Impact of glycaemic index of high carbohydrate diets on exercise energy metabolism and capacity and fasting concentration of plasma lipids in healthy physically active individuals

摘要

The present thesis describes the impact of glycaemic index of high carbohydrate diets consumed for 5 days on exercise energy metabolism and capacity and fasting plasma lipids in healthy physically active individuals. The thesis consists of a literature review (Chapter 1), general methods (Chapter 2), four experimental chapters (Chapter 3-Chapter 6) and general discussion and conclusion (Chapter 7). Chapter 3 presents a pilot study aimed to investigate whether high carbohydrate meals with high and low glycaemic index of foods present within meals developed by using the glycaemic index values from the published glycaemic index tables, produce significant differences in postprandial glucose response. Eight healthy active women consumed prescribed high carbohydrate diets with either high or low glycaemic index in a randomised counterbalanced order. The experimental meals which consisted of breakfast, morning snack and lunch were consumed after an overnight fast. Plasma glucose responses were measured at baseline and every 30 minutes for 300 minutes after baseline. We concluded that high carbohydrate meals with high and low glycaemic index prescribed using the glycaemic index values from the existing glycaemic index tables in the literature produced a significant difference in postprandial plasma glucose responses. Thus, for further studies high carbohydrate diets with high and low glycaemic index were developed using glycaemic index values from available glycaemic index tables. The aim of Chapter 4 and Chapter 5 was to investigate the extent to which the glycaemic index of high carbohydrate diets consumed for 5 days reduces the rate of fat oxidation during endurance exercise and exercise capacity during running conducted in the fasted state in men and women. To determine this, 9 healthy physically active men (Chapter 4) and 9 healthy physically active women (Chapter 5) performed three treadmill runs to exhaustion at 65% max after their habitual diet, after 5 days on a high carbohydrate high glycaemic index diet, and after 5 days on high carbohydrate low glycaemic index diet, in a randomised counterbalanced order. Blood samples for the measurements of glucose, insulin, glycerol and non-esterified fatty acids, and expired air samples for the measurements of the rates of fat and carbohydrate oxidation were obtained at 15, 30, 45, 60, 75, 90 minutes and at the point of exhaustion. Running capacity was measured as time to exhaustion and distance covered. It was found that in both men and women, the extent to which high carbohydrate diets consumed for 5 days reduced the rate of fat oxidation during running in the fasted state was not influenced by the glycaemic index of the diet, and that glycaemic index of high carbohydrate diets consumed for 5 days had no impact on running capacity. Chapter 6 aimed to investigate the impact of the consumption of high carbohydrate diets with high and low glycaemic index for 5 days on fasting plasma concentration of lipids, insulin sensitivity and biomarkers for endothelial function (i.e. intercellular adhesion molecule-1 and vascular cell adhesion molecule-1) in physically active individuals. Fasting blood was collected from 17 healthy individuals on three occasions in a randomised counterbalanced order: after consuming habitual diet, after 5 days on high carbohydrate high glycaemic index diet and after 5 days on high carbohydrate low glycaemic index diet. It was found that the extent to which high carbohydrate diets consumed for 5 days increases fasting plasma concentration of triglyceride and reduces the concentration of high density lipoprotein cholesterol was not influenced by the glycaemic index of the diets. It was also found that glycaemic index of high carbohydrate diets consumed for 5 days had no impact on insulin sensitivity or on biomarkers of endothelial activation. In conclusion, consideration of the glycaemic index of high carbohydrate diets consumed by physically active healthy men and women for 5 days has no impact on insulin sensitivity and fasting concentration of plasma lipids, it does not influence the rate of fat oxidation induced by high carbohydrate diets during running conducted in the fasted state and has no influence on running capacity. Thus, when physically active individuals increase carbohydrate intake for the purposes of muscle glycogen accumulation, consideration of the glycaemic index is not important. Future studies are needed to determine whether the glycaemic index of high carbohydrate diets modify exercise energy metabolisms in top grade athletes.