Respiratory Muscle Strength and Physical Function in Japanese Patients with Type 2 Diabetes

Background: Type 2 diabetes mellitus (T2DM) is associated with impaired skeletal muscle mass and function; however, respiratory muscle involvement has not been fully investigated, particularly in Japanese patients. This study aimed to evaluate respiratory muscle strength and identify its associated factors in Japanese patients with T2DM. Methods: This cross-sectional study included 55 patients with T2DM without respiratory disease. Maximal inspiratory pressure (PImax) and maximal expiratory pressure (PEmax) were measured and expressed as percentages of the predicted values (PImax %pred and PEmax %pred). Clinical variables, including age, sex, body mass index (BMI), diabetic polyneuropathy (DPN), skeletal muscle index (SMI), handgrip strength, and 6-min walk distance (6MWD), were assessed. Correlation and multiple regression analyses were performed to identify factors associated with PImax %pred. Results: PImax %pred was significantly lower than the predicted value (69.5% [95% CI: 63.1–75.9], p < 0.001), whereas PEmax %pred did not differ significantly from the predicted value (95.6% [95% CI: 86.8–104.3]). PImax %pred was significantly associated with age, BMI, SMI, handgrip strength, and 6MWD. In multiple regression analysis, age, sex, BMI, and handgrip strength were independently associated with PImax %pred, whereas DPN was not. These findings remained unchanged after adjusting for smoking habits. Conclusions: Inspiratory muscle strength was selectively reduced in Japanese patients with T2DM and was independently associated with general physical characteristics and overall muscle strength. Inspiratory muscle weakness may reflect generalized skeletal muscle dysfunction rather than solely a neuropathic impairment. This assessment of respiratory muscle strength may provide additional insights into the physical function of patients with T2DM.