Author: Alison Ludzki

Editors: Zena Lapp, Alex Taylor, and Sarah Kearns

Gyms are riding out their busiest season, as patrons hang on to their New Year’s resolution exercise programs. But will it last? It seems inevitable that exercise participation wanes from January through December, except for maybe a blip prior to “beach body” season.  High-intensity interval training (HIIT) is one newer exercise option that could help your resolution stick.

HIIT is a new flavor of workout allowing you to get some of the health benefits of exercise in less time – if you work extra hard. This is especially appealing to people whose biggest challenge in meeting exercise guidelines is lack of time. The study of HIIT for its health impact (as opposed to for performance benefit) is relatively new, and there is research being done across the globe to understand its safety and efficacy in a range of clinical populations. Importantly, insight into how HIIT works will hopefully allow more people to have another safe and enjoyable option. More alternatives for exercise could help increase the number of Americans meeting the physical activity guidelines for health.

What is HIIT?

HIIT is any exercise session consisting of short bouts of high-intensity exercise interspersed with periods of recovery. HIIT is touted as an alternative to traditional endurance training that can result in similar improvements in fitness. Since becoming the new hot fitness movement several years ago, HIIT has received continued attention from researchers and the public. In fact, expert science and fitness journalist Alex Hutchinson recently suggested that HIIT has graduated from fad to trend. A key reason for this is increasing evidence that HIIT can yield similar effects to more commonly prescribed but more time-consuming endurance exercise programs. Current physical activity guidelines recommend 150 minutes of moderate-to-vigorous aerobic exercise per week. Given the increasing time pressures of our society, HIIT could be the means to address one of the most commonly cited barriers to regular exercise: a lack of time.

“The field has exploded with health-oriented HIIT research in the last 3-4 years,” Dr. Jonathan Little, a world-leader in HIIT research, said in an interview with me last year. “It is exciting to see so many research groups from around the world now employing various iterations of HIIT in so many different clinical populations in their work.” While low- and moderate-intensity continuous exercise have been commonly studied when it comes to health outcomes, high-intensity interval training was classically studied more in the context of sports performance. Now that it has become clear that some of the changes previously associated with endurance exercise also come with HIIT (e.g. better blood glucose control), there has been this surge in research assessing the health benefits of exercise in this “new” style. Unlike classic aerobic exercise, however, in order to reap the benefits of exercise in less time, you have to exercise very hard and endure discomfort. So a leading question remains: is this practical for sick or previously inactive people?

Practicality of HIIT

Original HIIT research studied “all-out” (maximal) exercise, commonly featuring 30-second maximal sprints with 4 minutes of recovery in between. However, many models of HIIT now employ more “practical” protocols – e.g. 1-minute intervals at 90% of the person’s maximal heart rate with 1 minute of recovery in between. Typical studies of HIIT bring participants into the lab to perform supervised bike or treadmill workouts. This allows researchers to easily ensure that participants achieve calculated target heart rates for the “hard intervals” (high-intensity bouts) by providing motivation and feedback throughout the exercise based on a heart rate monitor and prior exercise testing of those same subjects. This also allows experimenters to collect physiological data (e.g. blood samples, muscle samples, survey data) at precise times with respect to the exercise session, and it is through these data that the experimenters can properly determine the effects of HIIT.

There is now work showing that these practical HIIT protocols are well-tolerated among people with type 2 diabetes, older adults, and (female) cancer survivors. However, we have only scratched the surface of truly understanding the role of HIIT as a tool for the prevention and treatment of disease. For instance, there is ongoing debate over the practicality of HIIT as an exercise prescription for people with cardiovascular disease, and it is certainly not practical for everyone.

Aside from the question of safety, many of the existing HIIT studies examine only a few weeks and the longer-term health benefits of HIIT, and adherence to HIIT regimens, are not yet understood. Dr. Jenna Gillen, expert HIIT researcher working at the University of Toronto, Canada, summarizes:

“While there are numerous studies linking HIIT to improvements in cardiometabolic health, long-term randomized control trials are still needed to confirm these findings. Often research is conducted in a small group of subjects, over a relatively short-term training intervention (up to 12 weeks). We still lack well-controlled studies that assess the impact of HIIT over the longer-term, in both men and women. Equally important, more research is needed on long-term adherence to this type of exercise training. While we know HIIT elicits many favorable metabolic adaptations, there is little evidence on whether people will adhere to this type of high-intensity training on their own in a real-world setting.”

Ongoing Research

Various groups around the world are continuing to assess the practicality of HIIT, with questions centered on safety and adherence. Here at the University of Michigan, my research group – the Substrate Metabolism Lab – is conducting a study to compare the metabolic effects of 12 weeks of HIIT with traditional moderate-intensity exercise more comprehensively than ever before, in obese adults. The study is incorporating assessments of many tissues in the body (muscle, fat, liver, and blood). Dr. Jeffrey Horowitz, Principal Investigator of this study, explains:

“Although recent waves of research studies have provided good evidence about the beneficial health effects of HIIT, we are very excited about our “deep dive” to explore adaptations to HIIT at the cellular, tissue, and whole-body levels to better understand potential factors underlying HOW HIIT may improve metabolic health…A better understanding of the mechanisms underlying the health benefits of HIIT provides information needed to better optimize lifestyle programs for obese patients, and may also lead to development of pharmacological approaches to treat and/or prevent obesity-related diseases.”

Importantly, this study will include an extended follow-up period to identify whether people who participate in structured HIIT programs continue to exercise when left to their own devices; and what type(s) of exercise they choose to continue performing.

Our understanding of HIIT is limited by the study setup, as well as by study duration. Performing intervals on a bike or treadmill during a supervised exercise study is one thing, but when it comes to exercising in “real-world” conditions outside the confines of the laboratory, the reality is that most people don’t have easy or affordable access to this equipment. The ability to perform HIIT with more basic equipment (e.g. a flight of stairs at work) will be important to get at this question of accessibility. Researchers from McMaster University did just that, measuring aerobic fitness and blood glucose control before and after 6 weeks of HIIT: 3×20-second stair climbing, 3 days/week. The authors found that the stair-based HIIT increased aerobic fitness, but improvements in blood glucose control were unclear from the group of subjects studied.

It is important to understand how HIIT changes markers like fitness and blood glucose control in healthy adults; there is real untapped potential for HIIT to impact these and other health outcomes in people of all ages and health statuses. Having new and exciting exercise alternatives to help prevent and treat diseases like cardiovascular disease and diabetes may help increase adherence to exercise programs, but this cannot be widely recommended until the research catches up. For more information, the book The One Minute Workout book by Dr. Martin Gibala elaborates on the current status of HIIT research and provides unique insights on the process of doing much of this research, as the author is widely considered a pioneer of HIIT research.

Expanding the Exercise Menu

At this time, the most apparent benefit of HIIT is that it provides an alternative exercise option to traditional endurance training. Within reason, more exercise corresponds to better health, and for many people, the most important thing is to find a way to fit exercise into their lives. At this stage, we don’t yet have a complete understanding of the ways HIIT improves health, or of its safety and viability across populations. But overall, Dr. Little comments that “like most healthy lifestyle choices it is unlikely there is a ‘one size fits all approach’ – the best exercise, and the best HIIT, is the one that you enjoy doing and can incorporate into your life!” Dr. Gillen corroborates, noting that “it is important to remember that HIIT is providing one ‘menu option’ on a long list of physical activities that are beneficial for health.” In general, if there are more options on the exercise menu, the hope is that more people will be able to engage in regular exercise throughout the year and therefore be healthier. And so, research expanding our understanding of the practicality and safety of HIIT in different populations will be a key step towards its adoption by exercise experts and patients – which is an incredibly powerful step towards the prevention and treatment of disease.

For more information about HIIT, visit Cellucor’s blog.

About the Author

head shot-2Alison did her undergraduate in Kinesiology at McMaster University in Hamilton, Ontario. As a competitive runner, her academic interest focused on human performance. Through her Master’s studies in Guelph, Ontario, she became interested in clinical research while studying muscle metabolism in human and rodent models. In Guelph, she started the local Exercise is Medicine group, which seeks to connect researchers with health care providers to give them the tools to prescribe exercise for the treatment and prevention of disease. This inspired her to make knowledge translation a part of her career, and started her interest in science writing as a way to do that. Alison is currently a PhD student in the Substrate Metabolism Lab in the school of Kinesiology, where she studies the effects of diet and exercise on whole body and cellular health in humans. She is seeking a career in clinical research and knowledge translation.

Read all posts by Alison here.

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