There is growing evidence to suggest an association between type 2 diabetes and impaired pulmonary function. In this regard, several cross-sectional studies have appeared showing decreased indices of forced expiration, lung volume and diffusion capacity as the main lung dysfunctions detected in type 2 diabetic populations. In fact, diabetes is frequently co-morbid with chronic obstructive pulmonary disease, and data from the Atherosclerosis Risk in Communities Study showed a faster pulmonary function decline in type 2 diabetic patients than in other participants. This is important because the reduction of FEV1 has been demonstrated an independent cause of mortality in diabetic patients.
Interestingly, lung function measures start to decrease several years before the diagnosis of diabetes. In this regard an investigation found that insulin resistance is an independent determinant of pulmonary function in non-diabetic morbidly obese women. In addition, the results suggest that the metabolic pathways related to insulin resistance are crucial in initiating lung abnormalities in type 2 diabetic patients.
The reasons for the association between respiratory disease and diabetes are unclear. However, the relationship between type 2 diabetes and muscle strength, the impairment in lung elastic properties, and the presence of a low-grade chronic inflammation state are involved. In supporting these findings, thickening of the alveolar epithelia and pulmonary capillary basal lamina, fibrosis, centrilobular emphysema, and pulmonary microangiopathy have been detected in autopsies of diabetic patients. In addition, defects in the bronchiolar surfactant layer, which is involved in maintaining airway stability and diameter, may also be considered a contributing factor to the impairment of airway calibre regulation in diabetic patients. When the alveolocapillary barrier is damaged, surfactant proteins leak into the bloodstream. A recent population-based random sample study has described how increased circulating levels of surfactant protein A, the major surfactant-associated protein, were associated with altered glucose tolerance and insulin resistance. Therefore, surfactant defects in diabetic individuals may also lead to an increase in airway resistance and to a reduction in ventilatory patterns as observed in our studies. In addition, as experimental studies have shown that glucagon-like peptide 1 plays a role in the stimulation of surfactant production, its underlying deficit in type 2 diabetes could also enhance the airway resistance observed in these patients. However, the beneficial effects on pulmonary function using incretin-based therapies remain to be elucidated.
Clinical trial study hypothesis is that treatment with an incretin mimetic such as liraglutide may ameliorate lung function parameters in type 2 diabetics patients, independently of weight reduction. This hypothesis is based on the following factors:
1. \- There is growing evidence to suggest an association between type 2 diabetes and impaired pulmonary function.
2. \- In patients with type 2 diabetes, the incretin effect is severely reduced or absent, contributing to the reduced lung function parameters observed in type 2 diabetic patients.
3. \- GLP-1 stimulates surfactant production in "in vitro" studies and, in consequence, the increase in surfactant production induced by liraglutide could be the main factor involved in the respiratory improvement.
