Sodium-dependent transport sugars

Absorption of vitamin C from the small intestine is a carrier-mediated process that requires sodium at the luminal surface. Transport is most rapid in the ileum and resembles the sodium-dependent transport of sugars and amino acids, but the carrier is distinct for each class of compound. Some ascorbate may also enter by simple diffusion. With dietary intake less than 100 mg/d, efficiency of absorption is 80-90%. With intake equal to the RDA, plasma ascorbate is 0.7-1.2 mg/dL, and the ascorbate pool size is 1500 mg. Scurvy becomes evident when the pool is less than 300 mg, at which point plasma ascorbate is 0.13-0.24 mg/dL. Highest tissue concentrations of ascorbate are in the adrenal gland (cortex > medulla). 

K. J. Ullrich, G. Rum rich, and S. Kloss, Specificity and sodium dependence of the active sugar transport in the proximal convolution of the rat kidney, Pflugers Arch., 351 35-48 (1974). 

Plain water crosses the walls of the intestine by osmotic action alone. However, if sodium and sugars are also present then the active transport mechanisms described earlier operate and water absorption can be enhanced. The effect is dependant on concentrations the maximum rate of water uptake occurs when the concentrations give a slightly hypotonic solution (200-250 mOsm/kg, cf. 287 mOsm/kg for isotonicity) (Wapnir and Lifshitz, 1985). Conversely, when the lumen contents are significantly hypertonic, water is secreted from plasma into the intestine by osmotic action this is a dehydrating effect. 

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