Bitter melon (Momordica charantia; BM) has been shown to ameliorate diet-induced obesity and insulin resistance. To examine the effect of BM supplementation on cell size and lipid metabolism in adipose tissues, three groups of rats were respectively fed a high-fat diet supplemented without (HF group) or with 5 % lyophilised BM powder (HFB group), or with 0·01 % thiazolidinedione (TZD) (HFT group). A group of rats fed a lowfat diet was also included as a normal control. Hyperinsulinaemia and glucose intolerance were observed in the HF group but not in HFT and HFB groups. Although the number of large adipocytes (. 180 mm) of both the HFB and HFT groups was significantly lower than that of the HF group, the adipose tissue mass, TAG content and glycerol-3-phosphate dehydrogenase activity of the HFB group were significantly lower than those of the HFT group, implying that BM might reduce lipogenesis in adipose tissue. Experiment 2 was then conducted to examine the expression of lipogenic genes in adipose tissues of rats fed low-fat, HF or HFB diets. The HFB group showed significantly lower mRNA levels of fatty acid synthase, acetyl-CoA carboxylase-1, lipoprotein lipase and adipocyte fatty acid-binding protein than the HF group (P, 0·05). These results indicate BM can reduce insulin resistance as effective as the anti-diabetic drug TZD. Furthermore, BM can suppress the visceral fat accumulation and inhibit adipocyte hypertrophy, which may be associated with markedly down regulated expressions of lipogenic genes in the adipose.Bitter melon: Peroxisome proliferator-activated receptor: Thiazolidinedione: Adipocyte hypertrophy: Lipogenic genesThe metabolic syndrome has become a major public health problem in the whole world. It is characterised by the clustering of risk factors, including insulin resistance, obesity or abdominal obesity, hypertension and dyslipidaemia in an individual which dramatically increases the risk of developing CVD and type 2 diabetes mellitus 1 . Momordica charantia, the fruit of which is known as karella, bitter gourd or bitter melon (BM), is a common edible vegetable in Asia. Physiological benefits, including hypoglycaemia, hypolipidaemia, anti-virus and anti-carcinogenic effects, have been reported, but the mechanisms and functional components remain to be elucidated 2,3 .Using a transactivation assay, we found that an ethyl acetate extract of BM activates both PPARa and PPARg 4 . PPAR are ligand-activated transcription factors belonging to the nuclear receptor superfamily. Three subtypes (PPARa, b and g) have been identified and shown to play a key role in the control of lipid and glucose homeostasis as transcription factors regulating genes encoding enzymes involved in these processes 5 . Fibrate-class hypolipidaemic drugs and thiazolidinedione (TZD)-class anti-diabetic drugs are, respectively, specific PPARa and PPARg ligands, but efforts are now being made to screen for and develop PPARa and g dual agonists, focusing on the metabolic syndrome, to resolve the problems of insulin resistance,...