The present invention relates to a storage-stable aqueous or gel-like composition or solution comprising a magnesium compound selected from a magnesium salt or a magnesium complex compound, and L-carnitine, and the use thereof for the preparation of a medicament for supporting metabolism, in particular in muscle tissue, and for supporting muscle structure, and its use as food supplement or in the veterinary sector as animal feed additive, in particular for horses, poultry, pigeons, pigs, cattle, sheep and camels.