Pain and inflammation can be attributed to disease or injury. Arthritis causes pain and joint swelling in the joints, and can worsen as a result of obesity and stress. Although pain and inflammation can be reduced by taking anti-inflammatory medications, it can also be reduced by consuming certain foods. Foods that decrease pain and inflammation include salmon, sardines, herring, walnuts and soybeans. In addition, rainbow trout, oysters and ground flaxseeds help to decrease inflammation. These foods contain omega-3 fatty acids, which are categorized as polyunsaturated fats.