in such beverages? For example, does their inclusion in OHSs enhance the absorption of water and other solutes? Do they contribute to beverage palatability, osmoregulation, or maintenance of plasma volume?

The purpose of this paper is to review the effects of Na+on the intestinal absorption of water and carbohydrates during rest and exercise. It begins with a brief review of the methods employed in such studies and is followed by a discussion of the interactions among sodium, water, and glucose absorption. The emphasis will be on in vivo experiments on human subjects.

METHODS

Although there are several different methods of studying absorption (Leiper and Maughan, 1988; Modigliani et al., 1973), the segmental perfusion technique provides the most quantitative assessment of water and solute absorption in humans (Fordtran et al., 1961; Schedl and Clifton, 1963). This technique requires the subject to pass a multilumen catheter into the small intestine under fluoroscopic guidance. A mercury ball enclosed by an inflatable balloon is attached to the distal end to facilitate movement through the intestine. The test solution is infused at a constant rate (usually 8 to 20 ml/min) and contains a water-soluble nonabsorbable marker (usually polyethylene glycol).

When this technique was first introduced, it employed only a double-lumen tube with a mercury bag attached to its distal end. One lumen was used for infusion and the other was used to sample the perfusate at the end of the test segment. The drawbacks of the double-lumen tube are (1) reflux of the perfused solution proximally, and (2) contamination of the perfused solution by proximal endogenous secretions (Modigliani et al., 1973). To eliminate these problems, (1) an occlusive balloon proximal to the infusion port was included to prevent reflux and contamination of intestinal secretions, or (2) a third lumen was added to the catheter. The disadvantage of the occlusive balloon is that it may interfere with intestinal motility.

Most investigators who use the segmental perfusion technique employ a triple-lumen tube. By this technique, the test solution is perfused through the most proximal port and is sampled from the two more distal sites. The distance (10-15 cm) between the perfusion port and the first sampling site is called the mixing segment. It allows the marker to form a homogeneous solution with the intestinal contents before a sample is drawn (1 ml/min) from the proximal sampling site. The fluid then traverses the test segment (usually 20-40 cm) and is collected continuously by siphonage at the distal sampling site. It is not necessary to collect all of the solution, because the



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