Aerobic exercise training (AET) is recommended for lipid management. Several published government health authority guidelines prescribe minimum-intensity and -duration targets of physical activity intended to positively affect cardiovascular disease (CVD) risk biomarkers, such as the standard lipid profile comprising total cholesterol, triglycerides, high-density lipoprotein cholesterol, and low-density lipoprotein cholesterol. These guidelines may be of insufficient dosage to improve the standard lipid profile and lower CVD risk.

The aim of this thesis was to use quantitative methods ie systematic review with metaanalysis, to establish whether an optimal AET prescription for lipid management in adults exists. A literature review revealed that previous quantitative research estimating the effect size of AET on lipids had amalgamated heterogenous populations and AET protocols. This resulted in a large variation between estimated outcome measures as well as inconsistency of significance. The literature review also identified gaps where no research synthesis had been undertaken, such as analysis of the effects of AET on lipoproteins, apolipoproteins, and associated lipid ratios. To reduce the potential for confounding factors to under- or overestimate effect sizes, a rigorous synthesis and quantification using pre-determined and validated protocols was undertaken. The effect size of AET as an intervention to change lipids was estimated by pooling the outcome data of previously published randomised controlled trials. Intervention covariates, such as the intensity of AET effort, minutes per AET session, number of AET sessions per week, and duration of AET intervention, were investigated to determine if any of these explained the change in lipids. Both AET and population groups were differentiated: AET effort of intensity and duration of intervention were set at a required minimum for RCTs to be included for review, and RCTs were allocated to one of two reviews according to the health status of the population groups being studied.

Chapter 2 describes the protocol for a systematic review with univariate meta-analysis and meta-regression investigating the effects of AET on the standard lipid profile of adult populations free of chronic disease, and diagnosed either with or without Metabolic Syndrome. Chapter 3 describes the protocol for a systematic review with multivariate metaanalysis and meta-regression on novel lipid biomarkers in adult populations. Chapters 4-7 are the quantitative reviews investigating the impact of AET and intervention covariates on the standard lipid profile and novel lipid biomarkers. Chapter 8 presents the findings of this series of quantitative reviews.

The quantitative comparison of the aerobic exercise training protocols high-intensity interval training and moderate-intensity steady state training found neither protocol exerted more effect on total cholesterol, triglycerides, and low-density lipoprotein than the other, in heterogenous populations. High-density lipoprotein cholesterol was significantly raised by high-intensity interval training in comparison to moderate-intensity continuous training. Aerobic exercise training of a minimum intensity and duration similar to government recommended levels of physical activity significantly and positively impacted the standard lipid profile in adult populations free of chronic disease, resulting in a moderate reduction of CVD risk. In adult populations diagnosed with Metabolic Syndrome or Type 1 or 2 diabetes mellitus, the effect size of AET on the standard lipid profile was both significant and larger, as was the decrease in CVD risk, than that of adult populations free of Metabolic Syndrome or Type 1 or 2 diabetes mellitus, for similar AET protocols. Intervention covariates were not found to explain change in the latter population for any lipids, except the number of sessions per week explaining change in low-density lipoprotein cholesterol. However, intervention covariates potentially explained some of the change in triglycerides (intensity of AET effort), and some of the change in high- and low-density lipoprotein cholesterol (volume of total AET undertaken), of adult populations diagnosed with Metabolic Syndrome or Type 1 or 2 diabetes mellitus. Emerging lipid biomarkers such as lipoprotein fractions, apolipoproteins, and associated ratios were significantly and positively affected by AET, and intervention covariates explained some of these changes in antiatherogenic lipoproteins and apolipoproteins, as well as atherogenic lipid ratios, independent of population.

No optimal AET protocol was identified for populations free of chronic disease, although an increase in sessions per week may induce larger reductions in low-density lipoprotein cholesterol. However, this thesis has identified the aerobic exercise parameters which can be modified to induce greater effects on the standard lipid profile in populations affected by Metabolic Syndrome and diabetes. In addition, this thesis has identified aerobic exercise parameters which can be modified to induce greater changers in lipoprotein fractions, apolipoproteins, and associated ratios in heterogenous populations. These findings suggest future research is better equipped to discover tailored AET protocols which can better manage lipid profiles.

Objective To compare the effects of moderate intensity continuous training (MICT) and high intensity interval training (HIIT) on adult lipid profiles; to identify training or participant characteristics that may determine exercise-induced change in total cholesterol (TC), triglycerides (TRG), high-density lipoprotein cholesterol (HDL-C) and low-density lipoprotein cholesterol (LDL-C).

Design Systematic review and meta-analysis.

Data sources English language searches of several databases were conducted from inception until September 2019.

Eligibility criteria for excluding studies Inclusion: (1) published randomised controlled human trials with group population n≥5; (2) intervention duration ≥4 weeks; (3) comparing HIIT with MICT; and (4) reporting pre–post intervention lipid measurements. Exclusion: subjects with chronic disease, <18 years, pregnant/lactating, in elite athletic training; and studies with a dietary or pharmaceutical intervention component.

Results Twenty-nine data sets (mmol/L) of 823 participants were pooled and analysed. Neither HIIT nor MICT was better in decreasing TC (0.10 (−0.06 to 0.19), p=0.12, I²=0%), TRG (−0.05 (−0.11 to 0.01), p=0.10, I²=0%), LDL-C (0.05 (−0.06 to 0.17), p=0.37, I²=0%), or TC/HDL-C (−0.03 (−0.36 to 0.29), p=0.85, I²=0%). HIIT significantly raised HDL-C (0.07 (0.04 to 0.11), p<0.0001, I²=0%) compared with MICT.

Conclusion Neither HIIT nor MICT is superior for altering TC, TRG, or LDL-C, or TC-HDL-C ratio. Compared with MICT, HIIT appeared to significantly improve HDL-C. Clinicians may prescribe either protocol to encourage participation in exercise and reduce cardiovascular risk. To raise HDL-C, HIIT may result in a larger effect size compared with MICT.

PROSPERO registration number CRD42019136722.