Exercise wins back heart rate variability lost to type 2 diabetes

Type 2 diabetes commonly damages the nerves that regulate the heart, a complication called cardiac autonomic neuropathy. One way to measure that regulation is heart rate variability, the natural beat-to-beat variation in heart rhythm, which is known to be reduced in type 2 diabetes. Exercise can improve heart rate variability in healthy people, but whether it does the same in people with type 2 diabetes has been under debate. This review set out to pool all the available evidence and answer that question.

The authors searched PubMed, Cochrane, Embase and ScienceDirect up to 20 September 2020 for studies reporting heart rate variability in people with type 2 diabetes before and after exercise training. Twenty-one studies were included, 9 of them randomised, covering 523 patients in total: 472 who trained and 151 untrained controls. Training was endurance in 14 studies, resistance in 2, combined endurance and resistance in 4, and high intensity interval training in 4, with a wide set of variability parameters pooled and analysed by exercise type.

After exercise training, every heart rate variability parameter improved, spanning both the time-based measures that track beat-to-beat variation and the frequency-based measures of autonomic balance, while the untrained controls showed no change. When split by exercise type, endurance training improved most of the parameters, whereas resistance training and high intensity intervals mostly shifted the balance between the two branches of the autonomic system. Supervised training improved most parameters, and neither the duration nor the frequency of training appeared to influence the size of the benefit.

This pooled analysis moves a debated question towards a clearer answer: exercise appears to restore some autonomic regulation of the heart in type 2 diabetes, not just in healthy populations. The evidence is strongest for endurance training, with far fewer studies of resistance work and interval training available for comparison. Because fewer than half the included studies were randomised, some of the improvement could reflect differences in who trained, and the review cannot say how much a better variability reading translates into fewer diabetic complications.

For anyone managing type 2 diabetes, this is a reason to think of steady endurance exercise as training for the nervous system's control of the heart, alongside its better-known effects. The finding that supervised programmes seemed most beneficial suggests structure and accountability may matter as much as effort. And since neither duration nor frequency of training obviously changed the benefit, the evidence gives no reason to believe that only long or very frequent sessions count.