The present invention relates to a composition in the form of a powder or a viscous paste, and which includes at least one low viscosity alginate having a viscosity of less than about 100 mPaS in a 1 wt % aqueous solution, and at least one high viscosity alginate having a viscosity of more than about 100 mPaS in a 1 wt % aqueous solution. The composition can also include at least one suspending agent. The composition is readily soluble in water such that an aqueous preparation can be prepared without substantive mixing. Also, the aqueous preparation is suitable for use in the treatment and/or prevention of overweight for both therapeutic and non-therapeutic purposes.