Our gut is home to a myriad of microorganisms - bacteria, viruses, and fungi. These tiny creatures, which are found mainly on our skin and in our digestive system are mostly harmless to us and even important for our health. The microbiome is a term given to the genes of the microorganisms found on and in us. Much research has been done on these silent inhabitants in recent years, and it is becoming apparent that different medical conditions will result in a different composition of gut bacteria. Bowel diseases, diabetes, obesity, cardiovascular diseases, some types of cancer, and even anxiety and autism result in an altered microbiome composition. We have found significant changes in the composition and function of the gut microbiome in women with fibromyalgia.

The study will take place at the Alan Edwards Pain Management Unit, at the Montreal General Hospital. In this study we explore the efficacy of two dietary interventions for fibromyalgia as well as their effects on the gut microbiome.

Women 18 years or older who have fibromyalgia and who:

* are not pregnant;

* do not have any other significant medical conditions;

* did not take antibiotics, have changed their diet or were sick in the past month;

* do not follow a low-FODMAP or gluten-free diet

The gut microbiome is influenced by many factors (age, ethnicity) including gender so we only recruit women aged 18 or older in order to reduce the level of heterogeneity between participants.

You will be scheduled for three clinic visits:

• On the first visit you will be examined by a specialised pain physician. Then you will fill out questionnaires with the assistance of our research team. At home, you'll collect a small stool sample using an easy to use kit.
• On the second visit (a week after the first), you'll meet our clinical nutritionist for a thorough explanation of the diet that has been assigned to you. You'll give a blood sample and download a smartphone app that will help you adhere to the diet and report your dietary intake.
• You'll be asked to adhere to your assigned diet for 4 weeks. You'll be contacted regularly by the nutritionists who will help you follow the diet and you'll report your dietary intake.
• On the third visit (four weeks after the second), you'll collect a second stool sample, give a blood sample, and fill out questionnaires.

You may experience an improvement in the symptoms of fibromyalgia following the dietary intervention. If this happens, you will be able to continue following the diet after the study ends. If, following our analysis, we find that one diet is more efficient than the other, all participants who did not receive the more efficient diet will be invited to receive dietary counseling on it. This counseling will be offered by the study nutritionist team at no cost.

No serious risks are associated with this project. Dietary changes are sometimes associated with abdominal discomfort or changes in bowel habits. Giving a blood sample might be associated with mild discomfort.

The project takes place at the Montreal General Hospital.