Effects of sex and menstrual cycle on volume-regulatory responses to 24-h fluid restriction.

Reproductive hormones have significant nonreproductive physiological effects, including altering fluid regulation. Our purpose was to explore the impact of sex and menstrual cycle (MC) phase on volume-regulatory responses to 24-h fluid restriction (24-h FR). Participants (men: = 12, 20 ± 2 yr; women: = 10, 20 ± 1 yr) were assigned two randomized and counterbalanced fluid prescriptions [Euhy: euhydrated, urine specific gravity (USG) < 1.020; Dehy: 24-h FR, USG > 1.020]. Men completed both (MEuhy, MDehy), while women completed both in the late-follicular (; FDehy, FEuhy) and midluteal (; LDehy, LEuhy) phases. We measured body mass, plasma and urine osmolality (P, U), urine specific gravity (USG), urine color (U), and serum copeptin; 24-h FR yielded mild dehydration without influence of sex or MC ( > 0.05). Copeptin increased in men following Dehy (pre: 8.2 ± 5.2, post: 15.8 ± 12.6, = 0.04) but not in women (FDehy pre: 4.3 ± 1.6, post: 10.5 ± 6.9, = 0.06; LDehy pre: 5.6 ± 3.5, post: 10.4 ± 6.2, = 0.16). In FDehy, P increased following FR (pre: 288 ± 2, post: 292 ± 1, = 0.03) but not in men (pre: 292 ± 3, post: 293 ± 2, = 0.46). No MC differences were observed between body mass loss, P, U, USG, and copeptin ( > 0.05). These results suggest that volume-regulatory responses to 24-h FR were present in men but not in women, without apparent effects of the menstrual cycle.