A composition in the form of a dry powder and which comprises: a) at least one membrane lipid, and b) at least one biologically active compound comprising a xanthine and/or a carboxylic acid, and which forms structured lipid assemblies when dispersed/dissolved in an aqueous medium. A method of preparing such a composition is also provided, together with a dispersion of structured lipid assemblies suspended in a solution of at least one biologically active compound and a method of preparing the same. The compositions and dispersions are suitable for use in creams and lotions for skin care.