Medium-Term Effects of Increased Water Intake and Head-Up Sleep on Cardiovascular Health

Whether medium-term increased water intake alone, or in combination with co-adjuvant nonexercise interventions aimed to expand blood volume (BV), improve the human cardiovascular phenotype and cardiorespiratory fitness remains unexplored.

The purpose of this study was to determine the medium-term impact of increased (+40%) fluid (water) intake (IFI) or IFI plus head-up sleep (IFI + HUS) on BV and the cardiovascular phenotype in healthy individuals.

Healthy adults (n = 35, age 42 ± 18 years, 51% female) matched by sex, age, body composition, physical activity, and cardiorespiratory fitness were randomly allocated to IFI or IFI + HUS for 3 months. Body composition and BV were determined via DXA and indicator-dilution methods. Cardiac filling, output, and peak O₂ consumption (VO_2peak) were assessed via high-resolution echocardiography and pulmonary gas analyses at rest and during incremental exercise.

Intravascular volumes, comprising plasma and red blood cell volumes, were not modified by IFI or IFI + HUS. Cardiac volumes at rest, specifically left ventricular (LV) end-diastolic volume and stroke volume (SV), and systolic emptying rate were increased after IFI and IFI + HUS (P ≤ 0.007); the effects on SV and systolic emptying rate were larger in IFI + HUS vs IFI (P ≤ 0.037). Arterial elastance and cardiac afterload were similarly reduced by IFI and IFI + HUS (P ≤ 0.006). Moreover, resting LV diastolic filling rate and lateral wall e' velocity were only increased after IFI + HUS (P ≤ 0.031). During exercise, neither SV, cardiac output, and peak VO₂ were altered by IFI or IFI + HUS.

Medium-term increased water intake largely expands the resting volume and output of the LV while reducing arterial elastance and cardiac afterload, without altering intravascular volumes, cardiac or aerobic capacities. With the addition of HUS, relaxation properties of the resting LV are further improved.