Mannitol is an osmotic diuretic used to prevent acute tubular necrosis. [7] It is occasionally useful in enhancing the diuresis in patients with severe resistant oedema. [8] It has been administered also to MD patients with the purpose of possibly reducing the endolymphatic hydrops. [9, 10]
In healthy subjects, about 14% of mannitol administered orally is absorbed through the hydrophilic pores of the enterocytes. In the active phases of MD, we previously observed an increase of mannitol and lactulose intake as expression of the altered intestinal permeability. [4] We then speculated that the mucosal inflammation related with food or aeroallergens intolerance, unleashed by immunological cross-reaction in symptomatic MD patients, could have impaired the intestinal barrier function and induced a greater sugar absorption. Mannitol was the most sensitive between the two sugar challenges in this respect.
Mannitol behaves like an osmotic diuretic, i.e. it increases the excretion of water by the kidneys and the global volume of urine. Its mechanism of action does not involve a specific site in the kidney: it accumulates in the intercellular space, drawing water out of the cells due to increased local osmolarity. The fluids accumulated in the interstitial spaces are then rapidly eliminated as urine. An increased absorption of mannitol in case of altered intestinal permeability determines a decrease in sodium and an increase in serum osmolarity a consequently an increase in diuresis. [3]
The peculiar physical and chemical properties of mannitol make it an ideal tool to test the osmotic fluid retention in different condition of intestinal absorption, given the reliability of the "single paracellular permeation model”,, regardless of the cause of the altered intestinal permeability. [11]
Our new preliminary observations in a selected sample of MD patients in an active stage of the disease 1) confirm the hypothesis of an altered intestinal permeability on the basis of a single sugar test (instead of the double-sugar); 2) may suggest the assessment of intestinal permeability by a mannitol challenge without dosage of its urinary concentration but simply measuring the diuresis of the 4 hours following the water + mannitol load.
This proposed method would significantly reduce costs, compared to the validated non-invasive “double-sugar test”, providing an easy and self-administered way to identify altered intestinal permeability.
As far as we know, this is the first report of increased excretory urinary volume after mannitol challenge in a group of symptomatic MD patients.
The strength of this work is the very selected and homogeneous sample population, that includes only symptomatic, untreated MD patients, with no drug interferences.