The impact of N-3 pufa ingestion on metabolic, molecular and epigenetic responses to a short-term high-fat diet

Abstract

Obesity is widely considered a primary risk factor for type 2 diabetes (T2D). However, less is known about the early adaptive responses to short-term periods of high-fat energy excess (HFEE). Previous reports detailing whole-body adaptation to fat and energy oversupply are equivocal, perhaps, in part, owing to use of different experimental protocols, varying durations of dietary manipulation and participant cohorts with individuals of varying characteristics. In addition to use of different dietary protocols between studies, alterations in functional end-point measures due to the type of dietary fat consumed warrants consideration. Daily n-3 PUFA intake, commonly obtained from pelagic fish oil (FO) consumption, has been shown to positively associate with insulin sensitivity in epidemiological studies and thus may be a useful dietary strategy for slowing insulin resistance development. Chapter 2 of this thesis extends previous literature by demonstrating that 6 d HFEE (150 % habitual energy intake; 60 % of energy from fat) does not clearly alter whole- body insulin sensitivity, irrespective of FO consumption. However, investigation of metabolism at the tissue level, as presented in Chapter 3 of this thesis, offers insight into a potential tissue-specific level of regulation that precedes whole-body regulation. Skeletal muscle insulin signalling protein (e.g. protein kinase B (PKB)) activity, levels of certain ceramide species, and AMPK α2 activity were altered following HFEE and may explain the early maladaptive responses to short-term HFEE. Moreover, FO intake as 10 % of total fats mediated some of these molecular Sophie Wardle: Ph.D. Thesis ii responses, including PKB and AMPK α2 activity, reflecting possible functional effects of FO at the subcellular level. Regulation of these metabolic / molecular responses at both the tissue and whole- body level can be explained, in part, by genetic predisposition, environmental influence and more recently epigenetics, including microRNAs (miRNAs). In Chapter 4, we characterised the plasma and skeletal muscle miRNA responses to HFEE and oral glucose ingestion. We demonstrate transient changes in levels of certain miRNAs following oral glucose ingestion in both tissue types and in response to HFEE in skeletal muscle. However, no significant correlations between basal plasma and skeletal muscle miRNA levels were observed, suggesting that our candidate plasma miRNAs may be co-ordinating functional changes in other tissue types. Plasma miR- 145-5p and skeletal muscle miR-204-5p predicted a significant proportion of the variance in mean whole-body insulin sensitivity change in response to HFEE. These data indicate that these miRNAs may be useful biomarkers of insulin resistance development following HFEE. A constraint of this thesis is that all conclusions are made within the context of statistically unaltered insulin sensitivity. Therefore, future investigations of diet- induced maladaptation should consider establishing a time course of insulin resistance development in response to HFEE, or use different study populations. Populations that are more susceptible to T2D development, e.g., overweight, sedentary individuals would be of particular interest. These data would aid development of a working model of diet-induced insulin resistance that has more direct application to T2D progression and extends the data presented herein.