Sodium Free Fluid Ingestion Decreases Plasma Sodium During Exercise in the Heat

Abstract

assessed whether replacing sweat losses with sodium-free fluid can lower the plasma sodium concentration and thereby precipitate the development of hyponatremia. Ten male endurance athletes participated in one 1-h exercise pretrial to estimate fluid needs and two 3-h experimental trials on a cycle ergometer at 55% of maximum O 2 consumption at 34°C and 65% relative humidity. In the experimental trials, fluid loss was replaced by distilled water (W) or a sodiumcontaining (18 mmol/l) sports drink, Gatorade (G). Six subjects did not complete 3 h in trial W, and four did not complete 3 h in trial G. The rate of change in plasma sodium concentration in all subjects, regardless of exercise time completed, was greater with W than with G (Ϫ2.48 Ϯ 2.25 vs. Ϫ0.86 Ϯ 1.61 mmol • l Ϫ1 •h Ϫ1 , P ϭ 0.0198). One subject developed hyponatremia (plasma sodium 128 mmol/l) at exhaustion (2.5 h) in the W trial. A decrease in sodium concentration was correlated with decreased exercise time (R ϭ 0.674; P ϭ 0.022). A lower rate of urine production correlated with a greater rate of sodium decrease (R ϭ Ϫ0.478; P ϭ 0.0447). Sweat production was not significantly correlated with plasma sodium reduction. The results show that decreased plasma sodium concentration can result from replacement of sweat losses with plain W, when sweat losses are large, and can precipitate the development of hyponatremia, particularly in individuals who have a decreased urine production during exercise. Exercise performance is also reduced with a decrease in plasma sodium concentration. We, therefore, recommend consumption of a sodium-containing beverage to compensate for large sweat losses incurred during exercise. hyponatremia; electrolytes; fluid balance EXERCISE-INDUCED HYPONATREMIA (a plasma sodium concentration of Ͻ130 mmol/l, normal range 135-146 mmol/l) has been observed with increasing frequency during the last decade. Hyponatremia has been reported to occur in athletes during or after extraordinary physical efforts, especially in the heat, such as ultramarathons and ironman triathlons (7, 11, 12, 17, 18) and in the marathon (15, 23). Hyponatremia has also been reported with less strenuous exercise like a hike or a short march (24). The incidence of hyponatremia has been reported to be from 0 to 29% (18, 21). A high proportion of collapsed runners (9%) have, however, been found to have hyponatremia (18). As a consequence, it has been stated that hyponatremia, and not dehydration, has become the

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

“…Our major finding was that under environmental conditions in which fluid intake matched body mass loss, even a moderate amount of sodium added to the hydration solution was adequate to attenuate the decline in sodium concentration of plasma and to preserve plasma volume at pre-exercise levels. Our results regarding sodium concentrations are in agreement with those of Vrijens and Rehrer, 17 who showed that a low-sodium drink prevented the decline in plasma sodium concentration observed when a sodium-free fluid is ingested. In both their study and ours, fluids were ingested at a rate similar to the rate of water loss.…”

Fluid and fuel intake during exercise

“…A striking example of this imbalance is seen when a low serum sodium concentration (hyponatraemia below 130 mmol × l 71 ) causes fluid movement into the brain, causing swelling with symptoms that can progress from feeling strange, to mental confusion, general weakness, collapse, seizure, coma and death. Because the sodium concentration in sweat is much lower than that of plasma, the primary cause of hyponatraemia is dilution of body water by drinking a large volume of low sodium-containing fluid over several hours, and it can be exacerbated by also losing large amounts of sodium in sweat (Vrijens and Rehrer, 1999;Montain et al, 2001;O'Brien et al, 2001;Rehrer, 2001).…”

Fluid and fuel intake during exercise

“…A serious although infrequent problem is the development of hyponatraemia, especially in runners and walkers who drink excessively large volumes of low sodium fluids throughout prolonged exercise (Noakes et al, 1985;Barr and Costill, 1989;Vrijens and Rehrer, 1999;Montain et al, 2001;O'Brien et al, 2001;Speedy et al, 2001). Fast drinking in these slow-moving individuals (4-6 hour marathon times) causes them to gain significant body water over the course of the marathon distance because fluid replacement is far in excess of sweating rate.…”