Sodium replacement and fluid shifts during prolonged exercise in humans

Abstract

In the study presented here, we examined the affects of a close to complete replacement of sweat water and Na+ losses on fluid shifts during exercise. Six cyclists performed three 4-h rides at 55% of their peak oxygen uptake in a 20 degrees C environment while consuming 3.85 l of an 8% carbohydrate solution containing 5, 50 or 100 mEq.l-1 of Na+. Increases in Na+ intake reduced renal free water clearance from around 40 ml.h-1 to -8 and -121 ml.h-1 and led to a decrease in urine volume from approximately equal to 1.0 to 0.5 l (P < 0.05). In contrast, the 3.5-3.9 l fluid and 150-190 mEq Na+ losses in sweat were similar in each trial, as were the approximately equal to 80 mEq K+ losses in sweat and urine and the 282-288 mosmol.kg-1 plasma osmolalities. During the low-Na+ trial, plasma osmolality was maintained by a approximately equal to 1.3 l contraction of extracellular fluid (ECF) with the loss of approximately equal to 200 mEq Na+. However, in the other trials, approximately equal to 1.3 l of water was lost from the intracellular fluid. During the medium-Na+ trial, a loss of only approximately equal to 40 mEq Na+ maintained ECF volume, and during the high-Na+ trial, a gain of approximately equal to 160 mEq Na+ expanded the ECF by approximately equal to 0.8 l. However, corresponding changes in plasma volumes from -0.20 to 0.15 l had no effect on cardiovascular drift or thermoregulation. These data suggest that during prolonged exercise of moderate intensity under mild environmental conditions when sweat rates are approximately equal to 0.9 l.h-1, complete Na+ replacement maintains plasma volume and reduces dehydration, but when fluid intake matches sweat rate, has little effect on plasma osmolality.

Effect Of Physical Exercises On Serum Electrolyte

“…The findings in this work revealed that during exercise individual is very likely to experience a reduction in serum sodium concentration.A low in serum sodium concentration(<130mmol/L) is called hyponatramia (Tom,2003).The reduction in serum sodium level during exercise can be attributed to the loss of sodium in sweat during exercise.This is in accordance with the finding of Sanders et al(2001).However,it is expected that if the exercise duration and intensity is extended,and sweating increased,there will be increase in serum sodium level because of hydration and decrease blood volumes.Since the subjects were not taking any fluid during the exercise duration.…”

Sodium Replacement and Plasma Sodium Drop During Exercise in the Heat When Fluid Intake Matches Fluid Loss

“…We have no apparent explanation for this discrepancy, but it may reflect differences in the exercise protocol between the studies. Nevertheless, the conservation effect of sodium supplementation in plasma volume has also been confirmed in another study by Sanders et al 27 in which fluid consumption equaled fluid loss during prolonged exercise of moderate intensity. Specifically, they showed that, although a drink of 5 mmol/L sodium decreased extracellular fluid, a drink containing 50 mmol/ L sodium stabilized extracellular fluids.…”

“…7,18,27 In all these studies, drinking sodium-free solutions caused a marked decrease in plasma volume that was possibly more profound than sodium losses, resulting in normal blood sodium during exercise. When sodium was also provided, the conservation of plasma volume prevented an increase in blood sodium.…”

“…However, because sodium losses in the urine and sweat were not measured, definite conclusions cannot be drawn. Sanders et al 27 showed that although a solution of 50 mmol/L sodium did not suppress urine volume during prolonged exercise, it did increase overall sodium loss in the urine. They also found that sodium excretion in sweat was insensitive to sodium supplementation even at relatively high sodium intakes.…”

Athletic Participation by Children and Adolescents Who Have Systemic Hypertension

“…5,6 Accordingly, rehydration often requires deliberate concomitant intake of additional saltcontaining fluids and foods to ensure greater body-water retention and distribution to all fluid compartments. [7][8][9][10] Athletes should be advised that the use of diuretic medications and ␤ blockers, which are commonly used to treat hypertension, have been prohibited by some athletic governing bodies. These drugs can also decrease athletic performance in certain individuals.…”