Abstract
The dietary recommendation to replace saturated fat with carbohydrate as a means of reducing cardiovascular risk, has been hindered by the adverse effect of dietary extrinsic sugars on plasma lipids. The project described in this thesis, formed part of a BBSRC-funded trial to examine the impact of dietary extrinsic sugars on lipoprotein kinetics. A key objective of this project was to design and deliver two diets, high and low in extrinsic sugars, to test the hypothesis of the main trial; that dietary sugars influence plasma lipids, via differential effects on the kinetics of plasma lipoproteins, in groups with moderately raised and low liver fat. This thesis presents the results from five experimental Chapters (2-6): i) An independent, systematic review and meta-analysis of the relationship between dietary sugars and plasma lipids (Chapter 2), that confirmed significant positive and negative associations between dietary sugars, especially fructose, with plasma TAG and HDL-cholesterol, respectively; ii) the development of two experimental diets, high and low in sugars, based on the exchange of starch and sugars, using supermarket foods (Chapter 3). The exchange achieved, and came close to achieving, target intakes for sugar, that were representative of the upper 95th and lower 5th percentile of sugar intake in the UK; iii) a study of the impact of these two diets on plasma lipids and lipoproteins, in groups with raised and low liver fat (Chapters 4 & 5). The high sugar diet was accompanied by significant increases in plasma TAG, apoproteins (CII & E), and liver fat, and decreases in HDL-cholesterol, and lipoprotein particle size. In contrast, the low sugar diet was associated with opposite changes in all of these variables. The effects of both diets were shown to be more pronounced in the group with moderately raised liver fat (> 4. 2%), and were largely unaltered by the re-analysis of sugar intake by three different methods (Chapter 6). These findings provide evidence for the practical feasibility, and experimental utility of exchanging starch and sugars in real foods. They also provide further support for the recommendation to restrict the intake of dietary extrinsic sugars, especially in individuals with moderately raised liver fat.