METABOLIC RESPONSE TO DIFFERENT GLYCEMIC INDEXES OF PRE-EXERCISE MEAL

Correspondência: Rua José de Souza Maciel 192, Dona Suzana. Florestal, MG, Brasil. CEP: 35.690-000. valeriaefiufv@yahoo.com.br ABSTRACT Introduction: To ensure performance and health, the type of food and the time of pre-exercise ingestion should be considered by practitioners of morning physical activity. Objective: This study assessed the metabolic response after pre-exercise meals with different glycemic indexes (GI) and in the fasting state adopting different types of hydration. Methods: Twelve men performed four experimental tests; two with pre-exercise meals of high GI (HGI) and low GI (LGI), and two were performed in the fasting state with hydration: water (H2O) and carbohydrate drink (CHO). Each test consisted of a pre-exercise rest period of 30 minutes followed by 60 minutes of cycle ergometer with continuous load equivalent to 60% of the extrapolated maximal oxygen consumption (VO2MaxExt). During the exercise, participants were hydrated every 15 minutes with 3mL per kg body weight. During each experimental test, venous blood samples were obtained for fasting and at 15-minute intervals during rest, and every 20 minutes during exercise. The gas analysis was carried out in periods of 5 minutes every 20 minutes of exercise. Results: There was no difference in substrate oxidation. After 20 minutes of exercise, pre-exercise food intake procedures showed similar behavior, having only reduced blood glucose levels compared to fasting procedures (p<0.01). There was maintenance of blood glucose at stable and higher levels during exercise in relation to the other tests in the fast procedure with CHO. Conclusion: The data suggest that despite the similar metabolic behavior between LGI and HGI meals, the adoption of a LGI meal before the morning exercise seems to be a more suitable feeding practice due to higher tendency of rebound hypoglycemia after HGI meal and when morning exercise is performed on fasting, hydration with CHO seems to minimize the hypoglycemic risk arising from that state.


INTRODUCTION
The glycemic index (GI) of food reflects its absorption rate and subsequent glycemic response, which may occur more quickly and at higher levels in the case of high glycemic index (HGI > 70), or more slowly and at lower levels in the case of low glycemic index (LGI < 55) 1 .
This functional tool has been widely used to determine the most suitable pre-exercise meal for the maintenance of blood glucose levels during physical exercise 1 , avoiding the occurrence of hypoglycemia (blood glucose < 70 mg / dL) 2 .Special attention is devoted to the performance of morning physical activity because due to climatic conditions and labor activities, it is a time of great preference by the population, and it is the time of day in which the individual's muscle and liver glycogen reserves are depleted due to the overnight fasting period 3 .
In this context, it is known that in practical terms, the time prior to physical activity for food intake is restricted 4 , and breakfast can lead to rebound hypoglycemia 5 , which occurs due to high glucose uptake by the muscle, caused by the action of high insulin levels, which also inhibits lipolysis, added to the muscular action itself 1 .This condition obviously impairs an ideal performance during exercise, and should then be avoided.Moreover, training performed in total fasting state can also accelerate the onset of a hypoglycemia condition during exercise, which is also harmful.
In the case of training performed in total fasting state is important to consider a hydration with carbohydrate drink as a strategy to maintain normoglycemic values, however the studies are limited to evaluating it in relation to performance 6 .Moreover, according to a systematic review 7 conducted previously there are no studies that evaluate the metabolic impact of a meal offered 30 minutes prior to exercise.
Thus, given that hypoglycemia levels are related to sickness, nausea, malaise, and in some extreme cases, with fainting, harming not only the quality of the activity, but exposing the organism to a risk condition 8 , the aim of this study was to evaluate the metabolic response after pre-exercise meals with different glycemic indexes and in fasting state adopting different types of hydration.

METHODS
Twelve healthy and physically active men (age 22.9 ± 2 years, body weight 74 ± 5.5 kg, VO 2max 42.2 ± 8.9 ml.kg -1 .min - ) voluntarily participated in this study.The experimental procedure was approved by the Research Ethics Committee (REC) of the institution with process number 140/2011 and all participants signed the Informed Consent Form (ICF) endorsed by the REC.The following inclusion criteria were considered: the regular performance of physical activities, absence of cardiovascular risk factors and any chronic degenerative disease.los ejercicios, seguidos por 60 minutos de bicicleta ergométrica con carga continua equivalente a 60% del consumo máximo de oxígeno extrapolado (VO 2MaxExt ).Durante los ejercicios, los participantes fueron hidratados a cada 15 minutos con 3 ml por kg de peso corporal.Durante cada test, fueron obtenidas muestras de sangre venosa en ayunas y en intervalos de 15 minutos durante el reposo, y a cada 20 minutos durante los ejercicios.El análisis de gases fue realizado en períodos de 5 minutos a cada 20 minutos de ejercicio.Resultados: No hubo diferencia en la oxidación de sustrato.Después de 20 minutos de ejercicios, los procedimientos de ingestión de alimentos antes de los ejercicios presentaron comportamiento semejante, mostrando solamente reducción de los niveles de glucemia en comparación con los procedimientos en ayunas (p < 0,01).Hubo mantenimiento de la glucemia en niveles estables y más altos durante los ejercicios con relación a los otros tests en el procedimiento de ayunas con CHO.Conclusión: Los datos sugieren que, a pesar del comportamiento metabólico semejante entre comidas de BIG y AIG, la adopción de una comida de BIG antes de los ejercicios matinales parece ser una práctica de alimentación más adecuada debido a la mayor propensión de hipoglucemia de rebote después de la comida AIG y, cuando los ejercicios matinales son realizados en ayunas, la hidratación con CHO parece minimizar el riesgo hipoglucémico proveniente de ese estado.
To determine the workload, a metabolic gas analyzer (VO-2000, Aerosport, Medgraphics, St. Paul, Minnesota) was used during test in electromagnetic cycle ergometer (SciFit model ISO1000, Oklahoma, United States) with load increment up to 85% of the MHR calculated , which was obtained by the equation THR training heart rate = % (HR max -HRrest) + HR rest 10 , in which HR max was calculated by the equation MHR calculated = 202 -0.72 (age) 11 .This test started with a 3-minute warm-up exercise with load corresponding to the body weight of each volunteer, and from this load, 30 W were added every minute until reaching the target HR.With the aim of preserving the volunteer's physical integrity and according with Marsh (2012) 12 , extrapolated VO 2max (VO 2MaxEx ) was adopted to calculate the workload, as it was obtained from equation generated by a linear regression with HR and O 2 consumption values recorded during exercise up to the time it was stopped (85% of the MHR calculated ), and from it, the load corresponding to 60% of the VO 2MaxEx was determined.
The subjects were instructed to make a similar diet between tests and avoiding exercising on the test day.
Two dietary procedures according to the GI of meals were adopted, with nutritional composition detailed in table 1. Meals were LGI and HGI, both with hydration with water during exercise.
The GI of meals was calculated by the method of Wolever and Jenkins 13 with GI values taken from the International table of GI 14 , and in the absence of the exact food, the value of the food that most resembled was used.
Besides these procedures, two other procedures in the fasting state were also performed and are differentiated by the type of hydration: water or carbohydrate drink (CHO).
A standardized procedure was adopted for all experimental tests (figure 1), which was composed of a 5-minute warm-up exercise in cycle ergometer with progressive load from 45 to 55% of the VO 2MaxExt , performed after a rest period prior to exercise of 30 minutes, followed by 60 minutes of cycle ergometer with continuous load equivalent to 60% of the VO 2MaxExt .During exercise, participants hydrated every 15 minutes with 3 ml per kg body weight.
During each experimental test, fasting blood samples and in 15-minute intervals during rest and every 20 minutes during exercise were obtained.The equipment used was i-StatOne Blood Analyzer (i-Stat  Abbott, Illinois, USA) with cartridge CG8, obtaining blood samples by venipuncture in one of the forearm superficial veins with placement of scalp and collection of 1 ml of blood in each sampling time.Analyses of blood glucose, hematocrit, hemoglobin, calcium, sodium and potassium were performed.In addition, gas analysis was performed (VO-2000, Aerosport, Medgraphics, St. Paul, Minnesota) in periods of 5 minutes every 20 minutes of exercise.
To evaluate the fluid balance, body weight and urine density were recorded before and after exercise and urine volume was recorded after exercise, and weight loss was calculated (initial weight -final weight) 15 .
For statistical analysis were used the analysis of variance (ANOVA) and the paired t-test.The significance level adopted was 5% and the Sigma Stat software version 1.0 was used.

RESULTS
There was no significant difference in mean respiratory quotient (RQ) between procedures throughout exercise, as well as the oxidation values of CHO and fat (table 2).
There was a glycemic peak in the rest period 15 min after the ingestion of HGI and LGI meals in relation to fasting procedures (p <0.01), but not significantly different from one another.After 20 minutes of exercise, pre-exercise food intake procedures kept a similar behavior, only showing reduced blood glucose levels compared to the other procedures (p <0.01).At 40 and 60 minutes of exercise, fasting with H 2 O showed glucose concentration lower than that of fasting with CHO (p = 0.015).The glycemic response during the four procedures is shown in figure 2.
The hemoglobin and hematocrit concentrations showed no significant differences between procedures at all evaluation times; however, in all tests, exercise caused an increase in these variables in relation to rest (table 3).There were differences between groups regarding potassium concentration at 60 minutes of exercise, being higher in LGI procedure in relation to fasting with CHO (p = 0.019) (figure 3C).Calcium concentrations remained constant throughout the testing period in all procedures (figure 3A); however, sodium and potassium levels increased after the onset of exercise, and at the end of exercise, sodium levels returned to resting values, while potassium values remained high throughout the exercise (figure 3B and 3C).
Among the parameters related to fluid balance only the final urine volume differed between groups, being higher in HGI compared to fasting with H 2 O (p = 0.041).Moreover, there was a significant difference (p = 0.007) between initial and final urine density in the HGI procedure (table 4).
In the postprandial glycemic response, there was a peak 15 minutes after ingestion of both meals, with significant differences in relation to fasting procedures, but unlike the vast majority of studies 16,18,19 , it showed no difference between pre-exercise HGI and LGI meals (figure 2).
Although with no significant differences, pre-exercise HGI meal showed higher glycemic elevation in relation to LGI meal at 15 postprandial minutes and at 30 minutes, immediately before the onset of exercise, blood glucose levels in LGI procedure had already reached baseline while those of HGI procedure still showed higher values (figure 2).These results may have a clinical implication, since blood glucose and insulin levels are higher in response to HGI meal, and if exercise starts with levels still high, there may be a rapid decrease in blood glucose levels in the first moments of exercise, usually at 15 minutes 1 , which is a phenomenon called rebound hypoglycemia or reactive hypoglycemia 5 .
The decrease in plasma glucose levels actually occurred at 20 minutes of exercise (figure 2), but with both pre-exercise meals, HGI and LGI; however, the mean values remained within the normoglycemic range (70 to 99 mg / dL) 2 .Nevertheless, it is noteworthy that the intensity adopted in this study was low, 60% of the VO 2MaxEx , and that values below 70 mg / dL could be achieved during high-intensity exercise.
Considering the report of Jeukendrup and Killer 1 in which some individuals are more likely to develop hypoglycemia, it could be observed that in both HGI as LGI pre-exercise meal at 20 minutes of exercise, 33.3% of volunteers had glucose values below 70 mg / dL, and from these, only one showed the same hypoglycemic values in both meals, and only one volunteer, when submitted to HGI procedure, dropped out due to hypoglycemia symptoms previously mentioned 8 .These results point out to the need for monitoring the individual impact of pre-exercise diet, especially in athletes submitted to systematic training, since individual characteristics may or may not facilitate a condition of rebound hypoglycemia.
From 20 minutes and throughout the exercise, fasting with CHO obtained glucose values similar to those of HGI and LGI procedures,

C
which agrees with the results of Chen et al. 18 and Wong et al. 19 , who showed that the CHO consumption during exercise minimizes the metabolic difference arising from GI.
The data obtained in this study demonstrated that the risks in performing exercises in the fasting state can be minimized by hydration with CHO, when significantly higher blood glucose values compared to fasting with H 2 O at 40 and 60 minutes of exercise are observed (figure 2).It was also observed that at 60 minutes of exercise, only in fasting with H 2 O procedure, blood glucose levels below 70 mg / dL were recorded, and that this showed a tendency to decrease, which would allow a higher occurrence of hypoglycemic values in case exercise is prolonged.This behavior was also observed in a previous study 22 ; however, no significant difference was observed between fasting procedures.
The results of the fluid balance (table 4) showed greater final urine volume (p = 0.041) in the HGI procedure, which is consistent with the fact that the final urine density was significantly lower (p=0.007)compared to the initial volume in the same procedure.That was the only fluid change observed, which does not prevent us from concluding that the type of hydration adopted in experimental tests (3 ml/kg body weight) was sufficient to maintain the initial state of volunteers, since there was no significant body weight loss after exercise.However, in all procedures, participants were hypo-hydrated to begin the exercise (urine density > 1.020 g.ml -1 ), according to Casa et al. 23 .
Hydration was also sufficient to maintain hematocrit (40 to 54%) and hemoglobin values (13.5 to 18 g / dL) at normal levels for men 24 (table 3); however, exercise duration and intensity adopted in this experiment provided hemoconcentration during exercise, which warns us about the importance of adequate hydration especially during physical activities longer and more intense than usual, considering that individuals begin their physical practice dehydrated, as observed in the present study and reported by other authors 25,26 .
A progressive dehydration condition will cause a decrease in plasma volume during the exercise, leading to cardiovascular adaptation by increasing the heart rate in an attempt to maintaining sufficient cardiac output to meet the active muscles and assist in the thermoregulatory mechanism, since the venous return and consequently the systolic volume are affected 27 .In certain extreme dehydration conditions, the cardiovascular adaptation can trigger a collapse, producing a cardiac arrest 28 .
Of ions evaluated, only sodium maintained throughout the exercise within normal values (135 to 145 mEq / L) 29 , whereas ionic calcium (1.15 to 1.33 mmol / L) 29 and potassium (3.5 to 5.0 mEq / L) 29 showed values above physiological limit.Although no significant difference was observed between the four procedures for the total calcium and sodium levels, significantly higher potassium levels were observed at the end of 60 minutes of exercise in the LGI procedure compared to fasting with CHO (figure 3C).This can be attributed to the lower stimulation of insulin release caused by LGI food in contrast to stimulation every 15 minutes caused by CHO, as hypoinsulinemia can be considered as a possible cause of hyperpotassemia 30 , or due to the total composition of potassium in the LIG meal be greater when compared to HIG meal (964 mg vs. 599 mg).
The elevation of potassium levels at the beginning of exercise in all procedures occurs because during physical activity, it is released from the intracellular medium to the extracellular medium of the muscle tissue and then into the bloodstream, and as this tissue is the largest potassium deposit in the body 30 , changes caused by exercise are highlighted.

CONCLUSION
Despite the similar metabolic behavior between LGI and HGI meals, the adoption of LGI meal before morning exercise appears to be the most suitable nutritional strategy due to the greater propensity of rebound hypoglycemia after HGI meal.When performed in fasting, hydration with CHO seems to minimize the hypoglycemic risk arising from this state.However, although the insulinemic response has not been analyzed, further studies are needed to assess the potassium behavior during exercise after LGI meal.

Figure 2 .Figure 1 .-
Figure 2. Blood glucose concentration (mg/dL) during rest and exercise in the four procedures.a p<0.01:HGI and LGI vs. fasting procedures.b p = 0.015: fasting with CHO vs. fasting with H 2 O.

Figure 3 .
Figure 3.Total blood calcium (mmol/L), sodium and potassium concentrations (mEq/L) during rest and exercise in the four procedures.C: * p = 0.019: LGI vs. fasting with CHO.

Table 1 .
Nutrition Composition of pre-exercise meals.

Table 3 .
Hematocrit and hemoglobin levels during rest and exercise.