Effects of smoking on renal function in patients with type 1 and type 2 diabetes mellitus

Background. Smoking increases the risk of end-stage renal failure in patients with primary renal disease. Whether and to what extent smoking affects the kidneys in diabetic patients with normal renal function and variable degrees of proteinuria has not been fully studied. Methods. We followed 185 patients with type 1 or 2 diabetes mellitus and with or without signs of overt renal disease for at least 3 years, median 5.1 (3-6.8) years. Each patient had a baseline visit and at least four follow-up visits (average 4.8 +/- 0.3). Cases were patients who were smoking (n = 44) at the time the survey was started. Controls were patients who had never smoked (n = 141). Glomerular filtration rate (GFR) was estimated using the MDRD formula. Multiple logistic regression was used to correct for confounding factors. Results. At baseline, smokers were younger (47 +/- 14 vs 54 +/- 16 years, P < 0.01), and had a lower GFR (95 +/- 26 ml/min) than non-smokers (107 +/- 33 ml/min, P < 0.05). Mean GFR remained constant during follow-up in non-smokers (106 +/- 31 ml/min), but decreased significantly in smokers (83 +/- 22 ml/min, P < 0.0001), and this relationship persisted when adjusted for retinopathy, glycaemic control, age, body habitus, ACE-inhibitor treatment, blood pressure control or severity of proteinuria. The effect of smoking on GFR decline was stronger in patients with type 1 diabetes or male gender. Conclusions. Cigarette smoking causes a decrease in GFR in diabetic patients with normal or near-normal renal function, independent of confounding factors including severity of proteinuria. The latter finding suggests a mechanism independent of glomerular damage.