Insufficient water intake may influence body composition. Recent research explored the effects of water restriction and replenishment on body composition and evaluated the optimum amount of water that improves body composition. 

The study recruited 76 young adults aged 18–23 years (40 males and 36 females), with a 100% completion rate. Following overnight fasting for 12 h, at 8:00 a.m. on day 2, it conducted a baseline test, including anthropometric indices and urine and blood sample collection. It then subjected the Participants to water restriction for 24 h and provided three meals with ≤75% water content. At 8:00 a.m. on day 3, it determined the same indices as a dehydration test. 

It then randomized the participants into four groups: three water replenishment groups (WR groups 1, 2, and 3 received 1000, 500, and 200 mL of purified water, respectively) and one non-replenishment group (NR group, with no water). After 90 min, it performed the same measurements as a rehydration test. 

The study found that-

• During the dehydration test, compared with the baseline 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.
• Males showed significant differences in ECW, ECW/ICW (extracellular water to intracellular water ratio), ICW/TBW, and ECW/TBW.
• Females showed significant reductions in ICW, ECW, TBW, ECW/ICW, ICW/TBW, and ECW/TBW.
• Furthermore, males and females showed significant differences in ICW, ECW, ICW/TBW, ECW/TBW, ECW/ICW, TBW, and TBW/BW during the baseline and dehydration test.
• A comparison between the dehydration test and the rehydration test showed significant interactions between time × volume in ICW and TBW; in males, these were present only in ICW; in females, they were present in ICW and TBW.
• WR groups 1 and 2 showed higher ICW levels than the NR group
• WR group 1 showed higher TBW than the NR group.
• WR groups 1 and 2 showed no significant differences either in males or in females.
• In the rehydration test, males and females showed significant differences in body composition among the four groups.
• Water restriction showed adverse effects on body composition, and females showed more susceptibility to water restriction than males.
• Water replenishment showed improvement in the water content of body composition, alleviating the adverse effects of water restriction on ICW and TBW.
• After water restriction for 36 h, 1000 mL volume of water improved body composition among young male adults but not in females.

Zhang J, Zhang N, Du S, Liu S, Ma G. Effects of Water Restriction and Water Replenishment on the Content of Body Water with Bioelectrical Impedance among Young Adults in Baoding, China: A Randomized Controlled Trial (RCT). Nutrients. 2021; 13(2):553. https://doi.org/10.3390/nu13020553