This is a single-center randomized controlled trial evaluating the efficacy and safety of short (3 days) versus long (10 days) courses of intermittent hypoxic-hyperoxic training (IHHT) in patients with type 2 diabetes mellitus aged 50-74 years. Participants will be randomized into three groups: 3-day IHHT course (n≥100), 10-day IHHT course (n≥100), or control group receiving gas mixture with constant O2 21% for 10 days (n≥50). The primary outcomes are fasting glucose levels and HbA1c. Secondary outcomes include cardiovascular parameters, quality of life, cognitive function, and biological age.