Psoriatic arthritis causes swelling. So do certain foods, like fatty red meats, dairy, refined sugars, processed foods, and vegetables like potatoes, tomatoes, and eggplants. Avoid them and choose fish, like mackerel, tuna, and salmon, which have omega-3 fatty acids. Those have been shown to reduce inflammation. Carrots, sweet potatoes, spinach, kale, and blueberries are good choices, too.