BackgroundDiabetes is a risk factor for the development of vascular disease, chronic kidney disease, retinopathy, and neuropathy. Diabetes is a co-morbid condition commonly present in patients with respiratory disorders but the extent to which it influences ventilatory capacity, gas exchange, and functional capacity is not well known.Research questionDoes the presence of diabetes contribute to impairment in spirometry, gas transfer, and exercise capacity?MethodsRetrospective analysis of all subjects who performed incremental cardio-pulmonary exercise testing (CPET) between 1988 and 2012 at McMaster University Medical Centre. The impact of diabetes on physiological outcomes and maximum power output (MPO) was assessed using stepwise multiple additive linear regression models including age, height, weight, sex, muscle strength, and previous myocardial infarct as co-variates, and was also stratified based on BMI categories.Results40,776 subjects were included in the analysis; 1938 (5%, 66% male) had diabetes. Diabetics were older (59 vs. 53 years), heavier (88.3 vs.78.0 kg), and had a higher BMI (31 vs. 27 kg/m2). The presence of diabetes was independently associated with a reduction in FEV1 (− 130 ml), FVC (− 220 ml), DLCO (− 1.52 ml/min/mmHg), and VA (− 340ml) but not KCO. Patients with diabetes achieved a lower % predicted MPO[diabetic subjects 70% predicted (670 kpm/min ± 95% CI 284) vs. 80% in non-diabetics (786 kpm/min ± 342), p < 0.001]. With the exception of KCO, these differences persisted across BMI categories and after adjusting for MI.ConclusionThe presence of diabetes is independently associated with weaker muscles, lower ventilatory and gas transfer capacity and translates to a lower exercise capacity. These differences are independent of age, height, weight, sex, and previous MI.