This invention relates to the combination of .alpha.-amylase inhibitors prepared from Phaseolus vulgaris with .alpha.-glucosidase inhibitors obtained from Salacia oblonga and other species. The .alpha.-amylase inhibitor is accompanied by a quantity of lectins that reduces the amount of glucose originating from the starches present in the diet, and considerably reduces the appetite after repeated administration. The combination with .alpha.-glucosidase inhibitors, such as extracts of Salacia or the thiosugars present in it, further reduce the blood glucose, acting synergically, and consequently reduce the synthesis of fats from carbohydrates.