James Hewitt
700 words
2.5 minutes

Too much High Intensity Interval Training?

High-intensity interval training (HIIT) continues to grow in popularity as a fun, time-efficient way to improve fitness. It’s tempting to think that more intense training is better, but is this the case? How much is too much? A recent study sheds light on this question and also suggests a novel use for continuous glucose monitoring.

Lockdowns have led many of us to spend increasing amounts of time training indoors, but don’t take my word for it. Peloton’s stock chart looked like the profile of Alpe d’Huez (until some issues emerged with their treadmills). Health and safety considerations aside, I’m a big fan of training on indoor bikes and running machines. Workouts can be efficient and fun, particularly if you combine them with a service such as Zwift.  However, there is often a temptation to cram more intensity into a workout than you might in a typical outdoor session. Is this a problem?

Up to a point, HIIT can improve mitochondrial function

Exercise can improve the function of our mitochondria – the energy factories of our cells – increasing our energy capacity and enhancing glucose regulation. However, a recent study suggests that there might be an upper limit to the amount of high-intensity training that can be tolerated before things start to go wrong. The study, published in the journal Cell Metabolism, explored the relationship between training load, intensity and the function of mitochondria. The authors recruited both healthy participants and national level endurance athletes. An excessive training week, consisting of almost daily interval training sessions featuring close to all-out efforts, was associated with a disruption in both groups’ mitochondria’s function and glucose tolerance.

Is there a threshold for HIIT becoming harmful?

The study indicates that 90-minutes of high-intensity intervals per week was well tolerated and beneficial. This volume would be equivalent to three HIIT sessions featuring 6 x 5-minute intervals, for example. However, 152-minutes of high-intensity intervals per week, equal to five of these sessions, were associated with mitochondrial function and glucose regulation disruptions.

How to stay on the healthy side of HIIT

The authors note that, from a health perspective, they do not advise against high-intensity interval training. On the contrary, lots of good evidence indicates that moderate volumes of HIIT are associated with many positive health outcomes. However, the article highlights the importance of monitoring our response to training to ensure that the efforts lead to healthy adaptation rather than overload. This monitoring could be based on trends over time, such as running pace or average power. For example, if you’ve been training hard for some time and notice that you are struggling to maintain your usual speed or power, it may be an indication that you should reduce your intensity and training load.

Could continuous blood glucose monitoring be helpful?

Similarly, many people monitor their adaptation qualitatively, using perceived exertion. For example, ask yourself if sessions are feeling consistently harder than usual, even though pace or power is similar. This shift may also indicate that you need to back off. Finally, you could also use technology, such as by monitoring resting heart rate (sustained increases may mean that you are struggling to adapt), measuring changes in heart rate variability (sustained decreases may indicate that you are pushing too hard), or by using more novel technology such as continuous blood glucose monitoring (CGM).

Given the effect of excessive high-intensity training on blood glucose regulation, the authors of the Cell Metabolism paper suggest that CGM could be a helpful tool to find the ‘minimum effective dose’ of HIIT. Up to a point, HIIT should improve glucose regulation, or at the very least not disrupt it. However, suppose CGM began to indicate dysregulation, such as an impairment in glucose tolerance during a particularly high-volume training period. In that case, this may provide a helpful signal that it’s time to reduce training load and recover. I found this final suggestion particularly interesting. Several over-the-counter continuous blood glucose monitoring solutions target athletes, such as Supersapiens, https://www.supersapiens.com/, making this kind of data-informed decision-making accessible to more people. If you’re using a CGM to monitor your training and energy management, I’d be very interested to hear from you.


Flockhart M, Nilsson LC, Tais S, Ekblom B, Apró W, Larsen FJ. Excessive exercise training causes mitochondrial functional impairment and decreases glucose tolerance in healthy volunteers. Cell Metab. 2021;957–70.

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