by Louis Degnan EP
A tale as old as time: High Intensity Interval Training vs Low Intensity Steady State. In one corner, HIIT provides a short metabolically demanding burst of caloric expenditure that trains VO2 Max over time. In the other, LISS optimizes fat oxidation through sustained almost stressless effort. But which form of aerobic exercise is the most beneficial? Is there a clear victor in this fight? Or is the answer more nuanced than a unanimous decision.
High Intensity Interval Training refers to performing short bouts of very high intensity exercise (north of 80% max HR) with low to moderate intensity periods or even complete rest. This structure allows one to train at these higher heart rates for an overall longer period of time as opposed to simply sprinting yourself into the ground. Common protocols include 30 second “sprints” paired with 1-2 minute recover times or even less as well as the 4×4 Norwegian protocol that has been studied at length in cardiology. Because the demand for energy during HIIT is, well, high; our bodies need to rapidly metabolize energy sources to meet that demand. The fastest way to do that physiologically is from our glycogen stores or more simply put: carbohydrates. You may burn plenty of calories with only 2-3 HIIT workouts per week, but most of those calories would be coming from your glycogen stores or whatever you had eaten prior to your workout. The tradeoff is that this type of training is the most efficient way to improve your VO2 Max, which itself has a positive linear relationship with longevity outcomes.
Low Intensity Steady State refers to continuous long bouts of low intensity exercise (60-70% of Max HR). Its commonly performed on low impact modes of cardio such as incline walking, swimming, recumbent biking, and the elliptical. When the demand for energy is low and the duration of the exercise is longer than 20-30 minutes, our bodies can afford to metabolize energy at a much slower rate. Because fat is a chemically more complex compound than carbohydrates and is more calorically dense per gram; our bodies like to pull from stored fat to fuel this type of activity. However, because the heart rate remains at a sustainable level, one cannot improve VO2 Max over time nearly as efficiently with LISS as they can with HIIT.
To oversimply: LISS burns more fat, takes less effort, but doesn’t improve VO2 Max much. HIIT burns less fat and requires much more effort but greatly improves VO2 Max. So who wins? Well, the answer is complicated. When controlled for total caloric expenditure, exercise science suggests that there is no discernible difference between the two in terms of propagating fat loss. Although fat is the primary substrate metabolized to fuel LISS, its much more efficient for HIIT to create a larger caloric deficit; the main driver of fat loss. Therefore, similar fat loss can be achieved with less bouts of HIIT than more bouts of LISS and improve your aerobic capacity while doing so.
So HIIT wins?! Not so fast. The research comparing the two have controlled for numerous confounding variables and in doing so created conditions that are not realistic. HIIT is taxing on our nervous, muscular, and cardiovascular systems which requires longer recovery between bouts to maintain efficiency. Also, the massive caloric expenditure yielded during HIIT can cause spikes in hunger from its inherent glycogen depleting demand; making overeating an easy mistake for those wishing to trim bodyfat. LISS, while taking more time to see benefits from, requires little to no daily recovery between bouts and does not significantly increase appetite. Therefore, it is more practical to perform a higher training frequency and therefore higher volume of LISS than it is for HIIT.
In the end, the winner in the HIIT vs. LISS debate should be dependent on your goals: If you wish to improve your VO2 max and like training at a higher intensity, HIIT is the way to go. If you’re satisfied with your aerobic capacity and want to focus on improving body composition: LISS is your best option. Incorporating both forms of aerobic exercise have their own benefits to our longevity. Determining how much and what parameters of which is best for your longevity, be sure to ask our Exercise Physiologists during your next comprehensive exam at Princeton Longevity Center!