The present invention is a method for the treatment or prevention of preferably non-insulin dependent (NIDDM or so-called Type II) diabetes mellitus, or other conditions associated with impaired glucose tolerance such as obesity, and in particular to the use of phytanic acid derivatives for the said treatment and/or prevention. A method of making a composition for the treatment or prevention of non-insulin dependent diabetes mellitus and related diseases comprising combining phytanic acid or derivatives thereof with a pharmaceutically acceptable additive or adjuvant, and a composition for the treatment or prevention of non-insulin dependent diabetes mellitus comprising phytanic acid or derivatives thereof are also provided.