Dinesh Nagi, Editor
Drinking before exercise helps to delay the onset of severe dehydration, but the type of fluid taken should be chosen with care. Water empties from the stomach quickly but crosses the walls of the small intestine only slowly.
Adding sodium salts to water speeds up the transport of water into the systemic circulation because of the active transport of sodium. Adding some glucose also improves the absorption of fluid, but if the glucose solution is too concentrated then gastric emptying is delayed.”70 Commercially available carbohydrate-electrolyte solutions (sports drinks) with a concentration within the range 5-8 carbohydrates appear to be most effective at supplying both fluid and fuel. The gastric emptying rate of a solution is also influenced by the volume of fluid ingested. Other things being equal, a large volume empties more quickly from the stomach than a smaller volume.” 71 One strategy for rapid rehydration is to drink about 120-150 ml of fluid every 15-20 min so that the volume in the stomach does not fall to the point where emptying rate slows down.
Drinking carbohydrate-electrolyte solutions before exercise does produce, during exercise, rapid rises in blood glucose and insulin concentrations, followed by a sharp fall in blood glucose. However, as exercise continues, blood glucose concentrations normally return to pre-exercise values. It is interesting to note that, even on the occasions when blood glucose concentrations fall to hypoglycemic values during the early part of prolonged exercise, the subjects in these studies do not report any adverse sensations. 72 In summary, the weight of the available evidence does not support the commonly held view that drinking glucose solutions before exercise leads to a reduction in exercise capacity. Nevertheless, concentrated glucose solutions (10-25 per cent) are not recommended as a means of increasing carbohydrate stores within the hour before exercise because of the potential for causing gastrointestinal discomfort.
Drinking carbohydrate-electrolyte solutions immediately before and throughout exercise does not produce the same fall in blood glucose as that which occurs when the same solution is ingested within the hour before exercise. One of the reasons for this different response is the failure of insulin to increase in response to the elevated blood glucose concentration during exercise because the release of insulin from the pancreas is suppressed by the exercise-induced rise in plasma catecholamines. 73 Drinking carbohydrate-electrolyte solutions throughout prolonged exercise provides fluid and fuel, and so helps to delay the onset of severe dehydration and glycogen depletion. 74-76
The improvement in endurance capacity following the ingestion of a carbohydrate-electrolyte solution throughout exercise has been attributed to an increased rate of carbohydrate oxidation while maintaining normal blood glucose concentrations. 77 More recent studies, using running rather than cycling, show that ingesting glucose-electrolyte solutions exerts a glycogen-sparing effect and this may be the underlying reason for the improvements in endurance running capacity (for review see Tsintzas et al.78) This glycogen sparing may not be confined to skeletal muscles but may include liver glycogen stores. Drinking carbohydrate solutions immediately before and during exercise decreases hepatic glucose production that is sustained in proportion to the amount of carbohydrate ingested.79 The maximum rate of carbohydrate oxidation during exercise following the ingestion of carbohydrate solutions of various concentrations is approximately 1g min-J .79
Carbohydrate intake and recovery from exercise
Rapid recovery from heavy training or competition is particularly important to sportsmen and women who have to perform every day for several days or weeks, and it is essential that they adopt a nutritional strategy which will aid rapid recovery. Central to the recovery process is the restoration of muscle and liver glycogen stores, which may have been severely depleted during exercise.
Immediately after exercise, muscle begins resynthesizing the glycogen used up during exercise. The maximum rate of glycogen resynthesis occurs during the first few hours of recovery, and so ingesting carbohydrate during this period capitalizes on this process. Ivy suggested that, in order to maximize the glycogen resynthesis rate, the optimum post-exercise carbohydrate intake should be about 1 g kg-1 body mass.80 The practical prescription is 50 g of carbohydrate immediately after exercise and the same amount every 2 h up to the next meal.81 Depleted muscle glycogen stores can be repleted in 24 h when a carbohydrate-rich diet is eaten during the recovery period.82,83 This recovery diet should consist of 8-10 g carbohydrate kg-1 body mass, and should contain high-glycemic-index carbohydrates during at least the early part of recovery.
The key question, however, is whether or not performance capacity is restored along with muscle glycogen stores following high-carbohydrate refeeding, and several studies have attempted to address this question. The results suggest that, as long as carbohydrate intake is increased from about 6 g kg -1 body mass per day to 9 g kg -1, then endurance capacity is restored along with muscle glycogen stores.84
Even when the recovery period is only a few hours, and so too short to significantly increase muscle glycogen stores, there are benefits to be gained from drinking carbohydrate-electrolyte solutions. For example, Fallowfield and colleagues reported that, when runners drank a commercially available sports drink which provided the equivalent of 1 g kg-1 body mass of carbohydrate immediately after prolonged exercise, and again after 2 h, they were able to run for about 60 min, whereas after drinking a sweet placebo they were able to run for only 40 min.85 Furthermore, drinking a carbohydrate-electrolyte solution is a more effective rehydrating strategy than drinking water during recovery from exercise. 86
The type of carbohydrate consumed during the recovery period influences the rate of glycogen resynthesis.87,88 Burke and colleagues reported that a recovery diet that contained high-GI carbohydrates resulted in a larger muscle glycogen store 24 h after prolonged exercise than after consuming a low-GI carbohydrate diet.88 Although this study showed greater glycogen accumulation following the ingestion of a high-GI carbohydrate recovery diet, it did not include an assessment of the recovery of exercise capacity. Therefore, Stevenson and colleagues examined the influence of high- and low-GI carbohydrate recovery diets on subsequent exercise capacity.89 Recreational runners completed 90 min of treadmill running at 70 per cent V02max and were then assigned a recovery diet containing either high- or low-GI carbohydrates. Twenty-two hours later, after an overnight fast, they again ran on the treadmill, but on this occasion they continued to the point of fatigue. On the low-GI carbohydrate recovery diet they
ran for 109 min and after the high-GI carbohydrate diet they ran for only 97 min. All the subjects reported that they rarely felt hungry on the low-GI carbohydrate diet whereas on the high-GI recovery diet there were times when they felt that they could have eaten more of the energy-matched meals.
There is evidence to suggest that adding some protein to the carbohydrate solution increases the rate of post-exercise glycogen synthesis to a greater extent than can be achieved with a carbohydrate solution alone.90 The addition of protein increases the concentration of plasma insulin beyond that which is achieved with carbohydrate solutions alone after exercise. The presence of insulin stimulates the GLUT 4 transport proteins to remain active for longer than would be the case without an increased presence of this hormone.15 As a result there is a continued increased rate of glucose transport of glucose across the muscle cell membrane that enhances glycogen resynthesis. However, when larger amounts of carbohydrate (> 1.2 g/ kg body mass) are ingested during the recovery period, then the addition of protein appears not to provide an additional increase in the rate of glycogen resynthesis. 91
As mentioned earlier, glucose uptake by muscle is greater after exercise than before exercise. Exercise changes the characteristics of the muscle membrane so that glucose permeability is improved and muscles have increased insulin sensitivity. The two effects appear to be additive. In addition, glycogen synthase, the enzyme complex responsible for glycogen synthesis, is in its most active form immediately after exercise. There is an inverse relationship between muscle glycogen concentration and the amount of glycogen synthase in the active form,91 and athletes with the lowest post-exercise muscle glycogen concentrations show the greatest increase over the next 24 h.92
More recent studies have shown that the increase in post-exercise glucose uptake is associated with an increase in the glucose transporter protein, GLUT 4, after exercise.93 Training brings about an increase in the amount of GLUT 4 (by about 50 per cent) with a parallel increase in the activity of hexokinase. It is probable that the rapid uptake of glucose is mainly the result of the presence of an increased amount of glucose transporter proteins.94 These may enable an increase in the rate of glycogen resynthesis to occur, even when glycogen synthase levels have fallen to pre-exercise values.
This chapter has provided an overview of the relevant physiological responses to exercise and training. In addition it has included the nutritional strategies that help delay the onset of fatigue, namely how best to optimize the pre-exercise carbohydrate stores and the use of muscle glycogen during prolonged exercise. Taking regular exercise has huge health benefits that include the control of blood glucose in particular and an increase in functional capacity in general. For those people who are preparing for a prolonged period of heavy exercise, whether it is training or competition, then the recommendation is clear; they should taper their training during the week before the event and increase the carbohydrate content of their diet such that over the 48 h before the event they consume the equivalent of 8-10 g of carbohydrate kg -1 body weight a day. This prescription is the same for those people who have only 24 h in which to recover between training sessions or competitions. When recovery is limited to 24 h, then the high-GI carbohydrates are recommended immediately after exercise, followed by low-GI carbohydrates for the remainder of the recovery period. However, during recovery periods lasting several days or more, the type of carbohydrate consumed is not as important as during shorter recovery periods. One of the limitations to exercise, especially in the heat, is dehydration. Drinking well-formulated carbohydrate-electrolyte solutions (some sports drinks) containing no more than about 6-8 per cent carbohydrate is a good strategy to decrease the rate of dehydration during exercise and provides carbohydrate as extra fuel. The recommended amounts are of the order of 120-150 ml solution every 15-20 min. This practice improves endurance running capacity, probably by contributing to the carbohydrate metabolism in working muscles. However during exercise in very hot climates, the sports drinks should contain only about 2-4 per cent carbohydrate because under these conditions fluid is more important than fuel. After exercise rehydration is more rapid when carbohydrate-electrolyte solutions are consumed because when drinking water thirst is quenched before rehydration is achieved. One further point to note for those who have only a limited time in which to rehydrate after exercise is that they need to drink the equivalent of 150 per cent of the sweat lost. This translates into drinking in liters the equivalent of 150 per cent the body mass loss in kilograms.
The question about the optimum pre-exercise meal is still unanswered but there is growing evidence to suggest that it should contain low-GI carbohydrates. The advantages of a pre-exercise meal which contains predominantly low-GI carbohydrates is that it causes only minor perturbations of plasma glucose and insulin, and so favors a greater rate of fat metabolism. A greater rate of fat oxidation spares the limited muscle glycogen stores and so helps delay the onset of fatigue. Furthermore, when such meals are consumed 3-4 h before exercise they provide a sense of satiety for most of the postprandial period.
In conclusion, next to being born with the appropriate genes and undertaking the right training, a high-carbohydrate diet is one of the essential elements in the formula for success in sport and exercise. The nature of the carbohydrate may also play a significant part in preventing the onset of metabolic fatigue.
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The new edition of this acclaimed title provides a practical guide to the risks and benefits of undertaking sport and general exercise for patients with diabetes.
Fully updated to reflect the progress and understanding in the field, the book features new chapters and material on insulin pump therapy and exercise, physical activity and prevention of type 2 diabetes, dietary advice for exercise and sport in type 1diabetes, and fluid and electrolyte replacement.
Next week: New Joslin Diabetes Deskbook excerpt!
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