A medicinal preparation, a process, a nutritional composition and regenerative and insulin potentiating method for humans and also for mammals for prevention, treatment and management/glycemic control of diabetes mellitus by obtaining optimum glucose level in blood, by administering an extract of Costus pictus to produce above effect in a dose of 500-2000 mg/day in humans and 50-200 mg/kg/day in rats. A method for treating hyperglycemia in a patient by administering a medicinal preparation of Costus pictus D. Don. A method for long term management of diabetes by avoiding the problems associated with tight control of blood glucose concentrations, i.e., hypoglycemia tolerance and seizures, while simultaneously avoiding the problems associated with conventional moderate control of blood glucose concentrations, i.e., pathological complications associated with hyperglycemia, such as nephropathy, retinopathy, etc. The medicinal preparation further affords an insulin potentiating action in addition to decreasing blood glucose and/or glycosylated hemoglobin levels.