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 1 diabetes, and fluid and electrolyte replacement.
The following is an excerpt that answers the question, Does Carbohydrate Loading Work?…
In preparation for competition, most athletes taper their training in the week leading up to the event. Eating more carbohydrate during the 3-4 days before the competition is sufficient to increase muscle and liver glycogen stores to levels which are above normal values.47 The recommended amount of carbohydrate is about 600 gm a day (based on studies only in men). This amount of carbohydrate is clearly too great for women because it would account for almost the whole of their daily energy intake. A more helpful recommendation is one which is based on body mass, for example, 8-10 g kg -1 body mass per day for the 3-4 days before competition.
Dietary carbohydrate loading before cycling to exhaustion improves endurance capacity when compared with performances after a mixed diet. Early studies on carbohydrate loading reported improvements of 50 per cent in cycling time to exhaustion48 and the benefits of carbohydrate loading on endurance capacity during cycling have been confirmed repeatedly.49 There have been relatively few studies on the effect of a high-carbohydrate diet on running performance, but Goforth and colleagues were amongst the first to report an improvement in endurance capacity of runners (9 per cent) after carbohydrate loading.50 Improvements in endurance running capacity of about 25 per cent were also reported for male and female runners when they consumed a high-carbohydrate diet during the 3 days before a series of treadmill runs to exhaustion. One group supplemented their diet with simple carbohydrates (confectionery), and another group supplemented their diet with complex carbohydrates (pasta, potatoes and rice); the type of carbohydrate used had no influence on the subsequent improvement in endurance running capacity. Simply increasing energy intake in the form of additional protein and fat did not result in an improvement in endurance running capacity of a third group, confirming the importance of carbohydrate intake for improved performance.51
Competitors in a 30 km cross-country race clearly benefited from dietary carbohydrate loading during the 3-4 days leading up to this endurance competition. Ten runners completed the cross-country course on two occasions separated by 3 weeks.52 On one occasion, five of the 10 runners ran the race after carbohydrate loading, while the others maintained their normal mixed diets. On the second occasion the runners swapped dietary preparations and were paid to match or improve on their performance times for the first race. All the runners improved their times for the 30 km following preparation on the high-carbohydrate diet (135 vs 143 min). This is probably the most informative study published on the influence of carbohydrate loading on running performance because not only was the study conducted as part of a real competition, but also muscle biopsy samples were obtained from the runners before and after both races.
The high-carbohydrate diet for 3 days before the race significantly increased the pre-competition muscle glycogen stores. Furthermore, the carbohydrate-loaded runners completed the race in shorter times and without such a pronounced reduction in muscle glycogen as in the race preceded by the mixed diet. It is clear from this and later studies that the size of the carbohydrate stores alone will not dictate the outcome of an endurance race. Pre-race muscle glycogen stores must be sufficient to meet the demands placed on them by the endurance race; however, the benefits of carbohydrate stores in excess of this amount have not been established. Although absolute proof is lacking, the current practice is to raise carbohydrate stores as high as possible, within the constraints of time, training and dietary preparation. Other than a slight gain in body mass, there appear to be no disadvantages to dietary carbohydrate loading.
In races over shorter distances, high pre-competition muscle glycogen concentrations do not appear to improve performance. For example, there were no differences in performance times for a 20.9 km race, on an indoor 200 m track, when well-trained runners consumed either a mixed diet or a high-carbohydrate diet 3-4 days before the race.53
In contrast, starting exercise with a less than adequate glycogen store will significantly reduce exercise capacity, as has been demonstrated in laboratory studies.48.54 In real competitions, such as in a soccer match, those players who began the game with low muscle glycogen concentrations ran less than the rest of the team throughout the match.55
Most of the studies on the influences of dietary carbohydrate loading on exercise capacity have used men as subjects and some studies have failed to show the same benefits for women. Females use more fat for energy metabolism during submaximal exercise than males and the extent to which they are able to load their muscle glycogen stores may be somewhat less than has been reported for men.56-58
47.Sherman W, Costill D, Fink W, Miller J. Effect of exercise-diet manipulation on muscle glycogen and its subsequent utilization during performance. Int. 1. Sports Med. 1981; 2:114–118.
48.Bergstrom J, Hermansen L, Hultman E, Saltin B. Diet, muscle glycogen and physical performance. Acta Physiol. Scand. 1967; 71: 140-150.
49. Conlee R. Muscle glycogen and exercise endurance: a twenty year prospective. In: Pandolf K (ed.), Exercise and Sports Science Reviews. London: Collier Macmillan: 1987, pp. 1-28.
50.Goforth HW, Hodgdon JA, Hilderbrand RL. A double blind study of the effects of carbohydrate loading upon endurance performance. Med. Sci. Sport Exerc. 1980;12: 108A.
51.Brewer J, Williams C, Patton A. The influence of high carbohydrate diets on endurance running performance. Eur. 1. Appl. Physiol. 1988; 57: 698-706.
52.Karlsson J, Saltin B. Diet, muscle glycogen and endurance performance. 1. Appl. Physiol. 1971; 31: 203-206.
53.Sherman W, Costill D, Fink W, Miller J. Effect of exercise-diet manipulation on muscle glycogen and its subsequent utilization during performance. Int. 1. Sports Med. 1981; 2: 114–118.
54.Maughan RJ, Williams C, Campbell DM, Hepburn D. Fat and carbohydrate metabolism during low intensity exercise: effects of the availability of muscle glycogen. Eur. 1. Appl. Physiol. 1978; 39: 7-16.
55. Saltin B. Metabolic fundamentals of exercise. Med. Sci. Sports Exerc. 1973; 15: 366-369.
56.Tamopolsky LJ, MacDougall JD, Atkinson SA, Tamopolsky MA, Sutton JR. Gender differences in substrate for endurance exercise. 1. Appl. Physiol. 1990; 68: 302-307.
57.Tamopolsky M. Nutritional implications of gender differences in energy metabolism. Tn: Driskell J, Wolinsky I (eds), Energy-yielding Macronutrients and Energy Metabolism in Sports Nutrition. London: CRC Press, 2000, pp. 245-262.
58.Walker L, Heigenhauser G, Hultman E, Spriet L. Dietary carbohydrate, muscle glycogen content, and endurance performance in well trained women. 1. Appl. Physiol. 2000; 88: 2151-2158.
Next week: more from Psychology in Diabetes Care, 2nd Ed: Diabetes in Older Adults.
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