Cystic fibrosis (CF) results in thickened secretions in multiple organ systems including the lungs and gastrointestinal (GI) tract. Patients commonly suffer from nutritional deficiency, and achieving and maintaining adequate nutrition is an important goal of therapy because it is positively correlated with lung function. Lubiprostone activates chloride channels in the GI tract. Because its mechanism of action closely parallels the disease pathology, lubiprostone has the potential to provide GI benefits beyond the relief of constipation. This project is an observational study to examine the effects of lubiprostone on nutritional status and lung function in adults with CF. Our hypothesis is that lubiprostone will have beneficial effects on nutritional status.