FAST for DM - Fatty Acid Supplementation Trial (FAST) for Dermatomyositis (DM)

Dermatomyositis (DM) is a rare autoimmune disease that causes muscle weakness, skin rashes, and other symptoms. Researchers think both genetic and environmental factors play a role in this disease. They want to find out more about how diet and lifestyle choices affect people with DM/JDM. Objective: To see if omega-3 fatty acid supplements from fish oil, combined with a healthy diet, can help people with DM/JDM. Eligibility: Adults 18-60 years old, who live in the United States, can read English, and access Internet to complete questionnaires can participate. Design: Participants will have 5 or 6 inpatient visits. For 5 visits they may need to stay in the Clinical Center for up to 5 days. Participants will be screened. They will have a physical exam with blood, urine and stool tests. They will have tests of their heart and lung function. Their muscle strength will be measured. They may have an imaging scan of their thighs and pelvis. They will complete online questionnaires about their health and lifestyle. They may complete two optional skin biopsies. Participants will take 4 small capsules by mouth twice a day for up to 6 months. The capsules will contain omega-3 fatty acids from fish oil or a placebo. The placebo looks just like the regular capsule but contains no active ingredients. Participants will not know which capsules they are taking. They will follow a healthy diet based on the General Healthy Eating Pattern. Participants will receive dietary coaching and will have virtual check-ins throughout the study. For two 7-day periods, they will wear a watch-like device to track their daily activity and sleep patterns. Participants may opt to remain in the study for an additional 12 weeks. All will receive the fish oil supplements during this stage.