High-intensity intervals left anxiety and low mood unchanged in healthy adults

Exercise is routinely sold as a mental health intervention, and high-intensity interval training has picked up that claim along with its cardiovascular reputation. The appeal is obvious: short sessions, big physiological response, and a plausible story about mood following fitness. But most of the mental health evidence for exercise comes from moderate continuous activity or from clinical groups being treated for depression. Whether brief bursts of very hard work do anything for the anxiety and low mood of people who are not unwell had not been settled, which is the gap this review went after.

The reviewers searched PubMed, Scopus, Embase and PsycINFO for randomised clinical trials only, restricting the population to healthy individuals. Eight trials qualified, together evaluating 471 participants, 81% of whom were female. Results were pooled using a random-effects model based on standardised mean differences, comparing HIIT with passive control groups that did not exercise. Risk of bias in each trial was assessed with the RoB 2.0 tool, and the overall confidence in the pooled findings was graded using the GRADE approach.

HIIT showed no significant effect on anxiety symptoms compared with passive controls, and no significant effect on depressive symptoms either. Both pooled estimates pointed slightly in the direction of improvement, but the ranges around them comfortably included no difference at all, so neither result can be distinguished from chance. The reviewers graded the certainty of the evidence as very low, driven by the small number of eligible trials and their overall quality. Their conclusion is not that intervals are bad for mood but that the evidence does not currently support HIIT as a strategy for reducing either outcome.

This sits awkwardly against the broader literature in which physical activity is generally associated with better mental health, and it does so by narrowing the question. A healthy population has less room to improve on depression and anxiety scales than a clinical one, which makes a null easier to produce and harder to interpret. With eight trials and a female-dominated sample, the review is also honest that it is underpowered for subgroups such as men, older adults or people with high baseline symptoms. The open questions are whether longer interventions, different intensities or comparisons against active rather than passive controls would look any different.

The reasonable reading is that intervals remain a fitness tool whose mood benefits are unproven in people who are already well, not that they are useless. If you enjoy them, that enjoyment is its own reason to continue, and this review gives no cause to drop them. If you took up intervals specifically expecting your anxiety or mood to shift, the pooled trials do not support that expectation at present. And because everyone studied here was healthy, none of it speaks to how exercise is used alongside treatment for diagnosed conditions.