A nutritional supplement that includes fish oil and selenium has been found to provide a variety of activities beneficial in treating cancer and associated conditions. Such a supplement provides synergistic effects in reducing cancer cell proliferation when used in combination with chemotherapeutic drugs, and can reduce proliferation in drug-resistant cancer cells when used in combination with chemotherapeutic drugs to which the cells are resistant. Effects in reducing angiogenesis, reducing metastasis, reducing the number of circulating cancer cells, and modifying AXL signaling were also found. In addition, use of such a supplement was found to reduce wasting associated with cachexia and reduce circulating cytokines associated with inflammation. Overall effects were found to extend survival in clinical studies.