Timing of Sodium Bicarbonate Uptake During Exercise and Rest in Highly Trained Cyclists

Sodium bicarbonate (SB) has been extensively studied for its ergogenic effects in short-duration, high-intensity exercise (Grgic et al., 2021). More recently, SB has been increasingly used in endurance disciplines such as triathlon, road cycling, and cross-country skiing, to enhance performance during decisive, high-intensity efforts (Dalle et al., 2021). However, the influence of prolonged low-intensity exercise following SB ingestion on blood bicarbonate (HCO₃) uptake kinetics remains unclear. Therefore, this study aimed to assess the effects of low-intensity cycling compared with passive rest following SB ingestion on HCO₃ kinetics, including the timing and magnitude of peak HCO₃. Fourteen highly trained cyclists (female n = 5: body mass 53.0 ± 4.4 kg, height 169 ± 6 cm, VO2max 64.7 ± 4.1 mL·kg⁻¹·min⁻¹; male n = 9: body mass 70.9 ± 3.3 kg, height 180 ± 6 cm, VO2max 74.5 ± 7.7 mL·kg⁻¹·min⁻¹) completed three laboratory visits separated by ≥48 h. Visit one involved an incremental step and ramp test to determine lactate threshold 1 (LT1) and VO2max. On visits two and three, participants completed the exercise (EXC) and resting (REST) conditions, with identical time points for data collection. In both conditions, participants consumed a standardized breakfast at 08:00 AM, followed by SB ingestion (0.3 g·kg⁻¹) provided as mini-tablets within a carbohydrate hydrogel mixture at 08:45 AM. During EXC, participants cycled indoors for three hours at LT1 (3.0 ± 0.3 W·kg⁻¹) starting at 10:00 AM, with 50 g·h⁻¹ carbohydrate intake. During REST, participants refrained from exercise and food intake. In both conditions, HCO₃ was measured every 25 min from 08:45 to 10:00, and every 30 min from 10:00 AM to 1:00 PM using a blood gas analyzer (i-STAT Alinity, Abbott, USA). All data were assessed for normality using the Shapiro–Wilk test. Differences in HCO₃ time course were analyzed with repeated measures ANOVA, correcting for sphericity violations with Greenhouse–Geisser when necessary. Post-hoc pairwise comparisons were Bonferroni-adjusted. Effect sizes were calculated using partial eta squared (pη²). Peaks in HCO₃ were identified as the first point within ±0.6 mmol·L⁻¹ of the local maximum, corresponding to a 2% technical error of measurement at 30 mmol·L⁻¹ (Larcher et al., 2023), thereby capturing the initial plateau while minimizing measurement noise. Differences between peak and baseline (08:45 AM) concentrations (ΔHCO₃) and time to peak were compared between conditions using paired t-tests. All participants completed the protocol without adverse events, such as gastrointestinal problems. Data were normally distributed. A significant condition × time interaction was observed (F(9,117) = 5.36, p < 0.001, pη² = 0.29), indicating that the temporal pattern of HCO₃ differed between EXC and REST. Pairwise comparisons revealed higher HCO₃ concentrations during REST compared with EXC at 30 min (mean diff. = 1.2 ± 1.2 mmol·L⁻¹, p = 0.003) and 60 min (mean diff. = 2.9 ± 2.6 mmol·L⁻¹, p = 0.001) after 10:00 AM (exercise onset in EXC). No differences were observed at any other time point (all p > 0.05). A significant main effect of time was detected (F(9,117) = 197, p < 0.001, pη² = 0.94), reflecting the expected increase in HCO₃ following SB ingestion. However, there was no significant main effect of condition (F(1,13) = 3.95, p = 0.068, pη² = 0.23), indicating similar overall mean HCO₃ concentrations between EXC and REST. ΔHCO₃ did not differ between conditions (EXC: 9.0 ± 1.8 mmol·L⁻¹, CV 20%; REST: 9.3 ± 1.4 mmol·L⁻¹, CV 15%; p = 0.32). In contrast, time to peak HCO₃ was significantly longer during EXC (208 ± 42 min, CV 20%) compared with REST (146 ± 22 min, CV 15%; p < 0.001). Descriptively, responses were similar between sexes, with comparable ΔHCO₃ and time to peak HCO3 in females (EXC: 9.1 ± 1.9 mmol·L⁻¹; 213 ± 46 min; REST: 9.6 ± 1.5 mmol·L⁻¹; 147 ± 16 min) and males (EXC: 9.0 ± 1.9 mmol·L⁻¹; 205 ± 42 min; REST: 9.2 ± 1.3 mmol·L⁻¹; 145 ± 26 min). In summary, SB ingestion effectively elevated HCO₃, with peak concentrations being comparable between EXC and REST. However, low-intensity cycling delayed the time to peak by ~1 hour, emphasizing the importance of accounting for exercise during the absorption period when planning nutritional strategies for prolonged endurance events. These findings suggest that athletes and practitioners should adjust the timing of SB intake to align peak HCO₃ with critical performance periods during prolonged exercise.