Intra-Individual Reliability of Blood Bicarbonate Responses and Gastrointestinal Symptoms Following Sodium Citrate Supplementation

Background: Sodium citrate (SC) can elevate extracellular buffering capacity, yet the intra-individual reliability of its blood bicarbonate ([HCO₃¯]) kinetics and gastrointestinal (GI) responses is unclear, limiting individualized dosing strategies. Methods: Twelve healthy males (21 ± 1 yr) ingested a solution containing 0.5 g·kg¯¹ SC on two visits 3–7 days apart. Capillary [HCO₃¯] was sampled at baseline and every 30 min to 240 min to derive baseline and peak [HCO₃¯], time to peak (TTP), time to exceed +5 and +6 mmol·L¯¹ above baseline, and area under the curve (AUC). Reliability was quantified with ICC, typical error (TE), and CV; a Monte Carlo simulation estimated the probability of exceeding +5 and +6 mmol·L¯¹ at each time point. GI symptoms (12-item questionnaire) were recorded concurrently. Results: [HCO₃¯] rose significantly over time from 30 min in both visits (p < 0.001). Reliability was moderate for baseline [HCO₃¯] (ICC = 0.72 [0.25, 0.91]; CV = 3.5%) and AUC (ICC = 0.56; CV = 3.5%), but poor for peak [HCO₃¯] (ICC = 0.23 [−0.29, 0.68]; CV = 5.4%) and all time-based metrics, including TTP (ICC = 0.07; TE = 49.1 min; CV = 32.5%) and time to +5 and +6 mmol·L¯¹. Simulation showed an ≥ 80% probability of exceeding +5 mmol·L¯¹ from 120–240 min (83.9–85.8%), whereas +6 mmol·L¯¹ peaked at 69.7% (150 min). GI symptoms were common, unchanged across visits, and moderately reliable for overall burden (ICC = 0.61; TE = 2.63; CV = 46.6%). Conclusion: SC elicits a consistent group-level alkalosis, yet individual timing metrics are unreliable. Concentration-based indices are more stable for monitoring. Practically, a 2–3 h ingestion window maximizes the probability of achieving ≥+5 mmol·L¯¹, but individual profiling is recommended where precise timing is critical.