Subclinical intestinal inflammation and intestinal permeability have been reported in patients with ankylosing spondylitis. Gluten is one of the main triggers of zonulin releaseand gluten intake may contribute to the development of the disease by increasing intestinal permeability. This study aims to determine the impacts of a gluten-freediet on intestinal permeability, sub-clinical intestinal inflammation, disease activity, functional status and quality of life in patients with ankylosing spondylitis. The main questions it aims to answer are: Does a gluten-free diet have an effect on disease activity and functional status in patients with ankylosing spondylitis? Does a gluten-free diet have an effect on intestinal permeability and subclinical intestinal inflammation in patients with ankylosing spondylitis? The investigators will evaluate the effects of the gluten-free diet compared to a control group.