Diabetes mellitus is a global epidemic, characterised as a heterogeneous group of metabolic disorders associated with high risk of CVD. Green banana biomass, which is composed of resistant starches (RS) and cannot be hydrolysed by amylases, delays gastric emptying and modulates insulin sensitivity, thus contributing to improve metabolic disorders. The aim of the present study was to investigate the effects of consumption of RS from green banana biomass on body composition, fasting plasma glucose, glycated Hb (HbA1c) and homeostasis model assessment of insulin resistance in subjects with pre-diabetes or type 2 diabetes on top of treatment. Middle-aged subjects (n 113) of both sexes with pre-diabetes (HbA1c: 5·7–6·4 %) or diabetes (HbA1c ≥ 6·5 %) were randomised to receive nutritional support plus green banana biomass (40 g) (RS: approximately 4·5 g, G1, n 62) or diet alone (G2, n 51) for 24 weeks. Body composition, biochemical analyses and dietary intake were evaluated at the beginning and end of the study. In the experimental group (G1), consumption of RS was associated with reduction in HbA1c (P = 0·0001), fasting glucose (P = 0·021), diastolic blood pressure (P = 0·010), body weight (P = 0·002), BMI (P = 0·006), waist and hip circumferences (P < 0·01), fat mass percentage (P = 0·001) and increase in lean mass percentage (P = 0·011). In controls (G2), reductions were observed in waist and hip circumferences (P < 0·01), HbA1c (P = 0·002) and high-density lipoprotein-cholesterol (P = 0·020). In pre-diabetes or diabetes, non-significant differences were observed in the percentage reduction in HbA1c and fasting glucose in exploratory analyses. Our results indicate that the consumption of bioactive starches is a good dietary strategy to improve metabolic control and body composition.