13C-labelled glucose-fructose show greater exogenous and whole-body CHO oxidation and lower O2 cost of running at 120 versus 60 and 90 g·h-1 in elite male marathoners.

We evaluated the effects of carbohydrate (CHO) ingestion at rates of 60 (maltodextrin:fructose ratio 1:0), 90 (2:1), and 120 (1:1) g·h-1 on whole body substrate metabolism, exogenous CHO oxidation (via U-13C enriched glucose-fructose drinks) and gastrointestinal (GI) symptoms in elite male marathon runners [n = 8; marathon personal best (PB), 02:22:54 ± 00:05:37]. After 24 h of a high-CHO (8 g·kg-1) diet and pre-exercise meal (2 g·kg-1), participants completed 120-min running trials comprising 15 min at 95% lactate threshold (LT), 90 min at 94% lactate turnpoint, and a final 15 min at 95% LT. Mean whole-body CHO oxidation (120 g·h-1, 3.07 ± 0.54; 90 g·h-1, 2.46 ± 0.34; 60 g·h-1, 2.09 ± 0.09 g·min-1) and hour 2 mean exogenous CHO oxidation (120 g·h-1, 1.68 ± 0.16; 90 g·h-1, 1.31 ± 0.18; 60 g·h-1, 0.89 ± 0.11 g·min-1) were different between all trials (P < 0.01 for all pairwise comparisons), such that 120 g·h-1 > 90 g·h-1 > 60 g·h-1. Running economy was improved in the 120 g·h-1 condition, with a 8.1 O2·kg-1·km-1 lower O2 cost compared with 60 g·h-1 (P = 0.039). The incidence of moderate or severe (≥4) GI symptoms was high in all trials, though peak symptoms of nausea, stomach fullness, and abdominal cramps were greatest for 120 g·h-1. We report for the first time that CHO ingestion at 120 g·h-1 confers a metabolic advantage to male marathoners by better maintaining whole body rates of CHO oxidation, increasing exogenous CHO oxidation, and improving running economy. However, gut training strategies, preceding practical application, are warranted.NEW & NOTEWORTHY We used stable isotope methodology to evaluate exogenous rates of CHO oxidation in elite male marathoners. We report that 120 (maltodextrin:fructose ratio 1:1) g·h-1 CHO induces greater whole body and exogenous CHO oxidation compared with 60 (1:0)-90 (2:1) g·h-1 and reduces the O2 cost of running. However, the performance implications of such doses remain to be determined. The prevalence of GI symptoms across the doses suggests targeted fueling practice and gut training is warranted.