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Low glycemic index for improving health and exercise performance



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Objectives: Low-glycemic index foods have been promoted as a healthy choice for improving cardiovascular health and endurance exercise performance. This thesis assessed the effects of low glycemic-index foods and beverages during conditions that varied from extremes of inactivity (i.e. bed rest in study one) through moderate-intensity activity (i.e. brisk walking in study two) to high activity (i.e. prolonged endurance exercise in study three) on performance and markers of health. The overall purpose was to assess the effect of diets/beverages differing in glycemic index, on cardiovascular health, metabolic profile and endurance exercise performance. Study 1: Our purpose was to determine the effects of a low glycemic-index pulse-based diet (i.e., containing lentils, chick peas, beans and split peas) compared to a typical hospital diet on insulin sensitivity (assessed by the Matsuda index from the insulin and glucose response to a two-hour oral glucose tolerance test), insulin resistance assessed by the homeostatic model assessment of insulin resistance (HOMA-IR), bone resorption assessed by 24 h excretion of urinary n-telopeptides(Ntx) and cardiovascular risk factors (blood lipids, blood pressure, arterial stiffness and heart rate variability) during bed rest. Methods: Using a randomized, counter-balanced cross-over design with one-month washout, six healthy individuals (30 ± 12 years) consumed the diets during four days of bed rest. The Matsuda index, HOMA-IR, urinary Ntx and cardiovascular risk factors were determined at baseline and after the last day of bed rest. Results: Compared to the typical hospital diet, the pulse-based diet improved the Matsuda index (indicating increased insulin sensitivity; baseline to post-bed rest: 6.54 ± 1.94 to 6.39 ± 2.71 hospital diet vs. 7.14 ± 2.36 to 8.75 ± 3.13 pulse-based diet; p = 0.017), decreased HOMA-IR (1.38 ± 0.54 to 1.37 ± 0.50 hospital diet vs. 1.48 ± 0.54 to 0.88 ± 0.37 pulse-based diet; p = 0.022), and attenuated the increase in Ntx (+89 ± 75% hospital diet vs. +33 ± 20% pulse-based diet; p = 0.035). No differences for changes in cardiovascular risk factors were found between the two diet conditions, with the exception of decreased diastolic blood pressure during day three of bed rest in the pulse-based versus hospital diet (61 ± 9 vs. 66 ± 7 mmHg; p = 0.03). Conclusion: A pulse-based diet was superior to a hospital diet for maintaining insulin sensitivity, preventing insulin resistance, attenuating bone resorption and decreasing diastolic blood pressure during four days of bed rest in healthy adults. Study 2: Our purpose was to determine the effect of consuming low glycemic index (LGI) skim milk as a recovery beverage, compared to a high glycemic index (HGI) sports drink, following evening exercise on fat oxidation and blood lipids after a subsequent high fat breakfast. Methods: In this randomized counterbalanced crossover trial, 20 overweight or obese participants (BMI≥25 kg m− 2) underwent 4 conditions: 90-min exercise at 50% VO2peak followed by sports drink (EX-HGI); the same exercise followed by isocaloric skim milk (EX-LGI); the same exercise with water (Exercise); a control condition. Blood lipids, glucose, and fat oxidation were assessed before and for 6h after a standardized high-fat breakfast the next morning. Results: There was a condition main effect for fat oxidation (p=0.042) with means highest for EX-LGI (6.7±2.7 g/h) and lowest for EX-HGI (6.0±1.8 g/h). Mean triglyceride concentration and total area under the curve for triglycerides with EX-HGI was higher than Exercise (1.7±1.6 vs. 1.3± 1.0 mmol/l, p=0.037, and 11.7±9.4 vs. 8.6±6.0 mmol l − 1 h, p=0.005, respectively). Mean glucose concentration with EX-LGI was lower than EX-HGI (4.1±1.1 vs. 4.4 ± 1.1 mmol/l, p=0.027). Homeostatic Model Assessment of Insulin Resistance was higher with EX-HGI than Control (2.32±1.15 vs. 1.86±0.97, p=0.005). Conclusion: Evening post-exercise skim milk consumption, compared with a high-GI sports drink, significantly reduced blood glucose and possibly increased fat oxidation after a high-fat breakfast the next morning in overweight/obese adults. Study 3: Tart cherries have antioxidant and anti-inflammatory properties and low glycemic index, and therefore may benefit performance and recovery from exercise. Our purpose was to determine the effects of consuming tart cherry juice versus a high-glycemic index sports drink on cycling performance, substrate oxidation, and recovery of low-frequency fatigue. Methods: Using a randomized, counter-balanced cross-over design, with one-month washout, 12 recreational cyclists (8 males; 35y) consumed cherry juice or sports drink twice a day (300mL/d) for 4d before and 2d after exercise. On the exercise day, beverages (providing 1g/kg carbohydrate) were consumed 45min before 90min of cycling at 65%VO2peak, followed by a 10km time trial. Blood glucose, lactate, carbohydrate and fat oxidation, respiratory exchange ratio (RER), O2 cost of cycling, and rating of perceived exertion (RPE) were measured during the initial 90min of cycling. Blood pressure was assessed after testing on the exercise day and overnight. Muscle soreness, maximal voluntary contraction (MVC) and low-frequency fatigue were determined at baseline and after the time trial on the exercise day, and 30min after beverage consumption 24 and 48h later. Results: There were no differences for time trial performance (17±3min cherry juice vs. 17±2min sports drink, p=0.27) or any other measures between drink conditions. There were time main effects (p<0.05) for isometric MVC (decreasing) and low-frequency fatigue (increasing; i.e. decreased force at low relative to high stimulation frequencies), changing significantly from baseline to post-exercise and then returning to baseline at 24h post-exercise. Conclusion: Tart cherry juice was not effective for improving performance, substrate oxidation during exercise, and recovery from exercise in recreational cyclists, compared to a high-glycemic index sports drink. Conclusion: Low-GI foods and beverages assessed in this thesis (i.e. pulse-based diets and skim milk) were effective for improving some aspects of cardiovascular and metabolic health during bed rest in study one where pulse-based diets were provided to healthy participants and in conjunction with acute exercise in study two where overweight/obese adults consumed skim milk after an evening exercise session, but tart cherry juice was not effective for improving endurance performance, substrate oxidation during exercise and recovery from exercise in study three where tart cherry juice was consumed by recreational cyclists four days before, on the day of and two days after exercise.



Glycemic index, physical activity, cardiovascular health, cycling performance



Doctor of Philosophy (Ph.D.)







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