High-intensity interval training (HIIT) workouts have become popular in recent years for a number of reasons. They don’t require as much time as a regular workout (some can take as little as 10 minutes), and research shows they improve fitness, lower blood pressure and help people better manage their blood sugar levels – which may aid in weight loss and prevent disease, such as type 2 diabetes.
And recently, a review has found that a form of HIIT workout called low-volume HIIT has benefits on cardiometabolic health. That means low-volume HIIT could induce similar – or greater – improvements in cardiorespiratory fitness, blood sugar control, blood pressure and cardiac function compared to continuous aerobic exercise (such as a five-mile run).
HIIT is characterised by alternating between low- and high-intensity intervals of exercise. For example, this might include cycling at an easy pace for a few minutes before increasing effort to a high or even maximal level for a short period of time before returning to an easy pace. This is then repeated throughout the exercise session with the total time spent at high-intensity typically low. Different categories of HIIT exist depending on the intensity of exercise required.
The researchers of this study performed a topical review of current evidence on low-volume HIIT and its benefits for heart health. Topical reviews provide an up-to-date overview of the latest information in a particular field or area of research that’s developing rapidly.
They looked at a total of 11 studies. They defined low-volume HIIT as exercise in which the total time spent in active intervals (not including rest periods) was less than 15 minutes. Overall, they found that low-volume HIIT improved a person’s capacity to burn fuel (such as carbohydrate and fat), which is directly related to blood sugar control – and may be important in preventing diseases, such as type 2 diabetes. They also found that supervised HIIT in healthy people and people living with obesity and type 2 diabetes is safe.
Low-volume HIIT was also shown to improve the heart’s structure – such as chamber enlargement. This increases the volume of blood the heart can pump to the rest of the body each heartbeat. These benefits were true for people without underlying health conditions, as well as for those with heart failure (where the heart is unable to pump blood around the body properly because it has become too weak or stiff).
The fact that this review has shown low-volume HIIT also improves cardiorespiratory fitness is significant. Even moderate improvements to heart health have been shown to reduce adverse cardiovascular events such as heart attack and stroke by as much as 30%.
These results show that even a short workout can improve health. Current guidelines from the World Health Organization recommend adults perform 150-300 minutes of moderate-intensity exercise or 75-150 minutes of vigorous exercise a week. Yet, lack of time is often cited as the main barrier to exercise for many people. Low-volume HIIT has the potential to be more time-efficient while offering similar or greater improvements in health outcomes as longer workouts.
My own research suggests that low-volume exercise interventions can be used without feeling overly difficult or unpleasant, which is important for motivating people to continue an exercise regime. It may also be good for people who are inactive or have long-term health conditions.
How does HIIT work?
Regardless of the type of HIIT, it’s thought the health improvements are caused by the rate – rather than the amount – at which skeletal muscle glycogen (carbohydrates stored by the body for energy) is used. Muscle glycogen is an important fuel reserve – so our body tries to replenish it as a priority.
HIIT workouts deplete muscle glycogen at such a rate that the body increases the number and activity of mitochondria (powerhouses of cells) in our muscles to allow us to meet the energy demands of exercise. This in turn leads to improvements in fitness, metabolic function, and health.
There are some limitations of research into HIIT. Most studies have been conducted in laboratory settings. This makes it hard to know how effectively HIIT would actually work as an exercise strategy in the real world.
This review also has its own limitations. Typically, when analysing the results of a wide body of research, experts use a systematic review or meta-analysis. These are considered to be the highest level of evidence within research designs. They systematically assess the quality of studies and use methods which limit bias. This allows us to draw reliable and accurate conclusions. But topical reviews don’t do this – meaning this particular paper doesn’t give the most objective possible view on the effectiveness of low-volume HIIT.
Also, when considering the time included for a warm-up and cool-down, in addition to the time spent recovering between high-intensity intervals, not all HIIT workouts can be considered to be more time-efficient than traditional exercise. In this review, the average total time per workout was approximately 40 minutes – of which no more than 15 minutes were active.
But this isn’t to say that HIIT can’t be an alternative to longer workouts – especially considering a growing body of evidence shows it has a number of similar benefits as other types of workouts. Current thinking also suggests that every bit of movement counts. So focusing on quality (intensity) of exercise, rather than duration, and finding ways to incorporate higher intensity movement into everyday activities might be helpful in improving our health and fitness.
Matthew Haines, Head of Division of Sport, Exercise & Nutrition Sciences, University of Huddersfield
This article was first published on The Conversation