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Abstract Background Body-fluid loss during prolonged continuous exercise can impair cardiovascular function, harming performance. Delta percent plasma volume ( d PV) represents the change in central and circulatory body-water volume and therefore hydration during exercise; however, the effect of carbohydrate–electrolyte drinks and water on the d PV response is unclear. Objective To determine by meta-analysis the effects of ingested hypertonic (> 300 mOsmol kg −1 ), isotonic (275–300 mOsmol kg −1 ) and hypotonic (< 275 mOsmol kg −1 ) drinks containing carbohydrate and electrolyte (Na + < 50 mmol L −1 ), and non-carbohydrate drinks/water (< 40 mOsmol kg −1 ) on d PV during continuous exercise. Methods A systematic review produced 28 qualifying studies and 68 drink treatment effects. Random-effects meta-analyses with repeated measures provided estimates of effects and probability of superiority ( p + ) during 0–180 min of exercise, adjusted for drink osmolality, ingestion rate, metabolic rate and a weakly informative Bayesian prior. Results Mean drink effects on d PV were: hypertonic − 7.4% 90% compatibility limits (CL) − 8.5, − 6.3, isotonic − 8.7% (90% CL − 10.1, − 7.4), hypotonic − 6.3% (90% CL − 7.4, − 5.3) and water − 7.5% (90% CL − 8.5, − 6.4). Posterior contrast estimates relative to the smallest important effect ( d PV = 0.75%) were: hypertonic-isotonic 1.2% (90% CL − 0.1, 2.6; p + = 0.74), hypotonic-isotonic 2.3% (90% CL 1.1, 3.5; p + = 0.984), water-isotonic 1.3% (90% CL 0.0, 2.5; p + = 0.76), hypotonic-hypertonic 1.1% (90% CL 0.1, 2.1; p + = 0.71), hypertonic-water 0.1% (90% CL − 0.8, 1.0; p + = 0.12) and hypotonic-water 1.1% (90% CL 0.1, 2.0; p + = 0.72). Thus, hypotonic drinks were very likely superior to isotonic and likely superior to hypertonic and water. Metabolic rate, ingestion rate, carbohydrate characteristics and electrolyte concentration were generally substantial modifiers of d PV. Conclusion Hypotonic carbohydrate–electrolyte drinks ingested continuously during exercise provide the greatest benefit to hydration. Graphical abstract
Rowlands et al. (2021) studied this question.