Table 3.

Intervention Studies on Hydration and Gastrointestinal Function ¹.

Citation	Study Objective	Population	Design	Intervention	Summary/Conclusion
Anti et al., 1998 [73]	Determine the effects of a high-fiber diet and fluid supplementation in patients with functional chronic constipation	117 adults with chronic functional constipation (age 18–50 y). Baseline fluid intake: Group 1: 1.0 L (SD 0.2) and Group 2: 1.0 L (SD 0.4)	Parallel RCT	Group 1 (n = 58, 20/38 M/F) consumed standard diet providing 25 g fiber with ad libitum fluid intake. Group 2 (n = 59, 23/36 M/F) consumed standard diet providing 25 g fiber with 2 L/d fluid for 2 months	Fluid intake was greater in Group 2 (average 2.1 L/d) vs. Group 1 (average 1.1 L/d). Group 2 had greater increases in stool frequency and decreases in laxative use compared to Group 1.
Chung et al., 1999 [75]	Examine the effect of excess fluid (isotonic and hypotonic) on the actual stool output as measured by stool weight while simultaneously monitoring the urine output in 15 healthy volunteers	15 adults age 23 to 46 y. Baseline fluid intake: Group 1: 1.38 L (SD 0.93) and Group 2:1.20 L (SD 0.29).	Parallel	Group 1 (n = 9, 4/5 M/F): Additional intake of near isotonic fluid (Gatorade); Group 2 (n = 6, 3/3 M/F): Additional intake of hypotonic solution (water). Both groups consumed additional 1 L/d of fluid for 2 days, followed by additional 2 L/d of fluid for the next 2 days.	No change in total stool weight in both groups. Stool frequency was not reported.
Ziegenhagen et al., 1991 [76]	Compare the long-term effects of wheat bran alone vs. wheat bran with fluid addition on gastrointestinal function in healthy subjects	11 adults (55% F), age 19–33 y	Crossover RCT	Period 1: 15 g wheat bran twice/d. Period 2: 15 g wheat bran + 300 mL tea or water twice/d. Basal fluid intake restricted to 1–1.2 L/d. 14 d intervention, 7 d washout.	Gastric emptying was slower with bran vs. control and bran + fluid. Whole gut (oroanal) transit was shorter, while stool frequency and stool weight were greater with bran and bran + fluid vs. control. No effects due to addition of fluid were reported.
Klauser et al., 1990 [74]	Investigate whether fluid deprivation has an influence on colonic function	8 healthy men (age 21–28 y)	Crossover RCT	Control week: Consume >2500 mL fluid/d. Intervention week: Consume <500 mL fluid/d. 1 week intervention, 1 week washout.	Stool weight and frequency decreased with fluid restriction. No change in oroanal transit time.
van Nieuwenhoven et al., 2000	Examine the effect of dehydration on various gastrointestinal parameters during strenuous exercise.	10 healthy men (age 18–30 y)	Crossover RCT	Euhydration/control arm: Habitual fluid consumption. Dehydration arm: 15-min periods in a dry sauna interspersed with 10-min cooling off periods until 3% body mass loss was reached	Gastric emptying was significantly slower during dehydration. Orocecal transit time, intestinal permeability, and intestinal glucose absorption were unaffected by dehydration. Hydration status during euhydration/control arm was not assessed. Habitual fluid intake was not reported. (Only results from the pre-exercise/resting stage are reported herein).

Abbreviations: d, day; F, female; g, grams; L, liter; M, male; min, minute; mL, milliliter; n, sample size; RCT, randomized clinical trial; SD, standard deviation; y, years. ¹ Intervention trials published since inception through April 2018.