Exercise and Fluid Replacement

Michael N. Sawka, FACSM; Louise M. Burke, FACSM; E. Randy Eichner, FACSM; Ronald J. Maughan, FACSM; Scott J. Montain, FACSM; Nina S. Stachenfeld, FACSM

Disclosures

March 02, 2010

In This Article

Modifying Factors

Sex

Women typically have lower sweating rates and electrolyte losses than men.[7,119,125] The lower sweating rates are primarily because they have smaller body size and lower metabolic rates when performing a given exercise task. In addition, women seem to have less wasted sweat when their skin is wet.[125]

Sex differences in renal water and electrolyte retention are subtle and probably not of consequence. The diuretic response to a water load can be greater in women than men, suggesting that women turn water over more quickly than men.[37] Women show reduced argininevasopressin (AVP) responses to osmotic stimuli, which should result in elevated renal water and electrolyte losses.[140] Paradoxically, within women, both endogenous estrogens and exogenously administered estrogens appear to increase AVP release and both estrogens and progesterone enhance renal water and electrolyte retention.[136,137,139]

Women appear to be at greater risk than men to develop symptomatic hyponatremia when competing in marathon and ultra marathon races.[4,71] While the explanation for this increased risk may be due to a number of biological and psychosocial factors, the cause for greater risk for hyponatremia has not been established with certainty. Previous information regarding fluid intake available to women have often been based on sweat loss data obtained from men, and as such, are too high for women and may have led to accentuated sodium dilution due to their smaller total body water.[103,134] This position statement provides some sweat rate data obtained directly from women (Table 2).

Although the kidney is important in the pathogenesis of hyponatremia, the target organs for morbidity and mortality are the brain and lungs. Studies to directly address the possible mechanisms for sex differences in how the brain handles water/electrolyte imbalances cannot be performed in humans. During AVP-induced hyponatremia, animal studies have shown significantly greater sodium transport in the male rat versus the female rat brain, suggesting impairment of the Na+-K+-ATPase pump activity in the female brain.[56,57] Therefore, this might aggravate hyponatremia induced cerebral edema. Likewise, sex hormones have been suggested to impair Na+-K+-ATPase pump activity in the female brain and account for women having increased morbidity and mortality from postoperative hyponatremia.[55]

Evidence Statement. Women generally have lower sweating rates than men. Evidence Category A. Sex differences in renal water and electrolyte retention are subtle and probably not of consequence. Evidence Category B. Women are at greater risk than men to develop exercise-associated symptomatic hyponatremia. Evidence Category C.

Age

Older (ages >65 yr) persons are generally adequately hydrated.[72] However, there is an age-related blunting of thirst response to water deprivation,[81,86,117] making older persons more susceptible to becoming dehydrated.[81] Older adults have an age-related increase in resting plasma osmolality and are slower to restore body fluid homeostasis in response to water deprivation[110] and exercise[86] than younger adults. If given sufficient time and access to water and sodium, older adults will adequately restore body fluids, indicating appropriate, albeit sluggish, control of body fluids.[84,86] Older persons are also slower to excrete water following fluid loads.[83,86,135,138,143] This slower water and sodium excretion increases sodium retention and can lead to increases in blood pressure.[84] Most, but not all, of the age related slower responses to water or saline loads or dehydration can be attributed to the lower glomerular filtration rate,[83] due to a progressive fall in the number of functioning nephrons.[49]

While thirst sensitivity to a given extracellular fluid loss is reduced in older adults, osmoreceptor signaling remains intact.[86,135,138] The osmotic and volume stimuli that results from dehydrating, impart important drives for thirst and drinking in older adults.[9] Thus, older adults should be encouraged to rehydrate during or after exercise, but they should also consider the risks of excess water (i.e., hyponatremia) or sodium ingestion (i.e., hypertension) because they may be slower to excrete both the water and electrolytes.

Prepubescent children have lower sweating rates than adults, and with values rarely exceeding 400 mL·h−1.[10,92] These lower sweating rates are probably the result of smaller body mass and thus lower metabolic rate. Sweat electrolyte content is similar or slightly lower in children than adults.[10]

Evidence Statement. Older adults have age-related decreased thirst sensitivity when dehydrated making them slower to voluntary reestablish euhydration. Evidence Category A. Older adults have age-related slower renal responses to water and sodium loads and may be at greater risk for hyponatremia Evidence Categories A and C. Children have lower sweating rates than adults. Evidence Category B.

Diet

Meal consumption is critical to ensure full hydration on a day-to-day basis.[1,2,72,131] Eating food promotes fluid intake and retention.[72] Sweat electrolyte (e.g., sodium and potassium) losses need to be replaced to reestablish total body water and this can be accomplished during meals with most persons.[85,105,128] Diet macronutrient composition has a minor influence on urine losses during rest and probably has even a smaller influence during exercise.[72] Therefore, diet macronutrient composition does not measurably alter daily fluid needs for individuals.[72]

Caffeine is contained in many beverages and foods and recent evidence suggests if consumed in relatively small doses (< 180 mg·d−1) it will likely not increase daily urine output or cause dehydration.[5,72] The influence of caffeine consumption on urine output during exercise or in dehydrated individuals is not well documented, but urine production is already decreased by dehydration, exercise and heat stress.[72,150] Therefore, it is doubtful that caffeine consumption during exercise would elevate urine output and induce dehydration during exercise. Since alcohol can act as a diuretic (particularly at high doses) and increase urine output, it should be consumed in moderation, particularly during the postexercise period when rehydration is a goal.[129]

Evidence Statement. Meal consumption promotes euhydration. Evidence Category A. Sweat electrolyte (sodium and potassium) losses should be fully replaced to reestablish euhydration. Evidence Category A. Caffeine consumption will not markedly alter daily urine output or hydration status. Evidence Category B. Alcohol consumption can increase urine output and delay full rehydration. Evidence Category B.

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