ABSTRACT: BACKGROUND: Rapid fluid delivery from ingested beverages is the goal of oral rehydration solutions (ORS) and sports drinks. OBJECTIVE: The aim of the present study was to investigate the effects of increasing carbohydrate and sodium content upon fluid delivery using a deuterium oxide (D2O) tracer. DESIGN: Twenty healthy male subjects were divided into two groups of 10, the first group was a carbohydrate group (CHO) and the second a sodium group (Na). The CHO group ingested four different drinks with a stepped increase of 3% glucose from 0% to 9% while sodium concentration was 20 mmol/L. The Na group ingested four drinks with a stepped increase of 20 mmol/L from 0 mmol/L to 60 mmol/l while glucose concentration was 6%. All beverages contained 3 g of D2O. Subjects remained seated for two hours after ingestion of the experimental beverage, with blood taken every 5 min in the first hour and every 10 min in the second hour. RESULTS: Including 3% glucose in the beverage led to a significantly greater AUC 60 min (19640 +/- 1252 delta per thousand vs. VSMOW.60 min) than all trials. No carbohydrate (18381 +/- 1198 delta per thousand vs. VSMOW.60 min) had a greater AUC 60 min than a 6% (16088 +/- 1359 delta per thousand vs. VSMOW.60 min) and 9% beverage (13134 +/- 1115 delta per thousand vs. VSMOW.60 min); the 6% beverage had a significantly greater AUC 60 min than the 9% beverage. There was no difference in fluid delivery between the different sodium beverages. CONCLUSION: In conclusion the present study showed that when carbohydrate concentration in an ingested beverage was increased above 6% fluid delivery was compromised. However, increasing the amount of sodium (0-60 mmol/L) in a 6% glucose beverage did not lead to increases in fluid delivery.