Studies suggest that metformin, widely used for treating Type 2 diabetes, possesses innate antineoplastic properties. For metabolic syndrome patients with hepatocellular carcinoma (HCC), metformin may provide antitumoral effects. We evaluated the impact of metformin on tumour growth and visceral fat composition using relevant preclinical models of metabolic syndrome. Studies were performed in three hepatoma cell lines, in HepG2 xenograft mice fed with standard chow (SC) diet, 60% high-fat diet (HFD) or 30% fructose diet (FR), and an ex vivo model of human cultured HCC slices. Visceral fatty acid composition was analysed by magnetic resonance imaging (MRI). Metformin had a dose-dependent inhibitory effect on cell proliferation and apoptosis in vitro through the deregulation of mTOR/AMPK, AKT and extracellular signal regulated kinase (ERK) signalling pathways. Tumour engraftment rates were higher in HFD mice than SC mice (hepatic: 79% compared with 25%, P=0.02) and FR mice (subcutaneous: 86% compared with 50%, P=0.04). Subcutaneous tumour volume was increased in HFD mice (+64% compared with FR and SC, P=0.03). Metformin significantly decreased subcutaneous tumour growth via cell-cycle block and mammalian target of rapamycin (mTOR) pathway inhibition, and also induced hypoxia and decreased angiogenesis. In ex vivo tumour slices, metformin treatment led to increased necrosis, decreased cyclin D1 and increased carbonic anhydrase-9 (CA-9). Metformin caused qualitative changes in visceral fat composition of HFD mice, with decreased proportions of polyunsaturated fatty acids (14.6% ± 2.3% compared with 17.9% ± 3.0%, P=0.04). The potent antitumoral effects of metformin in multiple preclinical models implicating several molecular mechanisms provide a strong rationale for clinical trials including combination studies in HCC patients.