It was all the rage a few years ago: high intensity interval training, aka, HIIT. It seemed like it was mostly young fit people trying to destroy themselves - intense exercise, lots of sweat, and promises of big results in less time. It seemed like just another fitness fad. But as it turns out, there was some pretty good science behind HIIT, and it’s actually pretty fun.
What exactly is HIIT? HIIT is short bursts of high intensity exercise followed by rest. It was developed using running, biking and rowing because those activities allow us to go as hard and as fast as we can and elevate our heart rate without experiencing muscular fatigue like we do when we squat or swing a kettlebell.
Here’s how it should feel: you should be pushing yourself as hard as you can sustain in the time allotted - focused, intent, and in the moment. Not necessarily a 100% effort but depending on the length of the interval it might mean an 85%-95% effort. At the end of each interval, you should be out of breath – talking should be difficult. It should feel a little bit terrible. It’s uncomfortable and painful maybe a little bit stress inducing, but it feels so good when you’re done.
Most HIIT sessions should consist of somewhere between 20 and 30 minutes of work. The key here is that HIIT training accomplishes more in a shorter amount of time than moderate intensity exercise.
And it accomplishes a lot! It helps reduce blood pressure, increase insulin sensitivity, increase brain derived neurotrophic factor (BDNF), a protein involved with memory, learning, and brain health. HIIT can also help improve markers – waist circumference, blood pressure, triglycerides and HDL - associated with metabolic syndrome. It induces EPOC or excess post-exercise oxygen consumption which means that your body continues to expend energy as it recovers. One study has shown HIIT to be more effective in a shorter amount of time for burning fat and reducing belly fat in postmenopausal women than moderate intensity cardio.
HIIT is also extremely effective at increasing cardiovascular fitness and increasing VO2 max. VO2 max is the maximum amount of oxygen your body can take in, transport, and utilize during exercise – it reflects the combined work of your heart, lungs, and muscles – it is a measure of fitness. VO2 max can be used to identify those at risk for heart failure, coronary artery disease, and other heart conditions and is inversely correlated with lower mortality in people with cancer. There is also a strong correlation between a high VO2 max and longevity.
Which must mean that doing HIIT will increase your lifespan, right? We know it improves VO2 max and cardiovascular fitness is strongly associated with longevity, and while that doesn’t necessarily prove that HIIT will help you live longer, one can hope.
Just to be clear: all forms of cardiovascular exercise are beneficial. If you like to walk or run around the lakes, great. If you like to get on the rowing machine in your basement and listen to a podcast, great. It’s all good for you.
But if you haven't tried HIIT, try it! Like I said at the beginning - it's fun, and you will feel amazing when you're done.
Sources
Mandsager K, Harb S, Cremer P, Phelan D, Nissen SE, Jaber W. Association of Cardiorespiratory Fitness with Long-term Mortality Among Adults Undergoing Exercise Treadmill Testing. JAMA Netw Open. 2018;1(6):e183605. doi:10.1001/jamanetworkopen.2018.3605
Abdalla HM, Dreher L, VanDolah H, Bacon A, El-Nayir M, Abdelnabi M, Ibrahim R, Pham HN, Bcharah G, Pathangey G, Wheatley-Guy C, Reddy S, Farina J, Ayoub C, Arsanjani R. VO2 Max in Clinical Cardiology: Clinical Applications, Evidence Gaps, and Future Directions. Curr Cardiol Rep. 2025 Sep 6;27(1):130. doi: 10.1007/s11886-025-02289-6. PMID: 40913731.).
Schmid, D and M.F. Leitzmann. Cardiorespiratory fitness as predictor of cancer mortality: a systematic review and meta-analysis. Annals of Oncology 26:2 Feb 2015, 272-278.)