Sex Differences In Response To Passive Dehydration Via 24-h Fluid Restriction

Abstract

Dehydration is a known stressor on the human body. It has been previously observed that a proportion of individuals may live in a chronic state of mild dehydration, either due to physical activity and lack of proper rehydration, or due to low habitual fluid consumption. This state of chronic dehydration propagated by low fluid consumption can be simulated via passive dehydration and fluid restriction. Physiological responses to fluid restriction have previously been investigated in men, leaving sex differences or unique responses in women uninvestigated. PURPOSE: The purpose of this investigation was to assess differences in response to 24-h fluid restriction (FR) between men and women. METHODS: Eleven participants (n=5 male: 21 ± 3 y, 69.97 ± 8.4 kg, 172.4 ± 3.8 m; n=6 female: 20 ± 2 y, 57.04 ± 3.62 kg, 163 ± 4.9 m) underwent passive dehydration via 24-h FR one time for men, and twice for women. Females were tested in the late follicular and mid-luteal phases of the menstrual cycle (days 12-14 and 19-21, respectively). Body mass loss assessed via nude body mass, plasma osmolality (Posm), and blood pressure were assessed pre- and post-FR. During FR participants also collected their urine which was assessed for urine color (Ucol), urine specific gravity (USG), osmolality (Uosm), and volume (L). RESULTS: No differences were found in this preliminary data set between men and women for body mass loss (%BML; male: 1.28 ± 1.14, female: .68 ± 1.2, p=.407), plasma osmolality post-FR (male: 291 ± 4 mOsmo·kg-1, female: 294 ± 3 mOsmo·kg-1, p=.319), Ucol (male: 5 ± 2, female: 6 ± 1, p=.594), USG (male: 1.026 ± 0, female: 1.027 ± 0, p=.862), urine volume (male: 0.30 ± .47 L, female: 0.16 ± 0.20 L, p=.501), or Uosm (male: 805 ± 313 mOsmo·kg-1, female: 740 ± 115 mOsmo·kg-1, p=.621). The only differences present between male and female participants were in pre-FR Posm (male: 293 ± 4 mOsmo·kg-1, female: 286 ± 3 mOsmo·kg-1, p=0.05). CONCLUSION: These preliminary data show no sex differences in response to 24-h FR, but this ongoing investigation and full dataset seeks to fully elucidate possible differences in response to FR simulating chronic mild dehydration that can affect young physically active, as well as elderly populations.