Effect of glycemic index

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Abstract

Low glycemic index (GI) foods may confer an advantage when eaten before prolonged strenuous exercise by providing a slow-release source of glucose to the blood without an accompanying insulin surge. To test this hypothesis, eight trained cyclists pedalled to exhaustion one hour after ingestion of equal carbohydrate portions of four test meals: lentils, a low GI food (LGI); potato, a high GI food (HGI), and glucose and water. Plasma glucose and insulin levels were lower after LGI than after HGI from 30 to 60 min after ingestion (p < 0.05). Plasma free fatty acid (FFA) levels were highest after water (p <0.05) followed by LGI and then glucose and HGI. From 45 to 60 min after ingestion, plasma lactate was higher in the HGI trial than in the LGI trial (p <0.05) and remained higher throughout the period of exercise. The rank order from lowest to highest for total carbohydrate oxidation during exercise was water, lentils, glucose and potato. Endurance time was 20 min longer after LGI than after HGI (p < 0.05). These findings suggest that a low GI pre-game meal may prolong endurance during strenuous exercise by inducing less postprandial hyperglycemia and hyperinsulinemia, lower levels of plasma lactate before and during exercise, and by maintaining plasma glucose and FFA at higher levels during critical periods of exercise.

A moderate glycemic meal before endurance exercise can enhance performance

The purpose of this study was to determine whether presweetened breakfast cereals with various fiber contents and a moderate glycemic index optimize glucose availability and improve endurance exercise performance. Six recreationally active women ate 75g of available carbohydrate in the form of breakfast cereals: sweetened whole-grain rolled oats (SRO, 7g of dietary fiber) or sweetened whole-oat flour (SOF, 3g of dietary fiber) and 300ml of water or water alone (Con). The meals were provided 45min before semirecumbent cycle ergometer exercise to exhaustion at 60% of peak O2 consumption (VO2 peak). Diet and physical activity were controlled by having the subjects reside in the General Clinical Research Center for 2days before each trial. Blood samples were drawn from an antecubital vein for glucose, free fatty acid (FFA), glycerol, insulin, epinephrine, and norepinephrine determination. Breath samples were obtained at 15-min intervals after meal ingestion and at 30-min intervals during exercise. Muscle glycogen concentration was determined from biopsies taken from the vastus lateralis muscle before the meal and immediately after exercise. Plasma FFA concentrations were lower (P<0.05) during the SRO and SOF trials for the first 60and 90min of exercise, respectively, than during the Con trial. Respiratory exchange ratios were higher (P<0.05) at 90and 120 min of exercise for the SRO and SOF trials, respectively, than for the Con trial. At exhaustion, glucose, insulin, FFA, glycerol, epinephrine, and norepinephrine concentrations, respiratory exchange ratio, and muscle glycogen use in the vastus lateralis muscle were similar for all trials. Exercise time to exhaustion was 16% longer (P<0.05) during the SRO than during the Con trial: 266.5 ±13and 225.1±8min, respectively. There was no difference in exercise time for the SOF (250.8±12) and Con trials. We conclude that eating a meal with a high dietary fiber content and moderate glycemic index 45min before prolonged moderately intense exercise significantly enhances exercise capacity.

Influence of high and low glycemic index meals on endurance running capacity

Purpose: The purpose of this study was to examine the effect of high and low glycemic index (GI) carbohydrate (CHO) pre-exercise meals on endurance running capacity.

Methods: Eight active subjects (five male and three female) ran on a treadmill at ~70% V?O2max to exhaustion on two occasions separated by 7 d. Three hours before the run after an overnight fast, each subject was given in a single-blind, random order, isoenergetic meal of 850 ± 21 kcal (mean ± SEM; 67% carbohydrate, 30% protein, and 3% fat) containing either high (HGI) or low (LGI) GI carbohydrate foods providing 2.0 g CHO·kg-1 body weight.

Results: Ingestion of the HGI meal resulted in a 580% and 330% greater incremental area under the 3-h blood glucose and serum insulin response curves, respectively. Performance times were not different between the HGI and LGI trials (113 ± 4 min and 111 ± 5 min, respectively). During the first 80 min of exercise in the LGI trial, CHO oxidation was 12% lower and fat oxidation was 118% higher than in the HGI trial. Although serum insulin concentrations did not differ between trials, blood glucose at 20 min into exercise in the HGI trial was lower than that during the LGI trial at the same time (3.6 ± 0.3 mmol·L-1 vs 4.3 ± 0.3 mmol·L-1; P < 0.05). During exercise, plasma glycerol and serum free fatty acid concentrations were lower in the HGI trial than in the LGI trial.

Conclusions: This results demonstrate that although there is a relative shift in substrate utilization from CHO to fat when a low GI meal is ingested before exercise compared with that for a high GI meal, there is no difference in endurance running capacity.

HIGH GI MEALS

Muscle glycogen storage after prolonged exercise: effect of the glycemic index of carbohydrate feedings

The effect of the glycemic index (GI) of postexercise carbohydrate intake on muscle glycogen storage was investigated. Five well-trained cyclists undertook an exercise trial to deplete muscle glycogen (2 h at 75% of maximal O2 uptake followed by four 30-s sprints) on two occasions, 1 wk apart. For 24 h after each trial, subjects rested and consumed a diet composed exclusively of high-carbohydrate foods, with one trial providing foods with a high GI (HI GI) and the other providing foods with a low GI (LO GI). Total carbohydrate intake over the 24 h was 10 g/kg of body mass, evenly distributed between meals eaten 0, 4, 8, and 21 h postexercise. Blood samples were drawn before exercise, immediately after exercise, immediately before each meal, and 30, 60, and 90 min post-prandially. Muscle biopsies were taken from the vastus lateralis immediately after exercise and after 24 h. When the effects of the immediate postexercise meal were excluded, the totals of the incremental glucose and insulin areas after each meal were greater (P < or = 0.05) for the HI GI meals than for the LO GI meals. The increase in muscle glycogen content after 24 h of recovery was greater (P = 0.02) with the HI GI diet (106 +/- 11.7 mmol/kg wet wt) than with the LO GI diet (71.5 +/- 6.5 mmol/kg). The results suggest that the most rapid increase in muscle glycogen content during the first 24 h of recovery is achieved by consuming foods with a high GI.

Purpose

  • Primary Purpose: to determine whether a CHO and protein hydrolysate (ProH) beverage would bring out improvements in performance during time-trial cycling versus a CHO beverage alone.
  • Secondary purpose: to determine whether treatment with CHO+ProH lessened signs of muscle disruption as compared with CHO.

Methods

  • Randomly counterbalanced double-blind

design

  • 13 recreationally competitive male cyclists
  • either a CHO+ProH beverage or CHO

beverage

  • 2 computer simulated 60 km time trials
  • Participants with 2 or more risk factors for coronary artery disease were excluded

Measurements

  • VO2 max
  • respiratory exchange rate (RER)
  • ratings of perceived exertion (RPEs)
  • blood glucose
  • heart rate
  • lactate
  • Plasma creatine phosphokinas (CK) and muscle soreness ratings were assessed before and after 24 hours

Results

  • CHO+ProH beverage improved time-trial performance
  • All occurring in the final lap (late stage) of the test
  • Plasma CK and muscle soreness ratings were high in the CHO trial,

Implication

  • Addition of ProH to the CHO drink may have prolonged the time to muscle fatigue.
  • ProH may have reduced markers of muscle disruption

Evaluation

  • Adequate background information and review of related literature.
  • Previous studies ambiguous and did not examine differences of late stages
  • Design strong enough to address previous ambiguities
  • explanation of protein hydrolysates
  • Conclusion valid
  • Results supported ProH

use

  • Use of more references,including contrasting literature
  • Small sample size of 13
  • Need larger sample for more accurate results
  • Can not be generalized to all athletes, only:
  • Males
  • Endurance cyclists

Abstract

The thermic previous termeffectnext term of meals consisting of predominantly carbohydrate (CHO) or previous termfat (FAT)next term followed by a graded previous termexercisenext term test was compared to a no meal (NOM) trial in 7 women. Oxygen consumption (VO2) increased similarly during the 45 min period following both meals (21%-CHO, 23%-previous termFAT)next term, however, total energy expenditure was greatest following CHO due to an increased respiratory exchange ratio (R). No differences between trials in VO2 or energy expenditure were noted during submaximal or maximal work loads. Following previous termexercisenext term VO2 decreased to postprandial, pre-previous termexercisenext term levels within approximately 30 min. Oxygen consumption remained elevated 13% in both meal trials at 3h post-meal (2h post-previous termexercise)next term, however, energy expenditure was greater in CHO during the final hour of recovery. It was concluded that CHO and previous termFATnext term induce similar increases in VO2, however, energy expenditure is slightly greater in CHO due to greater caloric cost of carbohydrate versus previous termfatnext term oxidation. There was no evidence of a diet plus previous termexercisenext term potentiation of VO2.

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