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.

* 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 criteria 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 activity \>= 2 cm out of 10 cm visual analog scale (VAS). * MMT-8 score 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 \ 2.0mg/dl or eGFR less than 50 mL/min per 1.73 m\^2. * Subjects with coagulation or bleeding disorders (such as hemophilia) or receiving anti-platelet or anti-coagulant medications, including daily aspirin, warfarin, or Plavix. * Life-threatening non-myositis illness that would interfere with the patient s ability to complete the study. * Known contraindications to O3FAs, excipients or placebo contents (e.g., allergy or known hypersensitivity to that drug or its excipients, including porcine gelatin, allergies to fish or shellfish, tocopherols, glycerin, or corn). Religious or ethical reasons to not consume fish, corn and/or porcine (pork) products. * Participants with any of the following: * Idiopathic anaphylaxis * Alpha-gal reaction * Known food allergies to beef, pork, lamb or other meat products, including cow s milk and dairy products, with a history of shortness of breath, rash, or hives within 6 hours of eating these foods. * Currently using O3FAs or consuming EPA/DHA in any form for the past 6 months. * Currently taking supplements or medications that affect lipoproteins for the past 6 months, including fish oil supplements, bile-acid sequestrants, plant sterol supplements, PCSK9 inhibitors, fibrates, statins, or niacin. * Use of medications or dietary supplements that interact with O3FA per pharmacy evaluation. A PharmD will evaluate the patient's current medication list for medications/supplements with the potential for significant interactions with O3FA. * No antibiotic usage in past 3 months, as well as no usage of anti-virals, antifungals, anti-parasitics in past 3 months (except antimalarials and Paxlovid or other COVID-19 anti-viral therapy allowed). * Subjects being treated with tamoxifen, estrogens or progestins that have not been stable for \> 4 weeks. * Uncontrolled diabetes with HgbA1C \> 8 or hospitalization in past 6 months for diabetes. * Uncontrolled hyperlipidemia with TC \> 400 mg/dL, TG \>150mg/dL. * Currently on a weight-loss program * Has experienced a weight change (gain or loss) of greater than 15 pounds or greater than 20 percent in the past 3 months * Currently taking a GLP-1 receptor agonist medication * No restrictive dietary habits per discretion of the study team. * Current use of medications or dietary supplements for weight or appetite control, including laxatives or diarrheal inhibitors within the past 4 weeks. * History of eating disorder. * Initiation of an exercise program within 4 weeks of screening visit. * Known or suspected history of drug or alcohol abuse within the past 6 months as determined by the medical record or patient interview. * Blood donation in the last 6 weeks or planned blood donation during study or requiring regular blood transfusion. * Pregnant females or nursing mothers within past 3 months, or those planning to get pregnant during the next 9 months. * Low total WBC \< 2000, platelets \< 100,000/mm\^3; hemoglobin \< 9.5 gm/dl. * Vitamin D level \< 20 ng/ml (at screening visit - necessitates addition of supplement and re-screen after minimum of 8 weeks). * Subjects with TSH levels greater than 1.5X upper limit of normal or clinical evidence of hypothyroidism (at screening visit- necessitates addition of supplement and re-screen after minimum of 8 weeks). * Participants with severe claustrophobia. * History of or anticipated poor non-cooperation with study requirements. * Participation in another clinical experimental therapeutic study within 30 days of screening visit or during the study. * Hospitalization within past 30 days (other than for routine infusions). * Prisoners or subjects who are involuntarily incarcerated. * Resident of a nursing home, ward of the state, or institutionalized during any part of the study period. * Persons with decisional incapacity/cognitive impairment. * Any history or evidence of severe illness or any other condition that would make the patient, in the opinion of the investigator, unsuitable for the study. * Participants who do not complete the ASA24 within 4 calendar days of screening will be excluded from the protocol. Additionally, participants will be excluded if their energy intake from the ASA24 is above or below established cut-off values for age and gender based on the 5th and 95th percentile of energy intakes from National Health and Nutrition Examination Survey (NHANES) data. Cut-off values for exclusion are \4400 kcal for women and \5700 kcal for men.