Aerobic exercise has a marked impact on cardiovascular disease risk. Benefits include improved serum lipid profiles, blood pressure and inflammatory markers as well as reduced risk of stroke, acute coronary syndrome and overall cardiovascular mortality.13–19 Additionally, aerobic exercise is effective in the prevention and management of insulin resistance and T2DM.20,21
A recent meta-analysis looking at the effect of different levels of light or moderate physical activity on all cause mortality demonstrated that 30 minutes of moderate exercise five times per week (the basis of most exercise prescription guidelines22) reduced all cause mortality by 19% versus no activity.23 Importantly, as light or moderate aerobic exercise can be carried out in an incidental manner, it is potentially accessible and nondisruptive to most of the population.
High intensity interval training
Many exercise programs prescribed for fat reduction involve continuous, moderate aerobic exercise (CME), as per Australian Heart Foundation clinical guidelines.22 However, such exercise programs have been shown to fail to result in significant fat loss.13,24
High intensity interval training (HIIT) involves repeatedly exercising at a high intensity for 30 seconds to several minutes, separated by 1–5 minutes of recovery (either no or low intensity exercise).25 The most common HIIT intervention used in studies is the Wingate Protocol developed in the 1970s.26 This involves 30 seconds of cycling at maximum effort (at an intensity of over 90% of maximal oxygen uptake, also known as 90% of VO2 max) separated by 4 minutes of recovery, repeated 4–6 times per session, with three sessions per week.25,27 This results in only 2–3 minutes of exercise at maximum intensity and 15–25 minutes of low intensity exercise per session, making it a time efficient method of exercise. Less demanding protocols may be utilised for sedentary, overweight patients, which is important to remember considering the target patient population for exercise as prevention and management of cardiovascular and metabolic disease.
HIIT vs continuous moderate exercise
High intensity interval training has been shown to significantly reduce subcutaneous fat, especially abdominal fat,27 as well as total body mass,28,29 and to improve VO2 max (a marker of physical fitness)30 and insulin sensitivity.31 In comparison with CME, HIIT burns more calories and increases postexercise fat oxidation and energy expenditure more than steady-state exercise.32 Further, HIIT decreased total cholesterol and LDL-cholesterol, while increasing HDL-cholesterol33 and VO2 max32 more than CME. Interestingly, in a 2008 study, fat loss was significantly increased after HIIT, while fat loss did not change in CME patients versus controls,31 ie. there was no difference in fat loss between subjects carrying out CME and the inactive subjects. In a study that highlights the efficacy of HIIT, subjects carrying out HIIT demonstrated improvements in endothelial function, VO2 max, body mass index (BMI), body fat percentage, blood pressure and glucose regulation, more so than a group receiving dietary and psychological advice in addition to CME.29 Perhaps most importantly, increased exercise energy expenditure (such as with HIIT) as assessed by metabolic equivalents (METs) has been shown to result in a reduced risk of cardiovascular events in both males18 and females,17 and decrease all cause mortality.34 However, long term studies are needed to specifically assess the effect of HIIT on overall mortality.
HIIT effects in high risk populations
In patients with cardiovascular disease, HIIT was shown to be superior to CME in reducing blood pressure,35 improving endothelial function,35–37 lipid profiles,38 VO2 max,38 left ventricular37 and overall myocardial function,35 as well as reversing left ventricular remodelling in heart failure patients.37 Patients with metabolic syndrome who carry out HIIT have been demonstrated to have improved endothelial function, insulin signalling, blood glucose and lipogenesis.29
Studies carried out in T2DM patients demonstrated reduced blood glucose and increased mitochondrial capacity and GLUT4 expression after only 2 weeks of three 20 minute sessions of HIIT per week,39 and have been shown to significantly improve glucose tolerance at 6 months with no such changes in CME subjects.40
Importantly, HIIT programs are not only effective, but are also safe. HIIT has been used effectively in patients with diabetes,39 stable angina,41 heart failure37 and after myocardial infarct,38 as well as postcardiac stenting42 and coronary artery grafting.43
Further research is still required into the effect of HIIT versus CME in cohorts with cardiometabolic diseases, especially observation of long term outcomes. Similarly, elucidation of the efficacy of HIIT in certain patient populations is needed, such as in those who have recovered from a cerebrovascular event or in those suffering from peripheral arterial disease.
A common reason given for not exercising is time constraints,44 and long term adherence to exercise programs is often less than 50% at 6 months.45 HIIT allows equal or improved outcomes for markedly less time investment and has the potential to be associated with higher rates of adherence46 due to the varied protocol leading to less boredom,29,47 although this remains controversial.48 In one study, similar changes were seen over a 6 week period in both HIIT subjects and CME subjects, although HIIT subjects performed only 20% of the exercise duration performed by the CME group,49 making it an extremely efficient intervention.
Potential disadvantages of HIIT
Injuries are often a concern when beginning any exercise program (particularly one such as HIIT), especially in elderly and sedentary patients. While musculoskeletal injuries may occur, they are not more common in groups performing HIIT50 versus other forms of exercise and can be minimised with careful selection of exercise equipment, for example cycling instead of walking. A recent systematic review demonstrated no cardiac or other potentially lethal events across seven HIIT studies in patients with coronary artery disease,51 suggesting HIIT is very safe when performed in a controlled environment, although prescription of such exercise must be considered on an individual patient basis.
Due to the extreme energy expenditure required in the interval phases of HIIT, high levels of motivation are required. While effective in controlled trials, and perhaps associated with higher adherence levels (as discussed above), studies to assess long term adherence rates to HIIT are still needed.
Importantly, as some types of exercise are contraindicated in certain patient populations52 and because HIIT is a complex concept for those unfamiliar to exercise, some patients may require specific assessment or instruction in HIIT from an exercise physiologist or physiotherapist.
High intensity interval training has been shown to have numerous clinical benefits for both healthy and ‘at risk’ populations. General practitioners are encouraged to discuss with their patients the concept of ‘evidence based exercise’ and using HIIT as part of their exercise program.
Conflict of interest: none declared.