The glucose product of sucrose digestion is transported across the epithelial cell membrane more rapidly than is free glucose, a phenomenon that may be related to specific glucose transporters that are not dependent on sodium. The fructose released by sucrose digestion is absorbed more slowly across the brush border membrane by a process called facilitated diffusion (carrier-mediated). Fructose appears to be absorbed better when ingested with glucose (separately or combined in sucrose) than it is by itself. This explains why 100 g fructose gives rise to osmotic diarrhea, whereas 250 g sucrose does not. The absorption of free fructose is incomplete when intakes exceed about 35 g per day.
Once inside the epithelial cell, glucose and fructose are presumed to traverse the enterocyte by diffusion. The basal-lateral membrane acts as a barrier preventing the free movement of monosaccharides into and out of the enterocyte. Movement across the membrane appears to be energy-dependent but sodium-independent.
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