Metabolic responses to short-term high-fat overfeeding

The main aim of this thesis was to increase our understanding of the metabolic responses associated with short-term high-fat overfeeding. To this end, four separate studies are described in this thesis; each of which involved the provision of a high-fat, high-energy diet to young, healthy, lean individuals. The first of these experimental chapters (Chapter 2) determined the effects of a 7-day, high-fat (65%), high-energy (+50%) diet on postprandial metabolic and endocrine responses to a mixed meal challenge. This chapter demonstrates that 7-days of overfeeding impaired glycaemic control in our subject cohort but did not influence the response of selected gut hormones (acylated ghrelin, GLP-1 and GIP). In a mechanistic follow up study utilising stable isotope tracer methodology we then demonstrate that overfeeding-induced impairments in glycaemic control are attributable to subtle alterations in plasma glucose flux, rather than the overt tissue-specific adaptations (e.g. increased EGP, or reduced glucose disposal) that have previously been reported (Chapter 3). In an attempt to delineate the time-course of diet-induced impairments in glycaemic control, we then investigated the effects of 1-day of overfeeding (+80% energy with 73% of total energy coming as fat) (Chapter 4). Results demonstrate that a single day of overfeeding elicits responses which are comparable to 7-days of high-fat overfeeding; highlighting the rapidity with which excessive high-fat food intake can negatively influence glucose metabolism. In chapter 5 we utilised stable isotope tracer and muscle biopsy techniques to demonstrate that 7-days of high-fat overfeeding impairs glycaemic control but does not influence the fed-state mixed muscle protein fractional synthesis rate (FSR). In conclusion, the findings of this thesis demonstrate that while short-term high-fat overfeeding negatively influences whole-body glucose metabolism, skeletal muscle protein metabolism appears to be relatively unaffected in young, lean, healthy humans.