Low Versus High-glycemic Index Post-exercise Diets for Improving Metabolism and Body Composition

The increase in triglycerides after a meal (i.e. postprandial triglycerides) is a strong risk factor for cardiovascular disease. Overweight/obese individuals have a greater triglyceride response to the same meal compared to lean individuals; therefore they are at greater risk. If exercise is performed the evening before a high-fat breakfast, the postprandial levels of triglycerides after the breakfast are reduced; however, if high glycemic index (GI) foods are consumed after the exercise session, the benefits of exercise on next-day postprandial triglycerides is negated. The investigators recently showed (HSFC funded) that consuming low GI foods after exercise is similar to fasting after exercise and superior to consumption of high GI foods for lowering next-day postprandial triglycerides. Low GI food was also superior for increasing fat oxidation, and lowering insulin, and low and very low density lipoproteins and preventing a decrease in high density lipoproteins. The benefits of an acute exercise session followed by low GI food consumption is now clear; however, it is not known whether this practice over weeks of exercise training results in accumulated and sustained benefits.

The purpose of this study is to determine the chronic effects of consuming low compared to high GI foods after exercise sessions over six weeks in overweight and obese individuals.

Twenty-four overweight and obese males and females will be randomized to consume either high or low GI foods immediately after exercise sessions (four sessions per week, 90 minutes per session, six weeks). At all other times of the day over the six weeks, they will be supplied a moderate GI diet, with calories and macronutrients based on four-day food diaries completed before the study. Two weeks before the intervention, participants will be required to go on the moderate GI diet, as a lead-in. One week into the lead-in, a baseline test will be done to determine postprandial metabolic responses (i.e. triglycerides, low and very low density lipoproteins, high density lipoproteins, insulin, and fat oxidation) after a morning high-fat breakfast. A week later, the same postprandial testing will be carried out the morning after the first exercise and post-exercise consumption of a low or high GI recovery meal. Participants will continue the training and post-exercise food consumption for the next six weeks, with postprandial measures taken again following the final exercise and feeding session. Body composition and aerobic fitness will be determined before and after the six week intervention.

The investigators hypothesize that the low GI post-exercise feeding will be superior to the high GI post-exercise feeding for reducing body fat, improving fat oxidation, and reducing postprandial triglycerides, insulin and other harmful lipids. The results of the study will provide clinicians, exercise professionals and dietitians unique and experimentally tested strategies for their clients to lose body fat and improve metabolic profiles, to reduce the risk of heart disease.