CrossTalk proposal: High intensity interval training does have a role in risk reduction or treatment of disease

Traditionally the effect of exercise training and physical activity (PA) on risk classification and on the treatment of diseases has been studied at low to moderate exercise intensities (Fletcher et al. 2013; Levinger et al. 2015). However, in the last 15 years a growing body of evidence has suggested that exercise at high intensity may be equally effective, and in many cases outperform low to moderate intensities in terms of treatment effects and risk reduction (Swain & Franklin, 2006).

Epidemiological evidence of the importance of high intensity training in risk reduction

Epidemiological studies that compare the effect of physical activity of different intensities on all‐cause and disease‐specific mortalities demonstrate that the effect of moderate intensity is highly beneficial, but they also show that high‐intensity exercise may induce larger health benefits, relative to the time spent on physical activity (Gebel et al. 2015). For instance, a study from Taiwan of more than 400,000 individuals demonstrated that 15 min of vigorous PA daily gave similar all‐cause mortality risk reduction (∼25%) to 60 min of moderate intensity PA daily (Wen et al. 2011). In line with these results we found that a single weekly bout of physical activity at high intensity (∼90% of peak heart rate) gave similar or higher protection against premature all‐cause and cardiovascular mortality compared to several hours of moderate intensity exercise (Wisloff et al. 2006). These data are also supported by Lee et al. (2003), who found an inverse association between the relative intensity of physical activity and risk of coronary heart disease among men not satisfying current physical activity recommendations.

Peak oxygen uptake (${\dot{V}}_{{\mathrm{O}}_2\mathrm{peak}}$) is one of the strongest prognostic markers for future cardiovascular health and premature mortality (Myers et al. 2002; Keteyian et al. 2008; Nes et al. 2014). Interestingly, although physical activity levels are also associated with mortality, this association disappears after adjustment for ${\dot{V}}_{{\mathrm{O}}_2\mathrm{peak}}$ (Lee et al. 2011). Considering the strong association between ${\dot{V}}_{{\mathrm{O}}_2\mathrm{peak}}$ and health, it is important to know which type of exercise training is the most effective for improving fitness. There is strong epidemiological evidence that PA at high intensity gives substantially larger improvements in fitness. For instance, it is reported that Norwegian men with ∼49 min per week of very vigorous exercise (defined as > 90% of ${\dot{V}}_{{\mathrm{O}}_2\mathrm{peak}}$) achieved almost ∼2 metabolic equivalent of task (MET) (6 ml kg⁻¹ min⁻¹) higher ${\dot{V}}_{{\mathrm{O}}_2\mathrm{peak}}$ compared with those exercising at moderate intensities (∼70% of ${\dot{V}}_{{\mathrm{O}}_2\mathrm{peak}}$)(Nes et al. 2012). Even over 216 min per week of moderate intensity exercise gave ${\dot{V}}_{{\mathrm{O}}_2\mathrm{peak}}$ levels that were about ∼1 MET (3.5 ml kg⁻¹ min⁻¹) lower than that obtained performing very‐vigorous PA weekly for 49 min (Fig. 1). Similar findings were observed in women. From epidemiological studies there is now substantial evidence that high‐intensity PA has an equal or better effect in risk reduction for cardiovascular disease, ischaemic heart disease, stroke, some types of cancer and type 2 diabetes.

Figure 1. Weekly exercise time and peak oxygen uptake in ( ${\dot{V}}_{{\mathrm{O}}_2\mathrm{peak}}$ ) 2263 men performing physical activity regularly at different intensities .

Low intensity: Borg scale 6–11, ∼40–65% of ${\dot{V}}_{{\mathrm{O}}_2\mathrm{peak}}$; moderate intensity: Borg scale 12–13, ∼70% of ${\dot{V}}_{{\mathrm{O}}_2\mathrm{peak}}$; vigorous: Borg scale 14–15, 80–90% of ${\dot{V}}_{{\mathrm{O}}_2\mathrm{peak}}$; very vigorous: Borg scale 16–20, > 90% of ${\dot{V}}_{{\mathrm{O}}_2\mathrm{peak}}$. Figure modified from Nes et al. (2012).

Clinical and experimental data on high‐intensity interval training's role in treatment of disease

In a clinical setting, high‐intensity interval training (HIIT) is an effective way of performing high intensity exercise. The terminology used to describe high‐intensity training unfortunately varies across research groups. Here we use the definition suggested by us recently using HIIT when the intervals’ target intensity is between 80 and 100% of peak heart rate (Weston et al. 2014), whereas more sprint‐like training, using low‐volume supramaximal (i.e. all‐out) exercise is termed sprint interval training (SIT). Nearly all clinical studies investigating the effects of higher intensity exercise have used HIIT rather than SIT. These have consistently shown that exercise using HIIT compared to moderate intensity exercise is more effective on several important prognostic factors (${\dot{V}}_{{\mathrm{O}}_2\mathrm{peak}}$, ventricular function, endothelial function and quality of life) (Guiraud et al. 2012). HIIT has been found more beneficial in patients with heart failure (Wisloff et al. 2007; Freyssin et al. 2012; Guiraud et al. 2012; Fu et al. 2013; Haykowsky et al. 2013) or coronary artery disease (Rognmo et al. 2004; Moholdt et al. 2009; Guiraud et al. 2012), in individuals with the metabolic syndrome (Tjonna et al. 2008) or hypertension (Molmen‐Hansen et al. 2012), in obese individuals (Schjerve et al. 2008), and in patients with type 2 diabetes (Hollekim‐Strand et al. 2014). Also, numerous experimental animal studies support the clinical findings above, using HIIT protocols to improve health and reduce disease (Wisloff et al. 2009). Contrarily, a few experimental ‘SIT studies’ indicate detrimental effects on cardiovascular health (Holloway et al. 2015), and, in our view, more pre‐clinical trials in established animal models are warranted comparing HIIT, SIT and moderate exercise intensities on cardiovascular outcomes (including longevity) before larger randomized clinical trials should be initiated.

Application of HIIT in larger cohorts

Although the safety profile of HIIT has not been fully established, there are increasing data demonstrating that in stable and selected patients it can be performed safely with impressive improvements in physiology, functional capacity and quality of life. We recently provided the first evidence that the risk of a cardiovascular event is extremely low after both HIIT and moderate‐intensity exercise in a cardiovascular rehabilitation setting including more than 175,000 training hours (Rognmo et al. 2012). Also in a prospective study of over 12,000 US male physicians that were healthy at baseline, it was found that habitual vigorous exercise diminished the risk of sudden death during vigorous exertion (Albert et al. 2000). A recently published systematic review further supports the safety of HIIT (Levinger et al. 2015). The investigators included 11 studies with 156 patients and found the incidence of adverse responses during or within 24 h post‐exercise to be around 8%, ‘mild in nature’ and only ‘somewhat higher compared to the previously reported risk during moderate intensity continuous exercise’.

We are aware of the limitations of epidemiological studies, that they do not include purely interval based training, and the fact that they cannot establish cause–effect relationships. In addition clinical ‘HIIT studies’ are in general small and short‐term. For example, it has not been established in a randomized clinical trial that exercise training to increase ${\dot{V}}_{{\mathrm{O}}_2\mathrm{peak}}$ leads to more and/or healthier years. Such studies are needed and at least one has been initiated including more than 1500 elderly who exercise using either moderate or high intensity where the primary outcome is mortality (Stensvold et al. 2015).

In conclusion, we believe that for over 2 billion physically inactive adults worldwide (Hallal et al. 2012), HIIT may constitute an effective way of improving health and reducing mortality, particularly since the most commonly cited barrier to physical activity is lack of time. Due to the inverse dose–response relationship between vigorous activity and mortality, we argue that HIIT should play a central role in health activity guidelines to maximize the benefits of physical activity globally. It's a HIT!

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Additional information

Competing interests

None declared.

Author contributions

All authors have approved the final version of the manuscript and agree to be accountable for all aspects of the work. All persons designated as authors qualify for authorship, and all those who qualify for authorship are listed.

Biographies

Ulrik Wisløff is a Professor in Cardiovascular Physiology and Director of the K. G. Jebsen Centre for Exercise in Medicine at the Department of Circulation and Medical Imaging, Norwegian University of Science and Technology, Trondheim, Norway. Over the last years he has built up strong, international and transdisciplinary research groups (including physiologists, molecular biologists, sociologists, nurses, physiotherapists, medical doctors, bio‐engineers, nutritionists, bio‐statisticians, epidemiologists) with more than 50 staff members (∼55% women, 30% international) (ntnu.edu/cerg).

Øivind Rognmo is co‐director and scientist at the K. G. Jebsen Centre for Exercise in Medicine.

Jeff Coombes is a Professor in the School of Human Movement Studies at the University of Queensland, Brisbane, Australia. His research focuses on determining the optimal exercise prescription for improving health. His findings have emphasised the importance of cardiorespiratory fitness for health benefits and many of his current projects use high intensity interval training to improve fitness and investigate outcomes.