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The changes in plasma glucose concentration following the ingestion

Accepting that a high-carbohydrate buy fifa 15 coins pre-exercise meal is of benefit to performance, the next question to ask is whether there is an advantage in selecting one type of carbohydrate over another. The ingestion of different types of carbohydrates will produce markedly different changes in plasma glucose and insulin concentrations. These glycaemic and insulinaemic responses following the ingestion of different carbohydrates are the bases of a classification of carbohydrates that is more informative than describing them as simple or complex.

The changes in plasma glucose concentration following the ingestion of 50 g of available carbohydrate when compared with the glycaemic response following the ingestion of 50 g of glucose is used to describe the glycaemic index (GI) of that carbohydrate (Jenkins et al., 1981). For example, white bread has a high value whereas lentils have a low value, reflecting the differences in the size of the glycaemic responses following the ingestion of these two carbohydrates (Foster-Powell & Brand Miller, 1995; Wolever, Jenkins, Jenkins, & Josse, 1991). Although the concept was developed to help prescribe diets for diabetics, it is now being used in studies of the influence of carbohydrate pre-exercise meals on performance (Burke, Collier, & Hargreaves, 1999). One of the potential attractions of consuming low- rather than high-GI carbohydrate foods before exercise is that the normal suppression of fatty acid mobilization is less and so there is a greater contribution of fat to energy metabolism (Wu, Nicholas, Williams, Took, & Hardy, 2003).

In one of the first studies on the influence of high- and low-GI carbohydrate foods on exercise capacity, the low-GI food appeared to improve endurance capacity to a greater extent than the high-GI food. In this study, Thomas, Brotherhood and Brand (1991) used lentils as the low-GI food and potatoes as the high-GI food, and compared the responses to those obtained after drinking a glucose solution or water. The meals and solutions were ingested 1 h before cycling to exhaustion at an exercise intensity of between 65 and 70%

The carbohydrate content of the meals and glucose solution was equivalent to 1 g·kg BM−1 and the volume was adjusted to 400 ml. The exercise times for the lentils, the potato, the glucose, and the water trials were 117, 97, 108, and 99 min, respectively. There was a clear difference in exercise time for the lentils trial, but there were no differences in performance times between the potato, the glucose, or the water trials. Thomas et al. (1991) did not address the different rates of digestion and absorption of the three carbohydrates within the hour before the start of exercise, or the fact that they had matched only the amount of carbohydrate ingested in these trials and did not account for the other nutrients in the lentils and potatoes, including protein.

A more recent study addressed the question of whether there are performance benefits to eating low-GI carbohydrate meals before exercise—that is, whether the amount of work done in a fixed time is increased. Febbraio and Stewart (1996) fed their participants instant mashed potatoes (made up in water) as the high-GI carbohydrate and lentils as the low-GI carbohydrate (1 g·kg BM−1), and they used a low-energy jelly as the control meal. The three conditions were assigned in random order and the “meals” were consumed 45 min before exercise. The exercise test required the six participants to cycle at 70%

for 2 h before they cycled to complete fifa 15 coins as much work as possible in 15 min. There was no difference in the muscle glycogen concentrations at the start or at the end of exercise on the three test occasions The total amount of work accomplished during the last 15 min period was not different between the three conditions. Although these results do not confirm the benefits of a low-GI carbohydrate diet as reported by Thomas et al. (1991), a direct comparison cannot be made because different performance criteria were used in the two studies. Thomas et al. (1991) assessed endurance capacity, whereas Febbraio and Stewart (1996) assessed total work done in a fixed time (endurance performance).