Studies have shown that cigarette smoking can impair insulin action in persons with non–insulin-dependent (type 2) diabetes mellitus and in those who do not have diabetes. Other studies have demonstrated that persons who are not diabetic but are chronic cigarette smokers are insulin resistant and hyperinsulinemic when compared with nonsmokers. Cigarette smoking is believed to contribute to hyperglycemia in patients with type 2 diabetes and may possibly accelerate atherosclerosis. Targher and associates examined whether chronic cigarette smoking has an adverse effect on insulin sensitivity in patients with type 2 diabetes.
Forty adult patients of average weight with type 2 diabetes were enrolled in the two-part study. Twenty-eight study subjects were smokers, and 12 were nonsmokers. During the first part of the study, patients were given a 75-g oral glucose tolerance test in which plasma glucose, insulin and C-peptide levels were evaluated at baseline and 30, 60, 90, 120, 180 and 240 minutes after the glucose load for four hours. The second part of the study consisted of a four-hour euglycemic hyperinsulinemic clamp in which glucose was given as a prime-continuous infusion along with a 20 percent glucose solution to achieve a plasma glucose concentration of approximately 90 mg per dL (5 mmol per L). Blood samples were obtained after three hours of using the clamp and then every 10 minutes for the next hour.
Smokers had significantly higher plasma triglyceride and lower high-density lipoprotein (HDL) concentrations. Levels of plasma glucose did not significantly differ between the two groups, whereas levels of plasma insulin and C-peptide were significantly higher among the smokers. While rates of glucose metabolism did not significantly differ between the two groups, the insulin-mediated rate of total glucose disposal was much lower in the smokers.
Metabolic studies suggest that smoking acts by a pathogenetic mechanism to adversely affect insulin sensitivity both in healthy subjects and in smokers. The mildly higher fasting glucose levels in smokers might result in a higher rate of chronic diabetic complications. Patients who smoked also tended to have higher plasma insulin levels, higher plasma triglyceride levels, lower HDL cholesterol and higher systolic blood pressure, all of which are hallmarks of insulin resistance syndrome. Smoking appears to make the insulin resistance of these patients “blossom” with all the attributes of insulin resistance syndrome. As a result, their risk of cardiovascular disease may increase.
The authors conclude that chronic cigarette smoking decreases insulin sensitivity in patients with type 2 diabetes and appears to contribute to insulin resistance syndrome. Further study is needed because the weight gain associated with smoking cessation might also adversely affect insulin sensitivity.