. Journal of Applied Physiology. 58, 1622-1629

4. Burke et al. (1993). Muscle glycogen storage after prolonged exercise: effect of the glycemic index of carbohydrate feedings. Journal of Applied Physiology, 75, 1019-1023

5. Costill et al. (1981). The role of dietary carbohydrates in muscle glycogen resynthesis after strenuous running. American Journal of Clinical Nutrition, 34, 1831-1836

6. Daihyuk et al. (1994). Effect of passive and active recovery on the resynthesis of muscle glycogen. Medicine and Science in Sports and Exercise, 26, 992-996

7. Friedman et al. (1991). Regulation of glycogen resynthesis following exercise. Sports Medicine, 11, 232-243

8. Ivy et al. (1988). Muscle glycogen storage after different amounts of carbohydrate ingestion. Journal of Applied Physiology. 65, 2018-2023

9. Ivy et al. (1988). Muscle glycogen synthesis after exercise: effect of time of carbohydrate ingestion. Journal of Applied Physiology. 64, 1480-1485

10. Ivy, John L. (1991). Muscle glycogen synthesis before and after exercise. Sports Medicine, 11, 6-19

11. Keizer et al. (1986). Influence of liquid and solid meals on muscle glycogen resynthesis, plasma fuel hormone response, and maximal physical working capacity. International Journal of Sports Medicine, 8, 99-104

12. Parkin et al. (1997). Muscle glycogen storage following prolonged exercise: effect of timing of ingestion of high glycemic index food. Medicine and Science in Sports and Exercise, 29, 220-224

13. Sherman et al. (1983). Effect of a 42.2 km foot race and subsequent rest or exercise on muscle glycogen and enzymes. Journal of Applied Physiology, 55, 1219-1224

14. Zawadzki et al. (1992). Carbohydrate-protein complex increases the rate of muscle glycogen storage after exercise. Journal of Applied Physiology, 72, 1854-1859

15. R. A. Robergs, S. O. Roberts. (1997). Exercise Physiology: Exercise, Performance and Clinical Applications, Mosby Year-Book, 839 p.