Abstract
Insulin resistance is associated with a number of metabolic abnormalities that increase the risk of developing coronary heart disease. Dietary fat has been linked with insulin resistance, with alterations in the quality as opposed to the quantity of dietary fat now thought be more important in instigating improvements in insulin resistance. The aim of this work was to investigate the effect of alterations in the dietary fat intakes of middle-aged men (n = 32) on the insulin sensitivity of glucose disposal and postprandial lipid metabolism and to explore the mechanistic links between these insulin responsive pathways. Three separate dietary interventions were conducted; the first involved an increase in the intake of n-3 polyunsaturated fat, the second a decrease in saturated fat and an increase in carbohydrate and the third a decrease in saturated fat and an increase in monounsaturated fat intake. Compliance was monitored by the measurement of red blood cell phospholipid fatty acid composition, postprandial lipid metabolism was measured over 9 hours following a high-fat breakfast (80 g fat), and insulin resistance was measured using the short insulin tolerance test. The results of the study showed that while insulin sensitivity was inversely correlated with red blood cell saturated fatty acid concentration at baseline, the insulin sensitivity of glucose disposal was unaffected by any of the dietary interventions conducted. In measurements of postprandial lipaemia, improvements were observed following the low-saturated fat / high-monounsaturated fat diet and the n-3 polyunsaturated enriched diet, however the low-saturated fat/ high-carbohydrate diet was associated with a worsening of postprandial lipaemia through an increase in the concentrations of triglyceride-rich-lipoproteins. Changes in fasting biochemical measurements were most evident in the low-saturated / high-monounsaturated diet, with an 11 % reduction in total cholesterol and a 15.4 % reduction in fasting triglycerides. There were no observed changes in the activity levels or the gene expression of lipoprotein lipase. There was an unexpected positive association between the degree of insulin sensitivity and the extent of postprandial lipaemia, indicating that the link between these pathways is complex and warrants further investigation. Overall this work supports the view that dietary guidelines should be directed towards a change in the composition of fat, to a lower saturated fat intake, a higher monounsaturated fat intake and a lower n-6 : n-3 ratio through an increase in the intake of long chain n-3 polyunsaturated fatty acids.