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

* INCLUSION CRITERIA: In order to be eligible to participate in this study, an individual must meet all of the following criteria: * Adults 18-60 years of age with probable or definite EULAR ACR Myositis Classification Criteria 2017 for adult or juvenile dermatomyositis (DM, JDM). * Willingness to adhere to the general healthy diet pattern regimen, undergo dietary coaching on weekly to biweekly basis (10 sessions), and to complete online random reporting of dietary intake over a 6-month period. * Ability and willingness to comply with taking 4 study pills twice a day for 6 months. * Ability and willingness to wear ActiGraph device at home for 7 continuous days, twice in the study. * Willingness and ability to complete and consent to study testing, including blood, stool, and urine samples, and imaging studies. * Ability and willingness to complete a total of 5 study visits (screening, weeks -6, 0, 12, 24) onsite at NIH Clinical Center in Bethesda, Maryland. * Has the ability/transportation methods to attend on-site visits. Willing to pay for travel and out-of-pocket expenses. * Own or have reliable access to a computer, laptop or smart phone device (iPhone or Android) with internet access, and an active email address, to complete study consent form, online questionnaires, telehealth visits, and review online dietary education materials and videos. * Ambulatory * Must live within the United States. * Must be proficient in the English language and complete questionnaires in English (forms validated in English). Ability and willingness to complete forms online. * Moderately active DM/JDM defined by: * MD global VAS with a \>= 2.0 cm on a 10 cm scale and maximum value of 7.0 cm, and * At least 2 of the following criteria: * Patient Global Disease Activity Assessment \>= 2 cm out of 10 cm visual analog scale (VAS). * MMT-8 score of \<=138 out of 150. * Health Assessment Questionnaire disability index with a minimum value of \>= 0.50 out of 3.0 * Elevation of at least one of the muscle enzymes \[which includes creatine kinase (CK), aldolase, lactate dehydrogenase (LDH), ALT and AST\] at a minimum level of 1.3X the upper limit of normal. * Global Extramuscular disease activity score with a minimum value of \>= 1.0 cm on a 10 cm VAS scale (this measure is the physician's composite evaluation and is based on assessments of activity scores on the constitutional, cutaneous, skeletal, gastrointestinal, pulmonary and cardiac scales of the Myositis Disease Activity Assessment Tool (MDAAT). * Physician global damage and muscle damage both \<= 5.0 cm/10 cm VAS * If receiving prednisone and methotrexate, the dose must be stable for at least 4 weeks prior to the Week -6 visit, and daily prednisone \<= 20 mg/day. * Background therapy with other non-corticosteroid immunosuppressive agent, if required, must be at a stable dose for at least 6 weeks prior to the Week -6 visit, except with IVIG regimen should be stable 90 days prior to the Week -6 visit and for rituximab, stable regimen for 4 months prior to Week -6. * If an immunosuppressive agent was discontinued prior to the screening visit, then there must be a washout period before week -6 visit: * 4-week washout for prednisone, methotrexate, and IV methylprednisolone (IV pulse therapy) * 8-week washout for other immunosuppressive drugs, including azathioprine, mycophenolate mofetil, leflunomide, tacrolimus, cyclosporine, cyclophosphamide, colchicine, and hydroxychloroquine * 8-week washout for IVIG * For discontinuation of biologic or targeted drug therapies or dietary supplements, a washout prior to visit 1 (week -6) is required of 4 terminal half-lives. * Half-lives of most common biologics and targeted drug therapies used in the treatment of DM/JDM: * Etanercept Half-life 70 hours, Waiting period before enrollment (4 Half-lives) 12 days * Adalimumab, Half-life, 14 days, Waiting period before enrollment (4 Half-lives) 60 days * Rituximab, Half-life 32 days, Waiting period before enrollment (4 Half-lives) 130 days * Infliximab, Half-life 9 days, Waiting period before enrollment (4 Half-lives) 36 days * Abatacept, Half-life 17 days, Waiting period before enrollment (4 Half-lives) 68 days * Anakinra, Half-life 6 hours, Waiting period before enrollment (4 Half-lives) 1 day * Tofacitinib, Half-life 3 hours, Waiting period before enrollment (4 Half-lives) 1 day * Baricitinib, Half-life 12hours, Waiting period before enrollment (4 Half-lives) 2 days * Negative pregnancy test (urine or blood sample) if born female. * Body Mass Index (BMI) \> 18 and \<= 35 kg/m\^2 * Fish intake of less than 2 servings per week on average for the past 3 months. * Intake of meat products (beef, lamb, pork, venison, rabbit, cow's milk or dairy products) within 2 months of screening visit and of week 0 and have no reaction (no shortness of breath, hives, rash, or diarrhea) within 6 hours of ingestion of these meat products. EXCLUSION CRITERIA: An individual who meets any of the following criteria will be excluded from participation in this study: * Polymyositis; inclusion body myositis; cancer-associated myositis, defined as the diagnosis of myositis within 3 years of the diagnosis of cancer, except basal or squamous cell skin cancer or carcinoma in situ of the cervix if at least 5 years since excision. * Myositis in overlap with another autoimmune disease will be excluded under the following conditions: i. Myositis overlapping with another autoimmune disease that precludes accurate assessment of treatment response (e.g., difficulty assessing muscle strength in a patient with scleroderma and associated myositis). ii. Myositis overlapping with inflammatory bowel disease (IBD), including Crohn's disease, ulcerative colitis, or celiac disease, unless stable and well controlled with no changes in IBD-related medications for at least 3 months prior to enrollment, in which case the patient ma