No one has proven that any specific foods cause rheumatoid arthritis or make the symptoms worse.
Some people with rheumatoid arthritis claim to feel better if they avoid foods that come from the nightshade plants such as tomatoes, potatoes, peppers and eggplant. You can avoid these foods for a while to see if your symptoms improve, just be sure to replace them with other nutritious foods.
There are some foods that you should avoid because they aren't good for anybody. Stay away from foods that are high in saturated fat like red meat and foods that are high in sugar such as candy and pastries.
There are some foods that may help you feel better by reducing inflammation. Research shows that omega-3 fatty acids may help to reduce inflammation. Choose foods with healthy fats such as fish, walnuts, soy, canola oil, flax seeds and pumpkin seeds.
Eating fruits and vegetables may help to prevent rheumatoid arthritis. A study from the Mayo Clinic showed that women who ate diets higher in fruits and vegetables were less likely to develop rheumatoid arthritis than women who ate typical Western diets. This study doesn't show that eating fruits and vegetables will improve the symptoms of rheumatoid arthritis -- however, everyone should eat at least five servings of fruits and vegetables every day.
Calder PC. "n-3 polyunsaturated fatty acids, inflammation, and inflammatory diseases." Am J Clin Nutr. 2006 Jun;83(6 Suppl):1505S-1519S.
Cerhan JR, Saag KG, Merlino LA, Mikuls TR, Criswell LA. "Antioxidant micronutrients and risk of rheumatoid arthritis in a cohort of older women." Am J Epidemiol. 2003 Feb 15;157(4):345-54.