Abstract
The work of this thesis aimed to investigate dietary strategies for the amelioration of the lipid abnormalities found in an atherogenic lipoprotein phenotype/metabolic syndrome. The strategies involved the manipulation of the quantity and quality of dietary fat and carbohydrate, and the quantity of protein. This led to three different dietary interventions that included: (I) The OPTILIP study, a randomly controlled, parallel study designed to determine the optimal intake of dietary n-6/n-3 fatty acids with respect to blood lipids and lipoproteins. This was achieved by comparing four diets with varying n-6/n-3 ratios (3:1-5:1); one containing additional long chain polyunsaturated fatty acid n-3 (from fish, notably salmon, and a long chain n-3 enriched spread), two with additional linolenate (from a rapeseed oil based spread and cooking oil) and one with a combination of long chain polyunsaturated fatty acid n-3 and linolenate, with a control based on a estimated UK diet (10:1). Healthy male and postmenopausal females aged 45-70 were randomly allocated to one of the five diets for six months. (II) The effects of low-fat, high and low glycaemic index diets. This study was designed to test the effects of high and low glycaemic index (GI) low-fat diets. Fourteen normal, healthy men (aged 35-55 years) characterised with an atherogenic lipoprotein phenotype, were randomly assigned to one of two prescribed diets containing either a high proportion of foods with a high glycaemic index (> 80) or a low glycaemic index (< 60). Dietary prescriptions were achieved by instructing volunteers to follow two cycles of a 7-day menu that provided a choice of foods and mixed meals appropriate to either their high or low glycaemic index status. The study was free-living, with a crossover design. (III) The effect of low and high-carbohydrate hypoenergetic diets. This study was designed to test both low and high-carbohydrate weight-reducing diets against a control for 6-months. Subjects were overweight/obese subjects (180) randomised to 1 of the 3 diets, a low-fat, high-carbohydrate diet, a high-protein, high-fat diet or a control. . The main results for the OPTILIP study were reductions in pre and postprandial plasma triacylglycerol, small, dense low density lipoprotein and increases in the larger, lipid-rich high density lipoprotein two subfraction that was more pronounced in males on the two long chain polyunsaturated n-3 (fish-rich) diets. The low glycaemic index diet was accompanied by decreases in plasma cholesterol, triacylglycerol, high density lipoprotein-cholesterol and low density lipoprotein-cholesterol, body mass and body mass index, there were no significant differences in any of these variables after the high glycaemic index diet. There was no change in the distribution of low density lipoprotein subclasses as measured by its peak density or in waist circumference for either group. The low and high-carbohydrate hypoenergetic diets were similar with respect to weight loss (-9%), although the low-carbohydrate group increased lean body mass. Plasma triacylglycerol, low density lipoprotein peak density, triacylglycerol:high density lipoprotein-cholesterol, insulin, and Homeostasis model assessment decreased on both diets but to a greater magnitude on the low-carbohydrate diet; whereas plasma total cholesterol, low density lipoprotein-cholesterol, and high density lipoprotein-cholesterol were reduced to a greater extent on the high-carbohydrate diet relative to the low-carbohydrate diet. In conclusion, the results of the three dietary interventions described in this thesis all show, to varying degrees, favourable effects on markers of an atherogenic lipoprotein phenotype/metabolic syndrome and on traditional risk factors for cardiovascular disease, plasma total cholesterol and low density lipoprotein-cholesterol. The next obvious step would be to investigate the combined effects of a diet low in the ratio of n-6/n-3 fatty acids, high in protein with the source of carbohydrate derived from low glycaemic index foods. The precise proportions of macronutrients would need further work and may even depend on genotype and or phenotype and therefore warrant an individual approach to nutrition with prospective selection of subjects.