Sodium Bicarbonate Intake on Endurance Performance

Introduction Buffering Agents in Exercise Performance During high-intensity exercise, muscle fatigue occurs, negatively impacting both anaerobic and aerobic (endurance) exercise performance. One primary cause of muscle fatigue is metabolic acidosis, which arises from the accumulation of intracellular lactate and H+ ions (Freis et al., 2017; Krustrup et al., 2015). Since muscle acidity is regulated by intracellular and extracellular buffering systems, enhancing these systems is essential to sustain effective muscle contraction, delay fatigue, and thereby optimize performance in both anaerobic and high-intensity endurance exercises (Durkalec-Michalski et al., 2018a; 2018b).

Various ergogenic aids are utilized to boost intracellular and extracellular buffering capacity, with beta-alanine and sodium bicarbonate (SB) being two well-supported agents: Beta-alanine enhances intracellular buffering by increasing muscle carnosine levels, thereby preventing acidosis and improving performance (Trexler et al., 2015). SB primarily acts as an extracellular buffer, clearing H+ ions from the blood. This mechanism indirectly reduces H+ accumulation in muscles during intense exercise (Durkalec-Michalski et al., 2018a; 2018b). By preserving ATP production via glycolysis, this buffering action delays the onset of muscle fatigue during high-intensity exercises (Lancha Junior et al., 2015; Lopes-Silva et al., 2019).

Due to its ability to reduce lactate and H+ accumulation, research has predominantly focused on SB's effects on high-intensity, short-duration exercise (1-10 minutes) performance reliant on glycolytic metabolism (Delextrat et al., 2018; Driller et al., 2012; Gough et al., 2017). For example, 0.3 g/kg dose of SB intake improved total work output by enhancing buffering capacity in a 10 x 6-second sprint protocol (Miller et al., 2016). SB is thus commonly used for sports reliant on glycolytic metabolism, such as sprinting, middle-distance running, and combat sports. Additionally, SB may also be beneficial for longer endurance events (30-60 minutes) at the anaerobic threshold (AT), such as high-intensity interval training or long-distance endurance events concluding with a final sprint (Burke, 2013).

SB and Endurance Performance While long-duration endurance exercise (\>30 minutes) is primarily driven by aerobic metabolism, anaerobic glycolysis also significantly contributes to energy production, leading to elevated blood lactate levels (Egger et al., 2014; Jones et al., 1977; McNaughton et al., 1999). Consequently, SB can help maintain optimal pH levels during endurance exercise, enhancing high-intensity performance at or near the aerobic threshold (Egger et al., 2014; Freis et al., 2017).

Studies have examined the effects of 0.2-0.3 g/kg SB intake on endurance performance near the lactate threshold in cycling and running, often comparing it to neutral (sodium chloride) or acidic (ammonium chloride) supplements. Findings indicate that while SB improves time to exhaustion, acidic agents can reduce it (George \& MacLaren, 1988; Jones et al., 1977; Sutton et al., 1981). For instance, Egger et al. (2014) found that 0.3 g/kg SB intake one hour before exercise extended performance time during a test following a 30-minute exercise session at 95% anaerobic threshold.

While studies have supported SB's positive effects on endurance performance, there is mixed evidence. Some studies found that, despite increasing blood bicarbonate and pH or reducing lactate levels, SB did not improve endurance performance (Freis et al., 2017; McNaughton et al., 1999). These studies, however, were conducted under laboratory conditions. Research on SB's effect in field-based, real-world endurance settings has not yet been conducted.

Optimizing Sodium Bicarbonate Intake While SB has shown promise in enhancing high-intensity endurance performance, studies often focus on acute SB usage, typically recommending a 0.3 g/kg dose before exercise (Lancha Junior et al., 2015). This protocol effectively increases blood pH and bicarbonate levels (Price \& Singh, 2008) while reducing plasma H+ concentration (Renfree, 2007). However, this acute intake can lead to gastrointestinal symptoms (GIS) due to the rapid absorption of both bicarbonate and sodium (Siegler et al., 2012). In endurance sports, managing GIS is critical, as such symptoms can more severely impact performance than in short-duration events (Jeukendrup, 2017).

This has prompted the exploration of alternative SB protocols to minimize GIS, such as spreading the dosage over time, using lower doses, or administering over consecutive days. In a study conducted by researchers involved in this project, the effects of 0.2 g/kg SB on endurance performance were evaluated over four consecutive days in male cyclists. The results showed that while both acute and consecutive SB intake enhanced buffering capacity (Figure 1), only consecutive intake significantly extended time to exhaustion compared to placebo (Figure 2). Notably, GIS was observed with acute intake but was minimized with consecutive-day intake, suggesting the need for alternative acute SB protocols to further improve endurance performance by reducing GIS.

Figure 1. Effects of 0.2 g/kg Sodium Bicarbonate on Blood pH (A) and Blood Bicarbonate Levels (B) on Acute and Consecutive Days (Aktitiz et al., 2024). SB: Sodium Bicarbonate. §: Significant difference between Acute SB and PLA (p \< 0.05); ¥: Significant difference among all pairwise comparisons (Acute SB, Consecutive Days SB, and PLA) (p \< 0.05).

Figure 2. Effect of 0.2 g/kg Sodium Bicarbonate on Time to Exhaustion on Acute and Consecutive Days (Aktitiz et al., 2024). SB: Sodium Bicarbonate. \*: Significant difference between Consecutive Days SB and PLA (p \< 0.05).

In studies on alternative SB intake methods, consumption with carbohydrate-rich meals was found to be one of the most effective ways to prevent GIS. For instance, a study (Carr et al., 2011) showed that consuming 0.3 g/kg SB with 1.5 g/kg carbohydrate and 7 ml/kg fluid resulted in peak blood bicarbonate and pH levels within 120-150 minutes, with minimal GIS. This finding suggests that SB consumption with carbohydrates may enhance performance while minimizing adverse effects, providing a promising approach for endurance athletes.

SB and Female Athletes Although studies have demonstrated that SB can positively impact high-intensity endurance exercise performance (Egger et al., 2014; George \& MacLaren, 1988; Jones et al., 1977; Sutton et al., 1981), most research has been conducted on male athletes. Studies involving female athletes represent only 7.4% of the total SB studies (Smith et al., 2022). Due to their generally lower muscle mass and a reduced proportion of type 2 muscle fibers (Nuzzo et al., 2024), women may have reduced glycolytic capacity, meaning SB could have a distinct effect on female physiology (Green et al., 1984; Hegge et al., 2016; Janssen et al., 2000; Porter et al., 2002). Additionally, due to their monthly experience with gastrointestinal symptoms (GIS) such as bloating, abdominal pain, and diarrhea during the menstrual cycle, women may tolerate SB's side effects better (Carr et al., 2023). These findings suggest that SB might be more effective in female athletes.

Indeed, an acute dose of 0.3 g/kg SB taken 90 minutes before exercise was shown to enhance 1-minute maximal cycling performance in 10 physically active women (McNaughton et al., 1997). In another study involving female basketball players (n=15), a 3-day regimen of 0.4 g/kg SB improved repeated sprint performance and jump height during a simulated game (Delextrat et al., 2018). A meta-analysis also confirmed SB's positive effect on anaerobic performance in female athletes (Saunders et al., 2022). However, the impact of SB on high-intensity endurance performance in female athletes has not been investigated. Furthermore, existing studies are limited to highly trained women aged 19-26 (Saunders et al., 2022), and its effects on recreational-level and older female athletes remain unexplored.

In summary, while significant evidence suggests that SB can enhance high-intensity endurance performance, there is a notable lack of research specifically addressing female athlete physiology and alternative SB protocols that mitigate GIS in women.

METHOD Study Group The study will include 19 healthy female recreational runners aged 18-40, who have been consistently training (at least three days per week) for a minimum of one year. Participants will be recruited through outreach to university sports clubs in Ankara. Ethics approval for this study was obtained from the Hacettepe University Clinical Research Ethics Committee. Participants will be informed of the protocol, provided with informed consent forms, and the study will adhere to the updated Declaration of Helsinki (2013). Demographic forms documenting training history and an American College of Sports Medicine Physical Activity Readiness Questionnaire will be administered to confirm eligibility.

Inclusion Criteria:

* Female, aged 18-40. * Recreational runner with a minimum one-year history of regular training (at least three days per week). * Experience in races of 10 km or more (verified by an official race result). * Ability to complete a 10 km distance at an average pace below 6.00 min/km (confirmed through best-record evidence via applications like Strava).

Exclusion Criteria:

* Usage of any medications or supplements that could impact metabolism. * Presence of any acute or chronic condition limiting physical activity (musculoskeletal, cardiovascular, or respiratory issues). * Alcohol and/or smoking habits. * Adherence to a specific diet influencing metabolism during the study period (intermittent fasting, ketogenic, vegan diets, etc.). * Pregnancy or breastfeeding during the study period. * Severe menstrual irregularities or amenorrhea. * Body Mass Index (BMI) outside the range of 18.5-30. * History of gastrointestinal disorders (e.g., IBS, Crohn's disease). * Allergy or intolerance to sodium bicarbonate or related compounds. * History of disordered eating.

Study Design This randomized, double-blind, crossover study will involve each participant in four experimental exercise tests: one for body composition, one maximal oxygen uptake (VO2max) test, and two "performance tests" focused on evaluating 10 km completion time and other endurance-related variables. Participants will complete two interventions: sodium bicarbonate (SB) and placebo (PL). In the SB condition, participants will receive 0.3 g/kg SB, while in the PL condition, they will receive 0.03 g/kg table salt, administered double-blind. Performance testing will commence 1.5 hours after ingestion of a standardized 1.5 g/kg carbohydrate breakfast. After each test, a washout period of 3-7 days will occur, after which the groups will be crossed over, and the protocol repeated.

Body Composition Participants will visit the laboratory after an overnight fast, wearing no metal or electronic items, and will void their bladders before measurements. Body weight will be measured to a precision of 0.1 kg using an electronic scale (Tanita UBB SC 330, USA). Body composition, including fat mass, body fat percentage, lean body mass, and visceral adipose tissue, will be assessed using Dual-energy X-ray Absorptiometry (DXA, Lunar Prodigy Pro narrow Fan Beam, GE Health Care, Madison Wisconsin, USA). The DXA will be calibrated daily, and all measurements will be conducted by a licensed researcher. Body composition values will be analyzed using GE Encore v14.1 software and CoreScan for visceral adipose tissue.

Standardization of Dietary Intake One day of dietary intake records will be collected 24 hours before performance tests and analyzed using the BEBIS 6.1 software (Stuttgart, Germany). This standardization will ensure similar carbohydrate intake before tests. Additionally, participants will consume a standardized

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