Rod Tucker discusses the evidence behind high intensity interval training, otherwise known as HIIT, and its potential benefits.

According to NHS advice, exercise is a miracle cure that is able to reduce the risk of major illnesses such as heart disease, stroke, type 2 diabetes and even cancer. In order that we all benefit from exercising, UK government guidelines recommend that adults (aged 19 to 64) should undertake at least 150 minutes of moderate aerobic activities, such as cycling or brisk walking, every week, combined with strength training on two or more days per week.

Cardiorespiratory fitness - or simply, fitness - has been shown to be related to all-cause mortality. For example, in a recent study, researchers used a treadmill to measure the fitness levels of over 122,000 people with an average age of 53 years and followed them for 8.5 years. Amazingly, they observed that those with the highest level of fitness had an 80% reduced risk of death compared to the least fit individuals. In addition, the study found that being unfit increased the risk of death by more than having cardiovascular disease, diabetes or smoking.

Sadly, this message is still not filtering through to the general public. A report by the British Heart Foundation suggested that physical inactivity and sedentary lifestyles cost the NHS as much as £1.2 billion a year, and that around 39% of adults (about 20 million people) fail to meet the Government physical activity targets. There are a range of possible reasons why people don’t exercise, but one which is consistently reported among those who are not sufficiently active is lack of time.

Evidence for high intensity exercise

In recent years, there has been increased interest in high intensity interval training or HIIT. This technique involves intermittent periods of intense exercise, i.e. going flat out, separated by periods of recovery. HIIT has been the subject of numerous research studies but, perhaps most importantly, there is evidence that it gives rise to the same or even better improvements in fitness as produced by longer bouts of traditional exercise such as running or cycling.

Furthermore, although many of the HIIT studies have been short-term and undertaken in laboratories using expensive exercise equipment, it does seem that the results are transferable to the real world. In fact, you don’t even need to go to the gym to recreate HIIT training. In a 2017 study, 31 sedentary women completed three 20-second ‘all-out’ stair climbing with two minutes of recovery, for six weeks and saw a 12% improvement in fitness.

It is now established that physical activity reduces mortality, but is this also true for HIIT? There have been no studies assessing the impact on mortality and so it was interesting to see a new article describing the results of the Generation 100 study. This was the first, randomised controlled trial study, which began in 2012, designed explore the impact of exercise on health and mortality in people over the age of 70. The study ran for five years and the results have been published in the BMJ.

In the Norwegian study, participants were randomised to four different exercise regimes: the national guidelines for physical activity, which served as the control group (i.e., 30 minutes of moderate level activity every day); moderate intensity continuous exercise training (MICT); HIIT training and lastly a combination of MICT with two HIIT sessions per week. The MICT session involved 50 minutes of exercise at about 70% of individual’s maximum heart rate whereas the HIIT session involved what is known as 4 x 4 training and involves one to two minutes at 90% maximum heart rate with lower intensity breaks in-between. The study recruited 1,567 individuals with an average age of 72 and the primary outcome was the death or mortality rate in each group.

After five years, mortality rates were 4.7% (control group); 4.5% in the combined MICT and HIIT group; 5.9% in the MICT group and 3% in the HIIT group although these differences were not statistically significantly different. However, the authors reported a non-significant absolute reduction in risk of 1.7% between the HIIT and control group as well as a 2.9% absolute risk reduction between the HIIT and the MICT group. They also noted that the data suggest that HIIT lowers the risk of premature death compared with both MICT and recommended guidelines. The study also reported on the mental and physical health benefits, which found that these were highest for those assigned to HIIT.

Although HIIT is hard going, there is evidence that it benefits those with coronary artery disease, type 2 diabetes, heart failure and older people in general.

In conclusion, the Generation 100 study is the first to strongly suggests that HIIT training has a mortality benefit which may be greater than that derived continuous exercise. Given that a major barrier to exercise is the perceived lack of time, HIIT seems to offer a workable and ‘life-enhancing’ solution that is associated with improvements in physical and mental wellbeing. With all these benefits, perhaps it is now time that we all HIIT the gym.