Effect of exercise, diet and ethnicity on metabolic responses in postprandial state

Cardiovascular disease is a leading cause of mortality and morbidity worldwide. One of the key factors mediating cardiovascular disease risk, and the underlying atherogenic disease process, is disturbances to metabolism in the postprandial state, particularly with respect to lipoprotein metabolism. A number of studies have demonstrated that prior exercise can reduce postprandial triglyceride (TG) concentrations, with recent evidence indicating that increased clearance from the circulation of large very low density lipoproteins (VLDL1) plays an important role. However, it was unclear how exercise facilitated this potentially beneficial effect and this was the focus of the present work. The first experimental study in this thesis demonstrated, in 10 overweight/obese men, that 90 minutes of prior moderate exercise increased the affinity of VLDL1 for TG hydrolysis by lipoprotein lipase by 2.2-fold in the fasted state (p = 0.02) and 2.6-fold in the postprandial state (p = 0.001), but did not significantly alter the affinity of chylomicrons, a novel observation that adds to understanding of the mechanism by which exercise lowers TG concentrations. Postprandial responses to meal ingestion depend on the macronutrient composition of the food ingested. In the second experimental chapter, postprandial responses to ingestion of a test meal containing 75g glucose, or 75g fat, or a combination of 75g glucose and 75g fat were compared in 10 overweight/obese men. The main finding was that co-ingestion of fat with the glucose load reduced the postprandial glucose response, but not insulin response, compared with glucose ingestion alone. Co-ingestion of fat with the glucose load also substantially reduced the postprandial suppression of non-esterified fatty acids (NEFA) compared to glucose only ingestion. Postprandial TG responses were similar when only fat was consumed compared with co-ingestion of fat and glucose, but postprandial VLDL1 concentrations were lower in the latter condition. It is well established that ethnic differences exist in the prevalence of cardio-metabolic diseases. In particular, diabetes prevalence is high in Middle-Eastern populations. It is not known whether ethnic differences in postprandial metabolism contribute to these differences in risk. In the third experimental study, eight white European men and eight men of Middle-Eastern origin consumed a mixed-meal and postprandial responses were assessed. Postprandial insulin responses were higher in the Middle-Eastern men and postprandial TG concentrations were higher in the European men. This suggests that ethnic differences may exist in the inter-relationship between insulin resistance and lipoprotein metabolism. Thus, overall this thesis has provided insights into how postprandial metabolism is modulated by exercise, macronutrient intake and ethnicity