A diet food composition effective in prevention of lifestyle related diseases is disclosed. The composition comprises a soluble dietary fiber of 5 to 35 weight % mainly containing insulin and an insoluble dietary fiber of 95 to 65 weight %, selected from the group consisting of wheat bran, rice bran and soybean fiber. The insoluble dietary fiber and the soluble dietary fiber are mixed at a mixture ratio of 1 to 2-20 on the basis of weight portion. The composition per 100 kcal contains the dietary fiber of at least 10 grams and insulin of at least 2 grams. In a mixing process, the soluble dietary fiber can be previously mixed and whipped with water and oil before mixing with the insoluble dietary fiber and the gluten can be previously fermented before mixing with the insoluble dietary fiber in order to have a good finished product.
