Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Cancer-cachexia state frequently induces both fat and protein wasting, leading to death. In this way, the knowledge of the mechanism of drugs and their side effects can be a new feature to treat and to have success, contributing to a better life quality for these patients. Metformin is an oral drug used in type 2 diabetes mellitus, showing inhibitory effect on proliferation in some neoplastic cells. For this reason, we evaluated its modulatory effect on Walker-256 tumour evolution and also on protein metabolism in gastrocnemius muscle and body composition. Methods: Wistar rats received or not tumour implant and metformin treatment and were distributed into four groups, as followed: control (C), Walker 256 tumour-bearing (W), metformin-treated (M) and tumour-bearing treated with metformin (WM). Animals were weighed three times a week, and after cachexia state has been detected, the rats were euthanised and muscle and tumour excised and analysed by biochemical and molecular assays. Results: Tumour growth promoted some deleterious effects on chemical body composition, increasing water and decreasing fat percentage, and reducing lean body mass. In muscle tissue, tumour led to a decreased protein synthesis and an increased proteolysis, showing the higher activity of the ubiquitin-proteasome pathway. On the other hand, the metformin treatment likely minimised the tumour-induced wasting state; in this way, this treatment ameliorated chemical body composition, reduced the higher activities of proteolytic enzymes and decreased the protein waste. Conclusion: Metformin treatment not only decreases the tumour growth but also improves the protein metabolism in gastrocnemius muscle in tumour-bearing rats.16Fundacao de Amparo a Pesquisa do Estado de Sao Paulo - FAPESP [2010/00209-9, 2013/16115-1, 2014/13334-7]Conselho Nacional de Desenvolvimento Cientifico e Tecnologico - CNPq [304604/2010-0, 302863/2013-3]Fundacao de Amparo a Pesquisa do Estado de Sao Paulo - FAPESP, Sao Paulo, Brazil [2008/07737-0]Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)