The present invention relates to the field of poly- and oligosaccharides and their dietary effects. In particular it relates to a method of producing an α-glucan containing (α1→3) linked D-glucose units. The invention also provides a branched α-glucan comprising alternating (α1→4) and (α1→3) glucosidic linkages and having (α1→3,4) branching points, a food composition, and the use of an α-glucanotransferase enzyme for reducing the digestible carbohydrates of a starch containing food material. Further aspects of the invention are a bacteria, an enzyme and an expression vector.