The most recent 2018 International Olympic Committee (IOC) sports nutrition consensus statement recommendations have suggested that sodium bicarbonate (NaHCO₃) is one of five dietary supplements that has generally been shown to improve performance in the elite athlete (1). Indeed, there are a number of contemporary meta-analyses demonstrating the potential performance efficacy of supplementing with NaHCO₃ as compared to placebo in sports where perturbations in cellular buffering capacity influences performance (typically in events of 1–10 min) (2–5). Current consensus statement ingestion recommendations are to consume between 0.2 and 0.4 g·kg body mass (BM)⁻¹ with a small, carbohydrate (CHO) dense meal (~1.5 g·kgBM⁻¹ CHO) ~60 to 150 min prior to exercise (1). However, as contemporary papers have also highlighted, these recommendations serve only as a starting point when considering NaHCO₃ supplementation for the individual athlete (6–8). A recent review by Heibel et al. has addressed several practical issues associated with traditional NaHCO₃ supplementation approaches, identifying ingestion timing as potentially critical to maximizing the effectiveness of this supplement (6). Given the existing scientific support for the use of NaHCO₃ (2–5), as well as the high prevalence of use [e.g., rowing (9–12)], understanding the influence of ingestion timing under ecologically valid conditions may further improve the effectiveness of this supplement. To date no study has collectively investigated the relationship between NaHCO₃ timing, buffering capacity, gastro-intestinal (GI) distress, and pre-race nutritional recommendations coupled to performance outcomes in world-class athletes.

The premise for individualizing NaHCO₃ supplementation has both historical (8, 13) and more recent scientific support (14–16). Several contemporary publications have consistently demonstrated the high degree of inter- and intra-individual variability often observed during NaHCO₃ studies, despite dosing by body mass (kg) and standardizing pre-supplementation nutrition and fluid intake (7, 8, 17). Furthermore, a relatively recent study has profiled the large individual variations in blood bicarbonate (HCO3-) in response to ingesting 0.1, 0.2, and 0.3 g·kg⁻¹ NaHCO₃ (7). These authors also underscored the large variation in time-to-peak blood buffering capacity (e.g., highest recorded [HCO3-]) between the participants (range from 30 to 180 min) (7), despite a per kilogram body mass dosing regimen and 24-h dietary replication protocols. Subsequently, these data have been followed by a series of studies assessing the intra-individual reproducibility of blood buffering profiles (14, 16, 18) and exercise performance under varying doses of NaHCO₃ (14, 16). Collectively, these studies provide preliminary evidence suggesting that adjusting the start of a competitive effort to commensurate with an individual's peak blood buffering response at rest may result in better outcomes in terms of GI distress and exercise performance (14, 16, 19).

Although promising, the importance of timing performance trials to coincide with an individual's peak blood buffering capacity has not yet been investigated in world-class level athletes. For example, neither of the aforementioned studies investigating reproducibility and performance (14, 16) introduced NaHCO₃ as it would be to a competitive athlete (e.g., in a fed rather than fasted state, in capsule rather than liquid-based solution and under the pressure of competitive situations). Moreover, neither of these studies have investigated the effect of performance timing. Miller et al. compared an individualized NaHCO₃ ingestion protocol to a placebo and control trial in recreationally active individuals (16), whilst Gough et al. investigated the reproducibility of cycling time-trial performance under varying (but all individualized) dosages of NaHCO₃ (14). We therefore designed this proof-of-concept study to specifically address the question of whether or not ingestion timing influences time-trial performance [2,000-m rowing time-trial (TT)] in elite-level rowers adhering to their own individualized pre-race strategies (e.g., nutrition, warm-up, etc.). Incorporating the 2,000-m rowing TT under these conditions of high ecological validity also provided the opportunity to further explore the complex, inter-related issues surrounding NaHCO₃ ingestion, GI-distress, and acid-base balance; all concerns raised by previous investigations in this field (9, 11, 20). We hypothesized that 2,000-m rowing times would be improved when the TT commenced at an individual's peak blood buffering capacity as compared to a start time corresponding with the minimum IOC recommendations (60 min) (1).

A multi-center approach was utilized to maximize the number of international competitive rowers involved. Twenty three (n = 23) elite rowers [lightweight rowers (n = 4), body weight 73.6 ± 2.1 kg; open-class rowers (n = 19), 93.6 ± 5.8 kg; mean (M) ± standard deviation (SD)] were recruited across two research centers [Canadian Sport Institute Pacific (CSIP; Canada) and the New South Wales Institute of Sport (NSWIS; Australia)] with identical methods and protocols conducted at both institutes. Participants were male competitive rowers able to complete a 2,000-m ergometer TT at or below 6 min 20 s [Participant 2,000-m ergometer TT personal bests (PB) ranged from 5 min 39 s (open-weight) to 6 min 14 s (light-weight) and included 13 Olympic/World-Champs team members as well as one rowing ergometer world record holder]. Fifteen of the 23 athletes all had previous experience ingesting NaHCO₃ in various contexts, while the remaining eight (U23) were only aware of the potential benefits. All participants were informed verbally and in writing as to the nature and risks associated with the study, submitted to health screening and gave their written informed consent. All procedures in this study were approved by the respective ethics committees (Australian Institute of Sport Ethics Committee; approval code 20171205 and the University of Victoria Human Research Ethics Board; approval code 18-045) and were conducted in accordance with the Declaration of Helsinki.

A growing body of evidence suggests a number of factors may affect the efficacy of the supplement NaHCO₃ as a strategy to mitigate fatigue in the context of exercise performance (7, 8, 14, 16–18). These factors are primarily related to the complex interplay between ingestion strategies as they relate to changes in peak blood buffering capacity (e.g., dose-response), GI distress and ingestion timing. The aim of the present study was to specifically address the issue of ingestion timing by examining whether adjusting start times to coincide with an individual's peak blood buffering capacity after NaHCO₃ supplementation would influence 2,000-m rowing TT performance. Our approach of individualizing the time-to-peak was successful as Pre-WU [sHCO3-] was nearly 3 mmol·L⁻¹ greater (p = 0.02) for IP than CON (Figure 3B), although this difference diminished in the experimental trials after the addition of the warm-up (Pre-TT sHCO3- values: CON ~ + 5.5 mmol·L⁻¹; IP ~ + 6 mmol·L⁻¹). The present study also demonstrated a small but significant performance effect of an individualized NaHCO₃ ingestion strategy [IP (367.0 ± 10.5 s) vs. CON (369.0 ± 10.3 s); p = 0.007; d = 0.15]. Moreover, 18 of the 23 participants improved their times in the IP trial, with 11 participants at or above a 3 s improvement (Figure 4). Given the caliber of the athletes in this study, these findings provide preliminary support for individualizing ingestion timing strategies.

A number of recent independent investigations have clearly demonstrated the high degree of inter-individual variability associated with time-to-peak buffering capacity after NaHCO₃ ingestion, reporting peak HCO3- concentrations ranging between 10 and 150 min post ingestion after a similar dose of 0.3 g·kgBM⁻¹ (7, 18, 25). Indeed, the inter-individual variability has also been cited as a possible contributing factor toward inconsistencies of erogenicity observed in some studies (8, 17), and highlighted in contemporary reviews as an area for further study (6, 26). With specific reference to rowing, it is plausible that this inter-individual variability may have influenced the performance outcomes of many of the studies observing minimal to no effect of NaHCO₃ supplementation (9–12, 27). Although we observed less than a two second (~ 0.5%) difference in performance times (Figure 3), in practical terms this would equate to greater than a boat's length in competition. Even in this world-class cohort where we consistently observe 2,000-m CV's of 0.5 to 1.4% (28), the small but positive effect (d = 0.15) may be worth considering when developing a supplementation strategy for the elite competitor, particularly in light of the prevalence of small effects identified in this cohort (5). Though our findings tentatively support the concept of individualizing timing strategies with this supplement, further research is required to determine whether performance outcomes after individualization is due to differences in absolute changes blood [HCO3-] or some other mechanism (14, 16, 18).

Although speculative, as we only investigated the effect of timing, the results of the present study may also suggest that the absolute change in blood buffering capacity (Figures 3A,B), rather than timing exercise commencement to coincide at an individual's peak blood HCO3- concentration, may be more relevant when considering NaHCO₃ as a supplement. As demonstrated in a recent review (6), the ergogenic potential of consuming 0.3 g·kgBM⁻¹ NaHCO₃ appears to improve substantially when the concentration of blood HCO3- is >5 to 6 mmol·L⁻¹ above typical values (found in 17 of 19 studies reviewed). In the present study both CON (~ + 5.5 mmol·L⁻¹) and IP (~ + 6 mmol·L⁻¹) sHCO3- concentrations were elevated above 5 mmol·L⁻¹ prior to the start of each of the respective time trials. Furthermore, during the NaHCO₃ Loading Profile the time duration post-supplementation that participants were > 5 mmol·L⁻¹ was ~ 100 ± 20 min (range 40 to 160 min; Figure 2B), respectively; suggesting the potential for a buffering performance window. Though the theoretical premise of a minimal buffering threshold (e.g., 5 to 6 mmol·L⁻¹) or buffering window has merit, further research is required to directly test these hypotheses within the context of “real-world” performance parameters.

In support of the previous statement, the similar elevation in pre-TT [sHCO3-] observed in both performance trials (Figure 3B) also clearly demonstrates the effect of a typical high-intensity warm-up on blood acid-base kinetics, as only one participant achieved peak blood buffering capacity at the 60 min post-ingestion time point during the passive profiling trial (Figure 2B). This finding has practical significance, as the warm-up in this study was constructed by each individual athlete (within prescribed guidelines) and mimicked their own “pre-race” warm-up strategy. Presumably, either the proportion of high-intensity efforts or warm-up length facilitated an increased rate of HCO3- appearance in the blood as compared to a purely passive ingestion environment, essentially equating the buffering capacity between the two conditions. Irrespective of the similar pre-TT [sHCO3-], we ultimately cannot dismiss the performance improvement observed in the IP trial (Figure 4). Given the similar GI responses (Figure 5) and blood buffering concentrations between trials, we cannot speculate as to causation in this regard. In practice, however, it may be unnecessary to undertake the costly and time-consuming exercise of identifying an individual athlete's peak blood buffering capacity, when measured baseline and a “one-off” measure just prior to exercise will ensure HCO3- concentrations are significantly elevated (e.g., > 5 mmol·L⁻¹) post supplementation.

Presently, there are no universally accepted methods for determining peak blood buffering capacity after NaHCO₃ ingestion. Miller et al. used a single visit where peak [HCO3-] and pH values were visually determined during 160 min of sampling (16). Similar to our pilot study results (Figure 2A), after demonstrating a high degree of reproducibility in time-to-peak [HCO3-] (18), Gough et al. also used a single visit to determine time-to-peak HCO3- over a 180 min time frame post 0.2 and 0.3 g·kgBM⁻¹ NaHCO₃ ingestion (14). Commendably, these authors reported individual participant data to complement the mean ± SD absolute HCO3- change (mmol·L⁻¹) scores across the resting and two experimental trials (4 km cycling TT). Although mean differences in peak values were between ~ 0.8 and 0.9 mmol·L⁻¹, the range of difference scores across the three 0.3 g·kgBM⁻¹ NaHCO₃ trials was over 3 mmol·L⁻¹ within individuals at the high end (14), suggesting a large degree of intra-individual variability. The present study, using a combination of time-to-peak and TEM from the change scores (baseline to peak) over the 2 week pilot study (to objectively determine peak sHCO3-) still demonstrated a relatively large amount of variability across peak sHCO3- values (0.6 mmol·L⁻¹; 95% CI 0.4 to 1.6 mmol·L⁻¹). Given the inter- (7, 8, 17) and intra-individual variability associated with identifying time-to-peak HCO3- values, our small but significant difference in performance times between trials, and assuming the athlete will replicate all dietary practices prior to competition, we recommend identifying the time course where an absolute increase of > 5 mmol·L⁻¹ (HCO3-) occurs after ingesting 0.3 g·kgBM⁻¹ NaHCO₃ as a criterion standard (2).

Despite implementing rigid 24 h dietary controls, a pre-race standardized ingestion strategy (10 ml·kgBM⁻¹ fluid and 1.5 g·kgBM⁻¹ CHO) based on previous best practice recommendations (20) and athletes' preferred pre-competition diet, all athletes in the present study experienced at least some degree of GI distress, although the majority of it was minor (Figure 5). Regardless of severity, athletes will be reticent to use this supplement should it cause distraction during competition. Indeed, four of the top performing rowers in this study have not used NaHCO₃ during competition for this reason. Empirically, Saunders et al. have demonstrated improved exercise capacity when individuals do not experience GI discomfort after NaHCO₃ supplementation, albeit in recreationally trained participants (8). Given our participant cohort, we cannot speculate further as to whether the GI distress was directly related to the NaHCO₃, anxiety experienced by the rowers prior to the TT, the extreme intensity of the TT itself or a combination of factors. As suggested by numerous authors (2, 5, 6), GI distress associated with NaHCO₃ ingestion may be one of the primary factors in this supplement not reaching its ergogenic potential. Indeed, many contemporary studies have documented the commonly experienced side-effects (e.g., bloating, cramping, diarrhea, etc.) (8, 14, 18), and have even attempted to categorize these symptoms according to severity (16). However, as of yet there have been no direct causal links established between the severity of GI distress from NaHCO₃ ingestion and a decline in exercise performance. Moreover, the large disparity in the literature between ingestion protocols (e.g., capsules, liquid, fasted vs. fed-state), timing and nutritional control render between-study symptom comparison difficult. In the present study, although GI symptoms were similar regardless of condition, any opportunity to minimize the distractions associated with GI distress is logical and thus supports, where possible, prolonging the time period between ingestion and the start of competition.

One additional note in considering the ingestion strategy of the present study was the weight gain experienced by the athletes during both loading protocols. Although the BM increase was nominal for a weight supported sport such as rowing (<0.5%) and most likely a result of the 10 ml·kgBM⁻¹ fluid administered during pill ingestion, it is worth mentioning given the potential for this supplement to be used in weight dependent (e.g., triathlon team relay, middle distance running etc.) and weight category sports (mixed-martial arts, boxing, etc.). Moreover, when separated from the group, the weight gain appeared more pronounced in the lightweight (72.5 kg) rowers (~ 0.6 kg increase; n = 4). To our knowledge, only one study has investigated the potential fluid increases after incorporating NaHCO₃ (ingested within the typical range) into a rehydration strategy to offset the effects of acute dehydration (12). Kupcis et al. observed a greater increase in BM during a 2 h loading sequence compared to the present study (~ 1.5 vs. 0.4 kg), albeit total fluid intake was over double the total volume (10 ml·kgBM⁻¹ in the present study compared to 22 ml·kgBM⁻¹). However, these authors also did not observe any impact on performance (2,000-m rowing TT) compared to a nutrition/fluid-matched placebo control (12). Ultimately, when considering whether to implement a NaHCO₃ loading strategy it is worth noting that the expected body weight gain from NaHCO₃ ingestion is likely to be much less deleterious in weight supported (or independent) sports (rowing, canoe-kayak) than weight dependent sports (e.g., running, road cycling).

We acknowledge the perceived limitation in this study of not having a true placebo trial. However, the primary aim of this study was not to determine the efficacy of NaHCO₃ ingestion on 2,000-m rowing TT performance, as the potential ergogenic effect of this supplement has been shown previously across many published reviews and meta-analyses (2–5, 25, 29, 30). Rather, we sought to specifically address the question of whether or not ingestion timing impacts TT performance in an ecologically valid context (e.g., with dietary and warm-up conditions typically used by elite rowers). In an attempt to counterbalance our lack of a traditional placebo trial, deception was used to minimize the potential belief effect of timing (31). Thus, some level of expectancy was possible. When considering that coaching staff limited our time trial opportunities (i.e. two), and the fact that our time frame for carrying out the study was constrained, we agreed that the only way to mitigate the belief effect was to deceive the athletes about whether or not they were actually receiving NaHCO₃ in each trial. As nearly 65% of the athletes believed that the CON trial was indeed a placebo trial, we are confident this strategy was effective in “blinding” the athletes to the exact supplement they received. Given the limited peer-reviewed data available on this supplement in world-class athletes, and considering the inherent constraints of conducting research in a high performance environment (e.g., coach “buy-in,” training and competition schedules only allowing for 2 × 2,000-m TT-tests), we felt conducting the study using the 60 min timing as a control for comparison against individual peak was warranted despite the somewhat non-traditional research design.

Ultimately, the findings of the present study may support targeting the onset of a competitive effort to coincide with an individual's peak blood buffering capacity window after NaHCO₃ supplementation if working with competitive athlete cohorts. Similarities in blood buffering changes after warm-up, however, would suggest this effect was not a result of any meaningful differences in blood alkalinity. Although a number of meta-analyses have suggested that ensuring the athlete has reached a minimum of a 5 mmol·L⁻¹ absolute increase in blood [HCO3-] may also be important to maximize the effectiveness of this supplement (2, 26, 27), further study is required to directly test this threshold hypothesis. Regardless, understanding the total timeframe of this increase should allow for greater flexibility in the timing of NaHCO₃ supplementation (32). Moreover, given the extended time-frame above this 5 mmol·L⁻¹ mark observed in all athletes (Figure 2B), we would recommend future research investigate the potential of this “window of opportunity” rather than focusing solely on peak blood buffering values. The flexibility in supplement timing across a potential performance window may help with individual GI issues, pre-competition food intake timing preferences and/or sport rules dictating warm-up time, check-in time or similar logistical constraints. In terms of supplement tolerance, even when incorporating a “tried and true” pre-race nutritional strategy that includes adequate fluid, carbohydrate and delayed ingestion timing, practitioners can expect some athletes to experience at least minor GI disturbances. Indeed, future work in this area may eventuate in eliminating this issue altogether (15, 32). Finally, those athletes participating in weight dependent sports and considering NaHCO₃ supplementation should be conscious of acute increases in body weight associated with ingestion protocols designed to minimize GI distress.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The studies involving human participants were reviewed and approved by Australian Institute of Sport Ethics Committee; approval code 20171205 and the University of Victoria Human Research Ethics Board; approval code 18-045. The patients/participants provided their written informed consent to participate in this study.

SB, TS, GS and JS conceptualized and designed all aspects of the study. All authors contributed to data collection, analyses, writing of the manuscript, and agree to be accountable for the content of the work.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.