Daily water balance depends on the net difference between water gain and water loss. Water gain occurs from consumption (liquids and food) and production (metabolic water), while water losses occur from respiratory, gastrointestinal, renal, and sweat losses. The volume of metabolic water produced during cellular metabolism (~0.13 g·kcal−1) is approximately equal to respiratory water losses (~0.12 g·kcal−1),[38,93] so this results in water turnover with no net change in total body water. Gastrointestinal tract losses are small (~100–200 mL·d−1) unless the individual has diarrhea. Sweating provides the primary avenue of water loss during exercise-heat stress. The kidneys regulate water balance by adjusting urine output, with minimum and maximum urine outputs of approximately 20 and 1000 mL·h−1, respectively. During exercise and heat stress, both glomerular filtration and renal blood flow are markedly reduced, resulting in decreased urine output. Therefore, when fluids are over consumed during exercise (hyperhydration), there may be a reduced ability to produce urine to excrete the excess volume. With intermittent activities these effects may not be as strong on reducing urine production.
Over a protracted period (e.g., 8–24 h), if adequate fluid and electrolytes are consumed, the water losses will usually be fully replaced to reestablish the "normal" total body water (TBW). TBW is regulated within ± 0.2 to 0.5% of daily body mass.[1,31] TBW averages ~60% of body mass, with a range from approximately 45 to 75%. These differences are primarily due to body composition; fat-free mass is ~70 to 80% water, while adipose tissue is ~10% water. These water content relationships are independent of age, sex and race. Therefore, an average 70-kg person has approximately 42 L of total body water, with a range of 31–51 L. Trained athletes have relatively high TBW values by virtue of having a high muscle mass and low body fat and a small aerobic training effect. Additionally, individuals who glycogen load may experience a small increase in TBW, but this is not always observed. Furthermore, the surplus water associated with typical muscle glycogen increases is minor (~200 mL) when considering the small absolute muscle mass involved and assuming 3 mL water per gram glycogen (itself inconclusive). The precise fate of water liberated as glycogen is utilized is unknown, but the fact that any water bound to glycogen is part of the starting TBW pool suggests it is of little potential consequence to fluid intake recommendations.
When assessing an individual's hydration status, there is no one TBW that represents euhydration, and determinations need to be made of body water fluctuations beyond a range that have functional consequences. Ideally, the hydration biomarker should be sensitive and accurate enough to detect body water fluctuations of ~3% of TBW (or water content change sufficient to detect fluctuations of ~2% body weight for the average person). In addition, the biomarker should also be practical (time, cost, and technical expertise) to be used by individuals and coaches.
Table 3 provides an assessment of a variety of hydration biomarkers.[72,94] Dilution methods of TBW with plasma osmolality measurements provide the most valid and precise measures of body hydration status,[72,114] but are not practical for use by most persons. Other complex biomarkers such as plasma volume, fluid regulatory hormones, and bioelectrical impedance measures are easily confounded and/or not valid. Individuals can determine their hydration status by using several simple biomarkers (urine and body weight) that by themselves have marked limitations; but when these indicators are used together in the proper context, they can provide valuable insight.
The use of first morning body weight measurement after voiding, in combination with a measure of urine concentration should allow sufficient sensitivity (low false negative) to detect deviations in fluid balance. Urine biomarkers of hydration status can allow discrimination of whether an individual is euhydrated or dehydrated.[6,111,127] Urine specific gravity (USG) and osmolality (UOsmol) are quantifiable, whereas urine color and urine volume are often subjective and might be confounded. USG of ≤ 1.020 is indicative of being euhydrated.[6,12,111] UOsmol is more variable, but values ≤ 700 mOsmol·kg−1 are indicative of being euhydrated.[6,111,127]
Urine values can provide misleading information regarding hydration status if obtained during rehydration periods. For example, if dehydrated persons consume large volumes of hypotonic fluids, they will have copious urine production long before euhydration is reestablished. Urine samples collected during this period will be light in color and have USG and UOsmol values that reflect euhydration when in fact the person remains dehydrated. This emphasizes the need to use either first morning urine samples, or samples after several hours of stable hydration status, to allow valid discrimination between euhydration and dehydration.
Body weight (BW) measurements provide another simple and effective tool to assess fluid balance.[31,34] For well-hydrated persons, who are in energy balance, a first morning (after urinating) nude BW will be stable and fluctuate by < 1%.[1,31,64,65] At least three consecutive morning nude BW measurements should be made to establish a baseline value, which approximates euhydration, in active men consuming food and fluid ad libitum. Women may need more BW measurements to establish a baseline value, because their menstrual cycle influences body water status. For example, luteal phases can increase body water and BW by >2 kg. Lastly, first morning BW is influenced by changes in eating and bowel habits.
Acute changes in BW during exercise can be used to calculate sweating rates and perturbations in hydration status that occur in different environments.[1,34] This approach assumes that 1 mL of sweat loss represents a 1-g loss in body weight (i.e., specific gravity of sweat is 1.0 g·mL−1). The before-exercise BW measures are used with the postexercise BW corrected for urine losses and drink volume. When possible, nude weights should be used to avoid corrections for sweat trapped in the clothing. Other nonsweat factors contributing to BW loss during exercise include respiratory water and carbon exchange. Ignoring those two factors will over estimate sweat rate modestly (~5–15%) but generally do not require correction for exercise durations < 3 h. If proper controls are made, BW changes can provide a sensitive estimate of acute TBW changes to access hydration changes during exercise.
Evidence Statement. Individuals can monitor their hydration status by employing simple urine and body weight measurements. Evidence Category B. An individual with a first morning USG ≤ 1.020 or UOsmol ≤ 700 mOsmol·kg−1 can be considered as euhydrated. Evidence Category B. Several days of first morning body weights can be used to establish base-line body weights that represent euhydration. Evidence Category B. Body weight changes can reflect sweat losses during exercise and can be used to calculate individual fluid replacement needs for specific exercise and environmental conditions. Evidence Category A.
Cite this: Exercise and Fluid Replacement - Medscape - Feb 01, 2007.