Introduction
An abnormally elevated or lowered blood lipid profile, known as dyslipidaemia, is a significant risk factor of cardiovascular disease (CVD)1 2; ischaemic stroke3; non-alcoholic fatty liver disease (NAFLD)4; and chronic pancreatitis.5 6 Dyslipidaemia frequently coexists with other Metabolic Syndrome (MetS) factors such as obesity (Ob)7 and type 2 diabetes (T2D)8 9; and MetS is implicated in CVD risk.10 Moderate-intensity and vigorous-intensity aerobic physical activity positively impacts MetS factors, thus lowering CVD risk.11 12 Studies13 14 and systematic reviews15 16 have shown aerobic exercise reduces elevated total cholesterol (TC), triglycerides (TRG) and low-density lipoprotein cholesterol (LDL-C) and increases high-density lipoprotein cholesterol (HDL-C) in subclinical and clinical populations.
Much published work has examined and confirmed the beneficial physiological effects of aerobic physical activity or moderate intensity (55%–70% of maximal heart rate (MHR), rate of perceived effort (RPE) of 11–13 on the Borg scale)17 continuous training, known as MICT. The WHO recommends a minimum of 150 min per week of aerobic physical activity at moderate continuous intensity, or 75 min at higher intensity, to maintain or achieve health. However, WHO reports insufficient aerobic physical activity levels among adults>18 years.18 Poor adherence to such recommended aerobic activity or MICT protocols results from lack of time,19 and lack of support.20 Although enjoyment of exercise is positively associated with incidence of physical activity in adults, absence of enjoyment has not been significant in explaining lack of exercise, and attitudes towards exercise lack positive association with incidence of aerobic physical activity.21 Such findings have prompted searches for alternatives to MICT in order to address continuing insufficient aerobic physical activity levels.
High intensity interval training (HIIT) is a protocol of short work intervals<60 s–8 min22 of vigorous (70%–90% MHR or RPE Borg scale 14–16)17 to high intensity (≥90% MHR or ≥RPE Borg scale 17)17 interspersed with active (40%–70% MHR or RPE Borg scale 8–13)17 or passive (cessation of movement) recovery periods of 1–5 min.22 HIIT has been employed since the mid-20th century to improve athletic exercise performance.22 Contemporary protocols developed for non-athletes are intended to reduce session time and provide a greater stimulus for physiological and psychological adaptation compared with MICT.
HIIT has been shown to increase peak oxygen consumption (VO2MAX or VO2PEAK) compared with MICT in cardiovascular disease (CVD) populations,23 despite VO2MAX being only one component of positive changes to cardiorespiratory fitness.24 Studies indicate that a positive impact on biomedical health indices is protocol dependent in clinical25 and healthy26 populations.
To encourage individuals to undertake aerobic physical activity, both HIIT27 and MICT28 are promoted as enjoyable and effective, although no consensus exists as to which aerobic exercise protocol is more so. Studies have shown a minimum volume of weekly aerobic exercise for a minimum duration29 and a weekly aerobic exercise energy expenditure (EEE) threshold of 1200–2200 kcal30 is necessary to induce positive changes to lipids. Systematic reviews and meta-analyses of the effect of aerobic physical activity on lipid levels have established that longer intervention and session duration results in greater effects.31 32
A systematic review comparing HIIT against MICT found no difference on blood lipids in healthy and clinical populations, but no meta-analysis was conducted.33 A pooled analysis comprising only three studies and consisting of CVD, MetS and overweight populations unsurprisingly showed equivocal effects on serum lipids.34 Other systematic reviews16 35 36 and meta-analyses15 37–40 have investigated the effect of exercise on lipids, but have not compared HIIT against MICT. Thus no previously published meta-analysis exists that has examined the effects of HIIT versus MICT on lipids in subclinical populations.
The aim of this study was therefore to conduct a systematic review and meta-analysis comparing the effects of HIIT and MICT on TC, TRG, HDL-C, LDL-C and TC/HDL-C in subclinical populations and to examine whether one protocol surpassed the other.