Mitochondrial function and whole-body maximal oxygen uptake – a surrogate measure of metabolic function and health – are closely linked. Exercise boosts mitochondrial function, but some evidence suggests there is an upper limit to exercise’s benefits. A new study demonstrates that the benefits of exercise on mitochondrial function peak, after which the benefits plateau or even drop.

The study involved 11 healthy adults enrolled in a four-week high-intensity interval training (HIIT) program. The exercise load and duration gradually increased over the first three weeks of training (scaling up to 154 minutes per week) and then decreased for the final week to allow recovery. Researchers measured the participants' glucose tolerance and collected muscle biopsies throughout the intervention. They also assessed continuous blood glucose profiles in world-class endurance athletes.

They found that after the week with the highest exercise load, the participants' mitochondrial function had declined markedly, impairing their glucose tolerance and insulin secretion. Interestingly, the impaired mitochondrial function coincided with the loss of cellular Nrf2, a protein involved in the body’s antioxidant defense response. In addition, they found that the endurance athletes had worse glucose control than a matched control group.

The findings from this small study suggest an upper limit to the mitochondrial benefits of HIIT exists, potentially impeding performance and harming health. However, it’s noteworthy that the participants tolerated 90 minutes of HIIT per week and didn’t experience impairments until reaching 152 minutes. One of the unique advantages of HIIT is that a shorter routine can have robust beneficial effects comparable to those of endurance training. Learn more about HIIT in this episode featuring Dr. Martin Gibala.