| Summary: | Insufficient water intake may affect body composition. The purpose of this research was to explore the effects of water restriction and replenishment on body composition and to evaluate the optimum amount of water that improves body composition. A total of 76 young adults aged 18-23 years old (40 males and 36 females) in Baoding, China, were recruited in this randomized controlled trial, with a 100% completion rate. After fasting overnight for 12 h, at 8:00 a.m. of day 2, a baseline test, including anthropometric indices and collection of urine and blood samples, was explored. Participants were then subjected to water restriction for 24 h, and three meals with ≤75% water content were provided. At 8:00 AM of day 3, the same indices were determined as a dehydration test. Then, participants were randomly assigned into four groups: three water replenishment groups (WR groups 1, 2, and 3 given 1000, 500, and 200 mL of purified water, respectively) and one non-replenishment group (NR group, with no water). After 90 min, the same measurements were performed as a rehydration test. Compared with the baseline test, during the dehydration test, the intracellular water to total body water ratio (ICW/TBW) increased; and extracellular water (ECW), ECW/TBW (extracellular water to total body water ratio), and TBW decreased (all <i>p </i>< 0.05). For males, significant differences were found in ECW, ECW/ICW (extracellular water to intracellular water ratio), ICW/TBW, and ECW/TBW (all <i>p </i>< 0.05); for females, significant reductions were found in ICW, ECW, TBW, ECW/ICW, ICW/TBW, and ECW/TBW (all <i>p </i>< 0.05). Furthermore, significant differences were found in ICW, ECW, ICW/TBW, ECW/TBW, ECW/ICW, TBW, and TBW/BW between males and females during the baseline and dehydration test (all <i>p </i>< 0.05). Comparing the dehydration test with the rehydration test, there were significant interactions between time × volume in ICW and TBW (<i>F </i>= 3.002, <i>p </i>= 0.036; <i>F </i>= 2.907, <i>p </i>= 0.040); in males, these were only found in ICW (<i>F = </i>3.061, <i>p </i>= 0.040); in females, they were found in ICW and TBW (<i>F </i>= 3.002, <i>p </i>= 0.036; <i>F </i>= 2.907, <i>p </i>= 0.040). The ICW levels in WR groups 1 and 2 were all higher than in the NR group (all <i>p </i>< 0.05); the TBW was higher in WR group 1 than in the NR group (<i>p </i>< 0.05). No significant differences were found between WR groups 1 and 2, either in males or in females (all <i>p </i>> 0.05). In the rehydration test, significant differences in body composition were found between males and females among the four groups (all <i>p </i>< 0.05). Water restriction had adverse effects on body composition, and females were more susceptible to water restriction than males. Water replenishment improved the water content of body composition, alleviating the adverse effects of water restriction on ICW and TBW. After water restriction for 36 h, the optimum volume of water to improve body composition among young male adults was 1000 mL, but this was not the case for females.
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