| Summary: | Raised postprandial triglycerides (TG) is an independent risk factor for cardio-metabolic disorders. This is due, in part, to the increases in circulating remnant lipoproteins after TG have been transported for storage or hydrolysis. Raised TG, are also associated with an atherogenic lipoprotein phenotype (High TG, low high-density lipoprotein cholesterol (HDL-c) and small, dense low-density lipoprotein (LDL) particles). In addition, elevated TG contribute to ectopic storage of fatty acids in liver, adipose and muscle tissues, contributing to insulin resistance in all three tissues and consequent metabolic dysregulation. It is therefore paramount to prevent frequent and prolonged exposure to raised TG in the postprandial period, particularly in groups who are at increased risk of cardio-metabolic disease. The dyslipidaemic component of cardio-metabolic health can be inferred by assessing the capacity to breakdown and clear TG from circulation after high fat ingestion using an oral fat tolerance test (OFTT). The OFTT can also be used to assess the efficacy of interventions targeting reductions in postprandial TG. Exercise and nutritional interventions have been shown to alter postprandial TG excursions and provide insight in to underlying mechanisms of postprandial lipid metabolism. However, there are several topics within this area of research that require further clarification. These topics have been addressed within this thesis. The first experimental chapter of this thesis (chapter 3) aimed to investigate the repeatability of an OFTT (75g fat, 22g carbohydrate, 14g protein) designed to meet recommendations from an expert panel statement. This study also aimed to evaluate the repeatability of the postprandial response to an OFTT preceded by 1 hour of acute moderate intensity exercise (cycling at a work rate eliciting 90% oxygen consumption anaerobic threshold). After an overnight fast, 11 healthy adult male participants consumed OFTT meals on 4 separate occasions; 2 preceded by rest and 2 preceded by exercise. TG area under the curve (AUC) was calculated for each test and compared to the repeat condition using non-parametric Bland-Altman analysis. The 4-hour OFTT was repeatable in the rest condition, with 9 of 10 repeat measurements falling within ±15% of the median TG AUC (predefined as the upper limit of acceptable error). However, in the exercise condition repeatability was poor with only 2 of 11 repeat measurements falling within 15% of the median TG AUC. Adult offspring of type 2 diabetics (OT2D) show irregular TG responses to OFTT with high or low carbohydrate content, compared to healthy controls. Prior acute aerobic exercise may favourably influence these postprandial responses in OT2D. This feasibility study (Chapter 4) aimed to investigate the effects of carbohydrate content and acute exercise on TG AUC after OFTT in OT2D. On 4 separate days, 8 adult male OT2D ingested OFTTs with low (HFLC; 75g fat, 22g carbohydrate, 14g protein) or high (HFHC; 75g fat, 95g carbohydrate, 14 g protein) carbohydrate content. Participants rested or exercised (1-hour moderate intensity; 90% oxygen consumption at anaerobic threshold) the day before each OFTT. Recruitment to the single centre was slow, but participant adherence to the study was good. There were large effect sizes for lower TG AUC and incremental AUC (iAUC) in the HFHC with prior exercise. Insulin AUC was higher in HFHC conditions and there was a large effect size for lower insulin AUC in the exercise conditions. Given the large effect sizes observed for the effects of prior acute exercise on postprandial TGs, an adequately powered multi-centre study was deemed to be relevant and feasible. Consumption of strawberries appears to be beneficial in attenuating the postprandial lipaemic response to OFTT due to the high polyphenol content within strawberries. The mechanisms of this attenuation in postprandial lipaemia appear to be different from the mechanisms involved in exercise induced reductions in postprandial lipaemia. However, the combined effects of exercise and strawberry interventions in reducing postprandial lipaemic responses to OFTT has not been investigated. The final experimental chapter (Chapter 5) aimed to evaluate the combined effects of acute exercise and strawberry consumption on postprandial responses to OFTT (73g to 74g fat, 32g to 33g carbohydrate, 11g to 12g protein). On 4 separate days, ten overweight/obese males ingested OFTTs with 25g freeze dried strawberries or a placebo. Participants rested or exercised (40 minutes submaximal high intensity exercise, HIIE) the day before each OFTT. There was a 20% reduction in TG AUC in the exercise conditions and no differences in TG AUC in the strawberry conditions. This thesis offers key contributions to postprandial lipid metabolism research. First, the OFTT recommended by an expert panel statement is repeatable. Second, the variability observed in postprandial responses to OFTT with immediate prior exercise may explain the inconsistencies within the literature. Third, acute exercise showed a substantive effect in reducing TG AUC and iAUC with high carbohydrate OFTT in adult OT2D. These improvements could be explained by acute improvements in insulin sensitivity, however, a further adequately powered study is required to support the findings of this feasibility study. Finally, acute HIIE appears to be an effective strategy to reduce postprandial TG, but strawberry intake does not appear to improve postprandial TG.
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