Approximately 20% of adults have prediabetes in Germany. Prediabetes is defined as a condition with glucose levels outside the normal range but not yet meeting the criteria for type 2 diabetes. The pathogenesis of prediabetes, as well as of type 2 diabetes, involves whole-body insulin resistance associated with inadequate insulin secretion. These two central processes of glucose regulation are modulated by the brain. The brain communicates via the autonomic nervous system (ANS) with metabolically important organs in the periphery to modulate insulin sensitivity and insulin secretion. These processes are impaired in individuals with prediabetes and diabetes. An ANS sympathovagal imbalance has also been observed in individuals with prediabetes. There are no specific therapeutic approaches to improve ANS sympathovagal imbalance. It is assumed that resonant frequency breathing (RFB) maximizes heart rate variability (HRV) through rhythmization of breathing, heartbeat, and blood pressure. Through this state of coherence, the activity of the parasympathetic nervous system is upregulated, and the activity of the sympathetic nervous system is suppressed, leading to an increase in modulation of ANS activity. Several studies have demonstrated that heart rate variability-biofeedback (HRV-BF) interventions improve HRV, reduce stress and anxiety, and alleviate symptoms in patients with various medical conditions. To the best of current knowledge, no study has investigated the effect of HRV-BF-RFB on glucose metabolism. Therefore, the proposed randomized controlled non-blinded trial aims to gain evidence about the effect of HRV-BF-RFB compared to an anti-stress program on glucose metabolism in individuals with prediabetes. Glucose metabolism is characterized using the 75 g oral glucose tolerance test. There are two potential mechanisms by which HRV-BF-RFB may improve glucose metabolism in individuals with prediabetes: (a) a 0°-phase relationship between heart oscillations and breathing, maximizing the amplitude of respiratory sinus arrhythmia (RSA), and (b) activation of the cholinergic anti-inflammatory pathway. The investigators hypothesized that in individuals with prediabetes, the HRV-BF-RFB intervention will improve glucose metabolism and glucose variability.