1. The relative effects of changes in mucosal surface area and mucosal permeability on the passive uptakes of
mannitol and
raffinose have been studied in vitro using jejunal biopsies from 48 controls, 32 patients with coeliac disease and 11 patients with
Crohn's disease. Total mucosal permeation was corrected for surface area measured morphometrically to provide an index of mucosal permeability. 2. In untreated coeliac disease, permeation of
mannitol was reduced by 35% (P = 0.006) and that of
raffinose was increased by 66% (P = 0.0095) compared with controls, whereas mucosal permeability to
mannitol was increased twofold (P = 0.009) and to
raffinose fivefold (P = 0.0001). Mucosal permeability was similar for each
sugar. 3. In treated coeliac disease, permeation and permeability for
mannitol were normal, but remained elevated for
raffinose by 23% (P = 0.036) and 41% (P = 0.024), respectively. 4. In
Crohn's disease, permeation of
mannitol was reduced by 21%, but that of
raffinose and mucosal permeability to both
sugars were normal. 5. These findings suggest that surface area is quantitatively more important than mucosal permeability in determining the total permeation of
mannitol, while the converse is true for
raffinose. The findings are compatible with paracellular uptake of
raffinose, but with both paracellular and transcellular uptake of
mannitol. Both pathways are affected in coeliac disease, whereas only transcellular uptake is affected in
Crohn's disease.