Changes in body water content can exert profound effects on physiological function andperformance. For example, dehydration limits sweating and heat dissipation, causingincreased core temperature, reduced exercise tolerance and increased risk for heat-relatedinjury. Sophisticated mechanisms have evolved to maintain body fluid volume andcomposition despite sudden fluxes in water intake or loss. These regulatory mechanismsinvolve reflexes within the blood vessels, brain and intestines that act to modify rates of fluidintake and fluid output. Dehydration is a normal physiological state following prolongedexercise in the heat, which leads to the loss of water and increase of sodium in the blood.This state leads, in turn, to thirst, fluid intake and sodium/water retention by the kidneys.There are a number of “fluid retention” hormones that exert a profound control over this fluidregulatory system. Hormones are substances in the body that act to promote certain activityby specific organs. The fluid regulatory hormones act primarily on the brain and kidney tocontrol both intake and output of water and sodium. The most important hormone of thistype is the antidiuretic hormone, which responds rapidly to changes in body water status,and is responsible for controlling the rate of water retention by the kidneys. To complicatematters, antidiuretic hormone regulation maybe modified by a variety of factors, one ofwhich may be the female sex hormone estrogen.
Estrogen levels fluctuate widely throughout the normal menstrual cycle, with the majorestrogen peak occurring in the early phase, days 7-13 after the start of menstrual bleeding(just prior to ovulation). In addition, estrogen is one of the most commonly administeredhormones, in the form of oral contraceptives to young women, and estrogen replacementtherapy to post-menopausal women. Body water retention is common in high estrogenstates, such as that immediately preceding ovulation, during pregnancy, and while takingestrogen or estrogen-dominant oral contraceptives. Some of this excess water is retained inthe blood vessels (i.e., in the plasma), leading to plasma volume expansion, which haspotent effects on physical performance during heat stress. In fact, variations in plasmavolume observed following estrogen administration and during different phases of themenstrual cycle are comparable to the effects of posture, skin temperature and exerciseintensity.
There have been few studies directly examining the effects of sex steroids on body waterregulation. However, early research suggests that the greater body water content in highestrogen states is likely due to changes in both water intake and output. Thirst sensationand antidiuretic hormone are more sensitive to changes in body water status in pregnantwomen, in whom blood estrogen levels remain elevated. In addition, fluctuations inantidiuretic hormone parallels those of estrogen throughout the menstrual cycle, and areincreased following estrogen administration in postmenopausal women, suggesting that fluidretention is also likely to be increased. Thus, the combination of enhanced fluid intake andretention causes the increase in total body water content during high estrogen states.
Why is the regulation of body water important during exercise? Recall that body watercontent regulates sweating, and sweating provides our best method of heat loss duringexercise. So, adequate body water is essential to avoid heat injury (e.g., heat exhaustion orheat stroke). The effect of estrogen on the systems that control body water during exerciseis only now being studied, but there is reason to suspect that the greater water retentioninduced by estrogen may have beneficial effects during exercise. For example, manywomen find that their blood volume is greater and exercise body temperature responseslower during exercise in the heat when blood levels of estrogen are high. In support of thistheory, we find that women had greater antidiuretic hormone responses during estrogenadministration with oral contraceptives, compared to their responses while not on the pill.Finally, we find that estrogen administration in the form of oral contraceptives improvessweating during exercise and during body heating at rest.
These studies examining the effects of estrogen on the fluid regulatory and temperatureresponses to exercise in the heat in young women are ongoing. Further, estrogen affectsmost body systems (reproduction, cardiovascular, metabolic, central nervous system), so itwould be inadvisable to use this hormone solely for the purpose of altering or improvingbody fluid regulation. However, women taking this hormone for other reasons, such as birthcontrol or protection of bone content, may find an increase in water retention to be one ofthe more common side effects of this treatment.
Written for the American College of Sports Medicine by Nina Stachenfeld, Ph.D.
Estrogen levels fluctuate widely throughout the normal menstrual cycle, with the majorestrogen peak occurring in the early phase, days 7-13 after the start of menstrual bleeding(just prior to ovulation). In addition, estrogen is one of the most commonly administeredhormones, in the form of oral contraceptives to young women, and estrogen replacementtherapy to post-menopausal women. Body water retention is common in high estrogenstates, such as that immediately preceding ovulation, during pregnancy, and while takingestrogen or estrogen-dominant oral contraceptives. Some of this excess water is retained inthe blood vessels (i.e., in the plasma), leading to plasma volume expansion, which haspotent effects on physical performance during heat stress. In fact, variations in plasmavolume observed following estrogen administration and during different phases of themenstrual cycle are comparable to the effects of posture, skin temperature and exerciseintensity.
There have been few studies directly examining the effects of sex steroids on body waterregulation. However, early research suggests that the greater body water content in highestrogen states is likely due to changes in both water intake and output. Thirst sensationand antidiuretic hormone are more sensitive to changes in body water status in pregnantwomen, in whom blood estrogen levels remain elevated. In addition, fluctuations inantidiuretic hormone parallels those of estrogen throughout the menstrual cycle, and areincreased following estrogen administration in postmenopausal women, suggesting that fluidretention is also likely to be increased. Thus, the combination of enhanced fluid intake andretention causes the increase in total body water content during high estrogen states.
Why is the regulation of body water important during exercise? Recall that body watercontent regulates sweating, and sweating provides our best method of heat loss duringexercise. So, adequate body water is essential to avoid heat injury (e.g., heat exhaustion orheat stroke). The effect of estrogen on the systems that control body water during exerciseis only now being studied, but there is reason to suspect that the greater water retentioninduced by estrogen may have beneficial effects during exercise. For example, manywomen find that their blood volume is greater and exercise body temperature responseslower during exercise in the heat when blood levels of estrogen are high. In support of thistheory, we find that women had greater antidiuretic hormone responses during estrogenadministration with oral contraceptives, compared to their responses while not on the pill.Finally, we find that estrogen administration in the form of oral contraceptives improvessweating during exercise and during body heating at rest.
These studies examining the effects of estrogen on the fluid regulatory and temperatureresponses to exercise in the heat in young women are ongoing. Further, estrogen affectsmost body systems (reproduction, cardiovascular, metabolic, central nervous system), so itwould be inadvisable to use this hormone solely for the purpose of altering or improvingbody fluid regulation. However, women taking this hormone for other reasons, such as birthcontrol or protection of bone content, may find an increase in water retention to be one ofthe more common side effects of this treatment.
Written for the American College of Sports Medicine by Nina Stachenfeld, Ph.D.
