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@hippy
No they don't.
**Sources and Composition of Fluid Loss **
Body water loss can be categorized as insensible or sensible. Insensible water loss is evaporative water loss through respiration and water diffusion through the skin. Under normal conditions, respiratory water loss is approximately 250 to 350 mL/d (229) but can be higher in dry climates and when respiration rate is increased, such as during exercise or at altitude. Loss of water via diffusion through the skin is approximately 450 mL/d (275). All other water loss, including urination and thermoregulatory sweating is termed sensible because the person is aware of the loss as it is occurring. Urine production is the main avenue of body water loss under normal circumstances. Renal fluid output can vary considerably, as it is primarily a function of water intake. Obligatory urine loss (needed to excrete end products, such as creatinine and urea; and needed to excrete electrolytes to maintain electrolyte balance) is approximately 500 mL/d (5). However, urine output generally averages 1 to 2 L/d. Other factors that affect urine output include exercise, heat stress (decrease loss), cold, and altitude exposure (increase loss). Gastrointestinal (fecal) water loss in a healthy adult is small, approximately 100 to 200 mL/d (345). Table 2 lists estimated minimum daily water losses through respiration, urination, the gastrointestinal tract, and the skin.
Table 2. Estimation of Minimum Daily Water Losses and Production* ReferenceSourceLoss (mL/d)Production (mL/d)
Reprinted (with permission) from Institute of Medicine (239).
Hoyt and Honig (229)Respiratory loss250 to 350 Adolph (5)Urinary loss500 to 1000 Newburgh et al. (345)Fecal loss100 to 200 Kuno (275)Insensible loss450 to 1900 Hoyt and Honig (229)Metabolic production 250 to 350 Total1300 to 3450250 to 350 Net loss1050 to 3100
During exercise and/or exposure to a hot environment thermoregulatory sweat is the main source of water loss from the body. Evaporation of sweat secreted onto the skin surface by eccrine sweat glands is the primary avenue of heat loss during exercise and/or heat stress. Radiation (heat exchange between the body and the environment in the form of infrared rays), conduction (transfer of heat to or from the body through direct contact with an object), and convection [heat exchange between the body and surrounding moving air (wind) or body fluids (blood)] are other potential avenues of heat loss. However, when ambient temperature is greater than skin temperature, evaporation of sweat is the only means of body heat loss; which is important to attenuate the increase in body core temperature. With sweating, heat is transferred from the body to water (sweat) on the surface of the skin. When this water gains sufficient heat, it is converted to a gas (water vapor), thereby removing heat from the body. Evaporation of 1 kg of sweat from the skin will remove 580 kcal of heat from the body (508). It is important to note that sweat dripping from the body is wasted water loss because sweat must evaporate to allow effective cooling.
Metabolic heat production is directly proportional to exercise intensity. When exercise is performed, a large amount of heat is produced by the contracting muscles. In fact, less than 25% of all the energy produced by contracting muscles is used to perform work, with the remaining 75% converted to heat in the muscles. Thus sweating rates increase in proportion to work intensity. However, heat acclimatization, higher fitness levels, clothing, and higher ambient temperatures also increase an individual's sweating rate. By contrast, wet skin (from high humidity) can reduce sweating rate (189, 418, 419). Figure 3 illustrates predicted daily water requirements as a function of daily energy expenditure and air temperature.
Because sweating plays a critical role in attenuating increases in body core temperature, it is apparent that sufficient hydration (via drinking) is needed to maintain sweating, especially in extreme heat (7). Early observations have shown that if water supply is adequate, healthy humans can withstand and even thrive in extremely hot environments (provided that evaporation of sweat is not impeded by high ambient humidity or by the wearing of impermeable clothing) (7, 233). For instance, in 1910, healthy European men, observed during a very hot (≥45°C) and dry spell of weather, consumed ≥13.6 L of water per day and were able to walk and perform a considerable amount of physical exercise without difficulty (233).
In general, women typically exhibit lower sweating rates than men, primarily due to smaller body mass and lower metabolic rate achieved during activity (414). However, when expressed relative to body surface area, mean sweating rates are similar between sexes in temperate and hot-dry conditions (429). In hot-wet conditions, however, sweating rate per m2 surface area is lower in women than men (30, 429). The greater suppression of sweat in response to wetted skin results in less wasted sweat. By having a lower sweating rate in this condition women are losing less fluid and therefore minimizing hypohydration. On the other hand, men drip more sweat from their bodies (i.e., wasted water since it is not readily evaporated and does not contribute significantly to cooling) in humid conditions and become more dehydrated (30, 417, 429). However at the same level of hypohydration (5%), women exhibit similar physiological responses (e.g., increases in heart rate and body core temperature) to exercise-heat stress in both hot-dry and hot-wet conditions compared to men when matched for age, fitness, and percent body fat (417).
The loss of water due to thermoregulatory sweating is accompanied by loss of electrolytes. Sodium is the predominant electrolyte lost in sweat. The total amount of sodium lost depends on sweating rate and duration as well as sweat sodium concentration. Average sweat sodium concentration measured using the “gold standard” whole-body washdown procedure has been reported to be approximately 40 mmol/L, but ranges from as low as 15 mmol/L to as much as 90 mmol/L (35). Even those athletes with low or average sweat sodium concentration, can accrue a substantial sodium deficit by virtue of large sweat losses due to high sweating rates (≥2 L/h) or extended periods of strenuous exercise (two-a-day practices or ultraendurance events). The wide range is a result of the myriad of factors that influence sweat sodium concentration, including genetics, diet, heat acclimatization status, sweating rate, and hydration status (414). Sodium and chloride are reabsorbed in the duct of the sweat gland, thus sweat sodium concentration is lower than that of plasma. However, as sweating rate increases, the sodium secretion rate increases proportionally more than the rate of sodium reabsorption, thus sweat sodium concentration increases linearly with increases in sweating rate (68). Heat acclimatization improves sodium chloride reabsorption, thus resulting in lower sodium chloride concentration (>50% reduction) for any given sweating rate (10). Studies have found that moderate (∼3.5-4 g/d) to high dietary sodium (∼8-9 g/d) intake results in significantly higher sweat sodium concentration compared to that of low sodium diets (∼1-2 g/d) (11, 23, 221). Increased sweat sodium chloride concentration can also result from hypohydration (330), but neither sex nor aging seem to have a significant effect (331). Figure 4 illustrates predicted daily sodium requirements as a function of daily energy expenditure and air temperature. In addition to sodium, several other electrolytes are lost in sweat. These include, but are not limited to, chloride (∼30 mmol/L), potassium (∼5 mmol/L), calcium (∼0.5 mmol/L), and magnesium (∼0.1 mmol/L) (152, 414).
There are primarily two different types of hypohydration, depending on the route of water loss and the amount of osmolytes (electrolytes) lost in association with the water. Isoosmotic hypohydration (isotonic hypovolemia) occurs when fluid loss is iso-osmotic with plasma, that is, loss of water and osmolytes occurs in equal proportions. This type of body water deficit is associated with fluid losses induced by cold, altitude, diuretics, and secretory diarrhea. For example, fluid (urine) losses induced by administration of a diuretic, such as furosemide, will cause the intravascular and interstitial fluid compartments to decrease proportionally. Thus the end result is a state of hypohydration with no change in plasma osmolality. On the other hand, hypohydration induced by sweat loss due to exercise and/or heat stress results in a decrease in the extracellular compartment size and an increase in plasma osmolality (1, 273). This is because sweat is hypotonic compared to the plasma. An increase in plasma osmolality initiates fluid movement from the cellular compartment into the plasma to maintain osmotic balance. This results in cellular hypohydration, that is, cell shrinkage and hypertonicity. This is known as hyperosmotic hypohydration and occurs when loss of water is greater than the loss of osmolytes. Hyperosmotic hypohydration can also occur as a result of osmotic diarrhea. The effect on blood osmolality has important implications to cardiovascular and thermoregulatory physiology because hyperosmolality increases the temperature threshold for sweating and cutaneous vasodilation during exercise in the heat (172, 339).