Plasma volume changes in middle-aged male and female subjects during marathon running

Abstract

Circulatory fluid shifts were studied in middle-aged runners (6 males and 5 females, ages 32-58 yr) during a 42.2-km marathon race run in mild weather (dry-bulb temperature = 17.5-20.4 degrees C). Running times for the subjects were 3:12-4:40 (mean values were 3:34 for males and 4:10 for females). Venous blood samples were taken without stasis in all subjects seated at rest before the start of the race and within 3 min of finishing; eight of the subjects also paused for samples at 6 and 27 km during the race. At 6 km, body weight loss averaged less than 1%, whereas plasma volume (PV) had decreased by 6.5% in male subjects and 8.6% in female subjects. By the end of the race, hypohydration had reached 3.2% in male subjects and 2.9% in female subjects, but PV in both groups remained stable. Sweat rates during the race averaged 545 and 429 g X m-2 X h-1 for male and female subjects, respectively, with ad lib. water intake replacing 21-72% of fluid loss. Increases in plasma protein concentration throughout the race reflected the observed initial decrease in PV. The interpretation of PV responses to exercise and/or hypohydration is critically dependent on selection of base-line conditions; we were able to control for posture-exercise effects by treating the early exercise (6 km) sample as the base line for examining the effects of later fluid loss. Under these conditions, the vascular compartment resisted volume depletion. The ability to maintain stable PV can be explained in part by relationships among oncotic and hydrostatic pressures in the intra- and extravascular fluid compartments.

Ad libitum fluid intakes and thermoregulatory responses of female distance runners in three environments

“…The women in this and other studies (Christensen and Ruhling, 1980;Myhre et al, 1985) also experienced a plateau in rectal temperature after ~30 min of exercise when drinking ad libitum. Montain and Coyle (1992) demonstrated that replacing ~80% of sweat losses in men was adequate to maintain a plateau in rectal temperature beyond ~1 h of strenuous exercise.…”

“…The present ® ndings equate to intakes of 8± 13 ml´kg -1´h-1 (Table 1) without a single incidence of adverse gastrointestinal symptomatology. Fluid intakes were therefore generally higher than previously reported for female distance runners under similar conditions (Myhre et al, 1982(Myhre et al, , 1985Millard-Staþ ord et al, 1995). This may be the result of habitual entrained ¯uid intake practices of marathon runners who reside in warm climates, or possibly `dry mouth' (Greenleaf, 1992) resulting from the gas exchange measurements that immediately preceded the simulated ¯uid stations.…”

“…However, core temperature was only monitored continuously in one of the runners. Rectal temperature rose to 38.7°C by 40 min into the race, but remained between 38 and 39°C throughout the remaining 200 min of the run (Myhre et al, 1985). A plateau in rectal temperature 20 min into a marathon was also maintained by a female runner competing at a mean environmental temperature of ~17°C (Christensen and Ruhling, 1980).…”

“…In one study, ® ve female runners had sweat rates of 0.67 l´h -1 , ad libitum uid intakes of 0.28 l´h -1 (42% of sweat loss) and were dehydrated by 2.9% of body mass after a mild weather (~19°C) marathon (Myhre et al, 1985). However, core temperature was only monitored continuously in one of the runners.…”

Influence of Carbohydrate-Electrolyte Drinks on Muscle Metabolism and Endurance Running Performance in Man

“…Despite the relatively severe dehydration induced by the marathon in this study (5.5% weight loss), core temperature was maintained within expected range (36.6 -38.9 CC). This is consistent with previous studies (Pugh et al 1967;Maughan et al 1985;Myhre et al 1985;Noakes et al 1988). The rise in core temperature is directly related to the metabolic rate and thus running speed sustained during prolonged exercise (Maughan et al 1985;Noakes et al 1991a).…”