Influence of rehydration on short-term recovery from prolonged running and subsequent exercise capacity in humans

The aim of this research was to investigate the influence of rehydration with carbohydrate-electrolyte solutions, during a short-tern recovery period, on hydration status, physiological responses, and subsequent endurance capacity. The first study (Chapter 4) examined whether prescribed or ad libitum rehydration with a carbohydrate-electrolyte solution (CHO-E), during 4 h recovery from prolonged, submaximal running would influence the subsequent endurance capacity. Five women and two men performed the "recovery" protocol consisting of a 90 min run at 70%VO2 max on a level treadmill (TI) followed by 4 h rehydration-recovery (REC), and then an open-ended run to exhaustion at 70%VD2 max (1'2) as a measure of their endurance capacity, on two occasions, at least 7 days apart. During the REC, subjects were allowed to drink a 6.9% CHO-E ad libitum (AL) on one occasion. On the other occasion, the volume of the same fluid was prescribed (PI) from calculations of the body mass lost during TI. During T2, in the PI trial, the run time to exhaustion was 16% longer (P < 0.05) than during T2 in the AL nial (69.9 ± 9.1 vs. 60.2 ± 10.2 min). Thus, ingestion of a prescribed volume of CHO-E after prolonged exercise, calculated to replace the body fluid losses, restored endurance capacity to a greater extent than ad libitum rehydration during the REC. The second study (Chapter 5) investigated the influence of ingesting 50 g of carbohydrate (CHO) immediately after exercise, either with subsequent serial CHO feeding or water ingestion during the REC from prolonged, submaximal running on rehydration and subsequent endurance capacity. Eight male subjects performed the "recovery" protocol [i.e. 90 min run at 70% V02 max (TI), 4 h rehydration-recovery (REC), and open-ended run at 70% V02 max (T2)] on two occasions. During the REC, subjects ingested a prescribed volume of fluid equal to the body mass lost during TI in both conditions. Subjects ingested 50 g of CHO from a 6.9% CHO-E 15 min after TI on both occasions as their first prescribed fluid intake. Thereafter, subjects drank either the same solution (CE) or water CW) at each hour after TI during the REC. During T2, the run time to exhaustion was 54.2 ± 9.2 min in the CE trial and 52.2 ± 6.2 min in the W trial, respectively (NS). The volume of fluid retained expressed as a percentage of the volume ingested (% rehydration) during the CE trial was greater than that of the W trial (CE: 73.5 ± 4.2% vs. W: 63.0 ± 5.7%; P < 0.05). Serial CHO feeding during the REC was associated with increased CHO oxidation and suppressed fat oxidation during subsequent exercise. Thus, ingesting -150 g of CHO in a 6.9% CHO-E over a 4 h period following prolonged running is more effective in terms of rehydration compared to the same volume of fluid containing only 50 g of CHO and water, but does not have a greater effect on subsequent endurance capacity. The third study (Chapter 6) investigated the effects of rehydration per se and CHO ingestion, during the REC, on subsequent endurance capacity. Nine male subjects performed the "recovery" protocol on two occasions. During the REC, subjects drank either a 6.9% CHO-E (CE) or a CHO-free sweetened placebo (PL) every 30 min after Tl up to the beginning of the 4 h of the REC. Volumes prescribed (ml) were equal to 200% of the body mass lost during Tl. However, the total volume of fluid ingested during the REC was only 170.8 ± 12.6% and 172.6 ± 13.8% of the body mass lost after Tl (NS). During T2, in the CE trial, the run time to exhaustion was 54% longer (P < 0.01) than during T2 in the PL trial (69.3 ± 5.5 vs. 45.0 ± 4.2 min). After the REC, subjects were in positive fluid balance by 423 ± 215 ml in the CE trial and 446 ± 239 ml in the PL trial (NS). Thus, positive fluid balance can be achieved by ingesting a prescribed volume of either a 6.9% CHO-E or a placebo solution over the REC, calculated to replace approximately 170% of the body fluid loss. Despite this similar hydration status after the recovery in both conditions, ingesting a CHO-E is more effective in restoring endurance capacity compared to the same volume of placebo solution. The fourth study (Chapter 7) was intended to examine, and verify, the effects of ingesting different amounts of CHO in the form of a CHO-E during the REC on rehydration and subsequent endurance capacity. Nine male subjects performed the "recovery" protocol on two occasions. During the REC, a fixed volume of fluid equivalent to 150% of the body mass lost during Tl was consumed. Subjects ingested 50 g of CHO from a 6.5% CHO-E 30 min after Tl on both occasions as their first prescribed fluid intake. Thereafter, subjects ingested either the same solution (CE) or a CHO-free sweetened placebo (PL) every 30 min up to the beginning of the 4 h of the REC. During T2, the run times were 56.9 ± 8.1 min in the CE trial and 65.4 ± 7.8 min in the PL trial (NS). After the REC, subjects were almost equally euhydrated (CE: 0 ± 184 ml; PL: -27 ± 120 ml) in both conditions (NS). Serial CHO feeding over the REC was accompanied by enhanced CHO oxidation and suppressed fat oxidation. In conclusion, ingesting a placebo solution containing 50 g of CHO and placebo over a 4 h period following prolonged running, calculated to replace 150% of the body fluid loss, is equally effective in achieving approximate euhydration and restoring endurance capacity compared to the same volume of CHO-E containing -167 g of CHO. The studies reported in this thesis suggest that in order to achieve euhydration during recovery, a volume of fluid substantially larger (~ 150%) than that lost must be ingested. The provision of additional CHO (-150 to 170 g) would be expected to restore the body's CHO stores to a greater extent than a smaller amount of CHO (50 g) during the REC and, thereby, improve the subsequent endurance capacity. However, this was not the case. It appears that the ingestion of large amounts of CHO, during the REC, resulted in disturbances in fat and CHO metabolism which prevented an improvement in endurance capacity during T2, after consumption of the additional CHO.