The present study indicates that ingestion of foods of different GI values 30 min prior to exhaustive cycling exercise does not result in significant changes in exercise performance. Furthermore, consumption of carbohydrates of LGI and HGI does not alter β-endorphin levels during exercise and does not result in significant changes in carbohydrate and fat oxidation rate during exercise.
Ingestion of carbohydrates prior to exercise resulted in an increase in glucose and insulin (Figure and ). It is well known that when blood glucose increases the pancreatic beta cells increase their output of insulin in order to facilitate glucose uptake by the tissues. In our study an initial increase of glucose was observed and then plateaued whereas insulin continued to increase up to 30 minutes following the ingestion of foods. The same glucose and insulin response prior to exercise was seen in De Marco et al. study when the same amount of carbohydrates was ingested [17
]. This response of glucose and insulin is common since the initial increase in glucose constitutes the main stimulus for the delayed insulin increase.
Several studies attempted to alter the carbohydrate composition of a meal prior to exercise in an effort to improve performance. A number of those studies show no improvement in exercise performance [19
]. Febbraio et al. [19
] utilized a similar design with the one employed in this study and found no significant differences in exercise performance. Subjects received low and high glycemic foods (1.0 g. kg-1
of body weight) 30 min prior to a 120-min submaximal exercise bout that was followed by a 30 min time trial. Total work performed during the time trial was similar between the LGI, the HGI and the control condition. These results were evident despite the fact that carbohydrate oxidation was greater during the HGI condition. No significant differences in exercise performance were noted in two other studies by the same group [31
] when subjects received LGI and HGI foods (1.0 g. kg-1
of body mass) 45 min prior to submaximal exercise that was followed again by a time trial. Although differences in glucose and insulin levels were reported following consumption of the LGI and HGI prior to exercise, there were no apparent differences in the blood metabolites during the steady state exercise. Thomas et al. [33
] used four meals with different glycemic index foods (30, 36, 73 and 100) that each provided 1.0 g. kg-1
of body weight. The meal was consumed 1 h prior to cycling to exhaustion at 65-70% of VO2max
. The results showed no significant differences in time to exhaustion between trials. No enhancement in exercise performance was found when low and high glycemic index foods were provided 3 h prior to exercise even though there was a relative shift in substrate utilization from carbohydrate to fat following the LGI meal [22
]. As far as exercise performance is concerned, results from the present study coincide with those of earlier reports suggesting that although pre-exercise GI manipulation affects pre-exercise glucose and insulin levels, it does not presumably influence the rate of muscle glycogen utilization or exercise performance. Differences in glucose levels and carbohydrate and fat oxidation during steady state exercise could influence exercise performance during a subsequent short and intense exercise. Evidence indicates that increasing fat oxidation leads to sparing of glycogen [34
] and spared glycogen or higher blood glucose levels towards the end of exercise could be used to allow for high-intensity exercise to be continued for a longer time affecting exercise performance. In a recent study where low and high GI foods were consumed 15 minutes prior to exercise LGI food resulted in higher glucose levels at the end of exercise and performance was greater compared to a HGI food and a placebo condition [35
]. However, it has to be noted that the subjects in this study were not professional athletes and an abrupt increase in the exercise intensity following a steady state exercise could not be able to reveal performance and metabolic responses accurately. This is a limitation of the present study and further research should explore performance, metabolic and β-endorphin responses in well-trained athletes with a different time trial design (i.e. continues exercise at a submaximal intensity).
On the other hand, there are several studies that examined the effects of different GI foods, at different times prior to exercise, on exercise performance and substrate metabolism that suggest an improvement of exercise performance following LGI food consumption prior to exercise [17
]. Thomas et al. [36
] were amongst the first ones that expressed interest in the role of GI in sports nutrition. In their study, participants under four different conditions received three foods of different GI and water. Each meal provided 1.0 g. kg-1
of body weight and was given 60 min prior to cycling to exhaustion at 65-67% VO2max
. A significant 20 min prolonged workout was performed after consumption of the LGI foods that was accompanied by more stable glucose levels and higher free fatty acid concentration during exercise. De Marco et al. [17
] also showed a 59% increase in time to exhaustion after a 2-h submaximal bout in a LGI trial compared with a HGI trial accompanied by a relative hyperglycemia and lower RPE and RQ [17
]. Moore et al. [38
] administered low and high GI foods 45 min prior to a 40 km cycling trial and found a significantly improved performance following the LGI trial. Higher glucose levels at the end with no differences in carbohydrate and fat oxidation rates were noted between the two trials. In the study of Little et al. [37
], improved performance also appeared following the consumption of LGI and HGI foods (1.3 g. kg-1
of body weight) after the end of a simulated soccer game [37
]. Finally, consumption of HGI food (1.0 g. kg-1
of body weight) resulted in a 12.8% increase in time to exhaustion compared to a placebo trial [20
]. Discrepancies seen in the results reported by the aforementioned studies may be attributed to differences in meals' time of ingestion, amounts of foods (per kilogram of body weight) or methods of assessment of exercise performance.
In order to provide the same hydration status prior to each exercise trial subjects ingested the same amount of water (300 ml). However, the subjects during the GI trials ingested more volume (300 ml + GI meal) as compared to the control trial (300 ml). Eventhough the different ingested volume could affect gastric emptying and subsequently the metabolic responses this seems unlikely since none of the metabolic variables assessed in the control trial were changed prior to exercise. However, the different ingested volume between the control and the GI trials could have an effect during exercise and this is something that needs further attention in future investigations.
Previous research indicates a role of β-endorphin in metabolism and fatigue perception during exercise. For example, Fatouros et al. [4
] manipulated the carbohydrate intake of rats and found a higher concentration of β-endorphin in plasma and hypothalamus indicating that this peptide is affected by nutritional factors at peripheral and central level. Furthermore, manipulating the levels of peripheral β-endorphin by infusion of this opioid resulted in significant changes in glucose levels and pancreatic hormones during exercise further indicating that β-endorphin has effects on carbohydrate metabolism [6
]. Therefore, it was worth examining whether intake of carbohydrates of different quality (as far as glucose response is concerned) will result in different responses in β-endorphin at rest and/or during exercise. The results from the present study indicate that ingestion of different GI foods does not result in different β-endorphin levels at rest and during exercise. β-endorphin is rapidly responding to an intense bout of exercise [41
]. It was hypothesized that differences in GI foods would affect metabolism leading to different glycogen levels allowing for higher work output. More intense work, in turn, could lead to different beta endorphin responses. This hypothesis was rejected since no differences in performance or beta endorphin levels were observed.
One reason for the inability to observe significant differences in β-endorphin at rest following the consumption of GI foods could be related to the amount of carbohydrate consumed. Subjects received carbohydrates equivalent to 1.5 g. kg-1
of body weight and it seems that this amount of carbohydrates is not enough to alter the response of the pituitary and hypothalamus in the release of β-endorphin. Only one other study examined the response of β-endorphin to carbohydrate and fat meals and found similar results with this study since β-endorphin response changed in the obese but not in individuals of normal weight [5
]. β-Endorphin did not increase significantly until at the exhaustion time point. The inability of β-endorphin to increase during submaximal exercise could be related to the exercise intensity [10
]. Previous research indicates that β-endorphin contributes to the modulation of pain perception and fatigue during exercise [42
]. The results from this study revealed no differences in RPE and β-endorphin levels between the three trials contradicting the results from the aforementioned study.