Highly processed foods such as frozen meals, lunchmeat, baked goods, fast food, and packaged snacks usually have refined grains and added sugars, salt, and fat to help them last on store shelves and maintain their flavor. These can all increase inflammation.
Eating a highly processed diet is associated with increased obesity and insulin resistance, which could indirectly worsen arthritis symptoms.
Gluten is a protein in grains like wheat, barley, and rye. Some research has found that a gluten-free diet can benefit rheumatoid arthritis patients by reducing inflammation.
Studies have also found an association between people who have celiac disease and rheumatoid arthritis. Celiac disease is an autoimmune disease in which gluten triggers inflammation in the gut and elsewhere in the body.
Not everyone is sensitive to gluten, though. You might try avoiding it to see if your arthritis symptoms improve.