Abstract: The invention includes a substance based on nanocrystalline maghemite having a crystal size of between 0.5 and 4 nm, which is defined by a proportion of divalent iron ions less than five percent by weight of the total iron, and the transport of sodium and simultaneously phosphate in the gastrointestinal wall from the gastrointestinal contents into the bloodstream is reduced and thus can improve the imbalance of electrolyte and water and mineral balances of patients having impaired renal function when used orally in combination with suitable pharmaceutical adjuvants.