Glycaemic Impact of Low‐ and High‐Glycaemic Index Carbohydrate Diets in Ultra‐Endurance Athletes: Insights From Continuous Glucose Monitoring

Nine ultra-endurance athletes completed a randomised, crossover trial involving two 28-day dietary arms during which the athletes consumed a carbohydrate-rich diet (carbohydrate 58 ± 3, protein 15 ± 2 and fat 26 ± 2%) containing low- or high-glycaemic-index (LGI or HGI, respectively) carbohydrates. At the start and end of each dietary arm, participants performed a fasted 3-h submaximal run outdoors before ingesting either a low (GI = 32, isomaltulose [Palatinose]) or high (GI = 100, maltodextrin) glycaemic index drink (0.75 g/kg bm/h over 3.5 h). Participants then completed a treadmill run to exhaustion at 74 ± 1% v <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow><mml:mover><mml:mi>V</mml:mi> <mml:mo>˙</mml:mo></mml:mover> </mml:mrow> <mml:annotation>$\dot{\mathrm{V}}$</mml:annotation></mml:semantics> </mml:math> O_2peak, with pulmonary gas exchange measured over the first hour. Interstitial glucose [iG] was measured via continuous glucose monitoring (Supersapiens, Atlanta, USA). Data were analysed ANOVA and post hoc t-tests with Bonferroni adjustment as appropriate, with p ≤ 0.05 accepted as significant. Mean 24-h [iG] was similar between diets (LGI:102 ± 5 vs. HGI:100 ± 5 mg/dL). [iG] variability measures, including standard deviation (LGI:17 ± 1 vs. HGI:18 ± 2 mg/dL, p = 0.016) and coefficient of variation (LGI:16 ± 1% vs. HGI:18 ± 1%, p = 0.0003), were lower in the LGI diet, with a reduced percentage of time spent below the recommended range (LGI 2 ± 1% vs. HGI 4 ± 2%, p = 0.006. Level 1 [55-69 mg/dL] LGI 1 ± 1% vs. HGI 3 ± 2, p = 0.005). Carbohydrate oxidation during the first hour of the run test was reduced in the LGI diet arm (ΔLGI -0.14 ± 0.32 vs. ΔHGI 0.06 ± 0.28 g·min^-1, p = 0.016) but endurance capacity was similar across diets. Adopting a 28-day LGI carbohydrate-rich diet and incorporating isomaltulose improved glycaemic variability and reduced time spent below the target glycaemic range with evidence of similar endurance performance capability when compared to a HGI carbohydrate-rich diet.