Study design

Blood samples were collected at baseline and following placebo (CaCO 3 ), acute sodium bicarbonate (AcuteSB) ingestion, and modified sodium bicarbonate (ModSB) ingestion protocols to determine the effects on serum bicarbonate, sodium and lactate concentrations, as well as on serum pH and blood hematocrit. The study design was a randomized double-blind crossover. The order of ingestion protocols was counter-balanced using a Latin square to minimize the potential of a learning effect. All tests were administered between 8 and 11 AM, and were conducted at the same time of day for each participant. A period of 7–10 days was allowed between trials to ensure the ingested substances had returned to baseline prior to the subsequent trial [17]. Participants were instructed to maintain their normal training patterns throughout the study, and to refrain from intense training for at least 48 hours prior to each test. Participants were also instructed to eat the same high-carbohydrate breakfast approximately 3 hours prior to each post-ingestion assessment.

Participants

Ten elite middle-distance runners (6 men, 4 women) from running teams in the Los Angeles area volunteered to participate. Inclusion criteria were: 1). Peak oxygen consumption greater than 60 ml·kg− 1·min− 1 (men) or 50 ml·kg− 1·min− 1 (women); 2). Currently training, defined as at least 5 days·week− 1 of running; 3). Elite-level performance (750 or more points on the International Associations of Athletics Federations Scoring Table) for an 800 m–5000 m race during the preceding 6 months. Elite athletes were selected because they are a sample population that would actually utilize sodium bicarbonate supplementation.

Preliminary testing

Preliminary testing occurred at least 3 days prior to the 1st trial. Participants self-reported their training status and race results. Height and weight were assessed with a stadiometer. Body fat was assessed by hydrodensitometry [20], with residual lung volume measured by the oxygen dilution technique [21], in order to accurately assess body composition in this highly trained sample. After a brief rest, participants underwent maximal oxygen consumption (Vo 2peak ) testing using a modified treadmill protocol: Elevation was fixed at 8%; speed began at 4 mph and increased 1 mph every 2 minutes until volitional exhaustion. Based on pilot testing, this protocol was utilized so that a safe high-intensity treadmill running speed at 110% Vo 2peak could be calculated for elite runners. Minute ventilation, Vo 2 , and Vco 2 were determined during the Vo 2peak test by a Vmax 29 metabolic cart (Sensormedics, Loma Linda, CA). Heart rate was assessed during the Vo 2peak test by electrocardiography using CM 5 electrode configuration. Achievement of Vo 2peak was confirmed if at least two of the following criteria were met: 1). Respiratory exchange ratio greater than 1.05; 2). Heart rate within 10 bpm of age-predicted maximum; 3). Plateau in Vo 2 with increasing workloads.

Ingestion protocols

The ModSB protocol consisted of 4 progressively larger doses of SB administered with progressively shorter time intervals between doses consumed during the 19.5-h period prior to post-ingestion testing (Table 1). This protocol was developed in order to deliver a maximum amount of sodium bicarbonate in smaller individual doses that would not cause GI distress (≤ 200 mg·kg− 1), while maintaining short total consumption time to minimize sodium consumption. Calcium carbonate was chosen as the placebo because it had repeatedly been used successfully in previous sodium bicarbonate research [22]. The calcium carbonate ingestion protocol (placebo) simulated the ModSB protocol. In order to maintain the double-blind design of the study, the first three doses of the Acute SB protocol contained the placebo, and only the last dose contained 300 mg·kg− 1 of SB.

Table 1 Timing and Dosages of the Three Ingestion Protocols Full size table

For each trial, participants arrived at the lab 24 h prior to the post-ingestion test to have a baseline blood draw and receive 4 doses of the substance(s) to be ingested in numbered sealed bags with a detailed ingestion schedule and a 750 ml bottle of water. Beginning 19.5 h prior to the post-ingestion test, participants ingested each dose with 750 ml of water according to the ingestion schedule. Participants were asked to record the timing of each ingested dose for confirmation. The same number of capsules was used per dose across trials for each participant and doses were encased in opaque gelatin capsules (size “00”) to mask the flavor and granularity of the respective substances.

Participants self-reported their respective levels of GI distress on a Likert scale of 1–10 (1 = no GI distress, 10 = worst possible GI distress). The data were classified a priori as 1–3 equal to limited GI distress, 4–7 as moderate GI distress, and 8–10 as severe GI distress.

Blood analysis

Ten ml of blood was drawn via forearm venipuncture twice for each condition. Baseline blood draws occurred 24 h prior to the post-ingestion test in order to minimize the effect of diurnal variations in hematocrit, plasma sodium and bicarbonate concentrations [23]. The post-ingestion blood draws were taken 78 ± 7 minutes following the 4th dose.

Two capillary tubes were immediately filled from the blood samples to be tested for hematocrit via the microhematocrit method [24] in order to evaluate changes in blood composition following the ingestion protocols. The remaining blood was allowed to coagulate at room temperature for 25 minutes and was then spun in a refrigerated centrifuge for 10 min. The serum was pipetted into three small vials, one of which was placed in a − 80 °C freezer for later analysis of serum sodium and lactate concentrations. The serum in one vial was immediately analyzed for pH (Orion 720A+, Thermo Electron Corporation, Waltham, MA) and the other was used to measure bicarbonate concentration in duplicate with a carbon dioxide liquid stable reagent method (TR28321, Thermo Electron Corporation, Waltham, MA) using a spectrophotometer (Lambda 20, PerkinElmer, Waltham, MA) [25]. Once all the participants had been tested, the frozen samples were thawed and analyzed for sodium (Vitros DT60 II, Ortho-Clinical Diagnostics, Rochester, NY) and lactate concentrations (Vitros DT60 II, Ortho-Clinical Diagnostics, Rochester, NY) [26].

Statistical analysis

Means and standard deviations were calculated (X ± SD) for all measurements. Repeated measures ANOVA with factorial design (2 time points × 3 conditions) were analyzed to determine significant interactions between time (baseline, post-ingestion) and condition (Placebo, AcuteSB, ModSB) for blood parameters. Simple (condition) and repeated (time) within-participant contrasts were used to determine the location of any significant differences. Paired-sample t-tests were analyzed to determine differences in blood parameters at baseline. In order to evaluate differences following the ingestion of AcuteSB and ModSB, effect sizes were calculated for each variable. Because of the small sample size, corrected effect sizes were calculated using Hedge’s g. All data analyses were performed with SPSS 25.0. Statistical significance was established at p ≤ 0.05.