HIIT and LISS are acronyms for High-Intensity Interval Training and Low-Intensity Steady State respectively. They are two commonly used cardio styles and differ greatly in both practical application and physical effects.
HIIT consists of short, all-out sprint intervals alternated with periods of low-intensity recovery periods. An example would be a 30 second 100% effort sprint followed by a 1-2 minute steady pace walk, repeated 10 times (think sweat, LOTS of sweat). LISS, on the other hand, would involve a constant low-to-moderate paced walk, jog or bike ride at an intensity level that still raises your heat rate but allows for casual conversation.
HIIT, like weight training, is a form of anaerobic exercise, meaning it burns glucose for energy without oxygen being present. During bouts of HIIT training, your body’s fuel burning systems are put under a lot of stress. In order to meet the energy demands you are putting your body under, your body responds in two ways, 1) by increasing the activity of fat-burning enzymes to help free up available energy and 2) by increasing the mitochondrial capacity in your muscles. Mitochondria are the energy-producing units of your body. HIIT thus signals to your body to increase the total mitochondrial density in your muscles, meaning you have more fat-burning units at your disposal. All of these metabolic adaptations have a cascading and synergistic effect, resulting in enhanced metabolic and fat loss activity over time.
What this means in real-life is that by doing HIIT your body is becoming more physically efficient and effective at turning on and utilizing its fat burning mechanisms and that this fat loss process is not only kept elevated for hours after your training has ended- termed the “afterburn” effect- but rather that these fat-burn changes are stable and lasting.
In contrast, due to the lower-intensity nature of LISS, this cardio strategy does not pose this same metabolic strain, thus limiting its fat burn potential to solely those minutes (or hours) you actively spend at the gym. Your body also has the tendency to adapt quickly to LISS exercise, meaning you will eventually burn fewer and fewer calories following the exact same cardio workout. Now don’t get me wrong, I’m not saying that you should ditch LISS cardio all together. LISS cardio has many heart-protective and general health-promoting effects that should not be ignored (I personally find it quite therapeutic going for long runs or walks every week). All I’m saying is that slaving away for hours every day on a treadmill or bike hoping to see the physical results you want is not the best approach to cardio if your goal is fat loss.
So who wins? Well if your goal is to lean down and get fit fast- HIIT is your best cardio investment strategy that will ensure you get more burn-for-your-buck in return.