Background
For decades, it has been proposed that muscle acidosis is associated with muscle fatigue during intense exercise [
1]. Acidification has been suggested to negatively affect a myriad of steps in the excitation-contraction coupling in the muscle cell, such as activity in the myosin ATPase, Ca
2+ ATPase and Na
+-K
+ ATPase [
2], to attenuate the K
+ efflux through pH-sensitive potassium channels [
3,
4] and to impair the activity of metabolic enzymes [
1]. During high-intensity intermittent exercise performed in team sports, the high number of intense actions challenges the acid-base homeostasis in the muscle [
5,
6] and may, consequently, impair exercise tolerance. Interventions that promote the buffer capacity in the blood and/or muscles may therefore be beneficial in intense intermittent sports.
The Yo-Yo intermittent recovery test level 2 (Yo-Yo IR2) has been demonstrated to have a marked anaerobic component [
7]. For example, muscle pH levels below 6.8 have been measured at exhaustion, indicating a high taxation of the acid-base balance in the muscle cell. In addition, Yo-Yo IR2 performance has been shown to correlate to the amount of intense running during the most intense periods in team sport games [
8], making the test suitable for studying team-sport-specific exercise with a high anaerobic energy contribution.
Several interventions have been carried out to manipulate the acid-base environment prior to intense exercise, such as drug manipulations [
9,
10] and prior arm exercise [
11], demonstrating a significant effect on exercise tolerance. For example, β-alanine supplementation has been shown to be beneficial during high-intensity intermittent exercise by increasing the muscle carnosine buffer system [
10,
12]. The use of sodium bicarbonate supplementation in order to study fatigue resistance during intense exercise protocols has been widely applied and used by athletes for the last 80 years [
13], and a moderate effect size on the influence of the drug on exercise performance has been demonstrated [
14]. Bicarbonate doses of 0.3–0.5 g · kg
−1 body weight have been proposed for inducing consistent performance enhancements in a trained athlete population (for review, see [
15]). Some of these studies have also applied team-sports-specific testing protocols [
16,
17]. However, to the best of our knowledge no studies have investigated the consequence of sodium bicarbonate supplementation on team-sport-specific exercise that correlates with performance during the most intense periods in competitive match-play and where muscle acidosis has been demonstrated to by high [
8].
Thus, the aim of the present study was to examine the effect of oral sodium bicarbonate (0.4 g · kg−1 body weight) ingestion on Yo-Yo IR2 performance and physiological response in trained athletes familiar with high-intensity intermittent exercise in training and competition. We hypothesized that sodium bicarbonate intake would result in a more alkaline physiological environment and improve high-intensity intermittent exercise tolerance.
Discussion
In the present study, we observed that prior intake of sodium bicarbonate in capsular form using a protocol with gradual intake enhanced high-intensity intermittent exercise performance in young trained males. The performance improvement after sodium bicarbonate ingestion was accompanied by an elevated blood alkalosis and concentration of bicarbonate. In addition, blood lactate concentrations at exhaustion and peak values reached during the experimental protocol were higher, while the rating of perceived exertion was lower during intense exercise after sodium bicarbonate supplementation. In contrast, blood glucose, plasma K+ and Na+ as well as cardiovascular loading during high-intensity intermittent exercise were unaffected by sodium bicarbonate intake.
Performance in the Yo-Yo IR2 test increased by 14 % after sodium bicarbonate intake, which is comparable to a 16 % increase after caffeine intake in a comparable athlete population [
18], but lower than a 34 % improvement after β-alanine supplementation in football players [
23]. In comparison, training studies report a 19–29 % improvement in Yo-Yo IR2 performance observed after short-term periods (4–8 weeks) with speed endurance training in moderately trained [
20] and well trained males [
24]. Support for the performance improvement in the present study for the sodium bicarbonate intervention is provided, for example, by enhanced performance in the final sprints of a repeated sprint test [
16,
25] and simulated boxing performance [
17] after sodium bicarbonate intake, indicating an ergogenic effect during high-intensity exercise conditions performed under metabolic stress comparable to the short, intermittent and anaerobic Yo-Yo IR2 exercise protocol [
7,
8,
21]. However, some studies applying intense exercise regimes such as Wingate testing report no beneficial effect of sodium bicarbonate supplementation (for review, see [
14,
15]), indicating a predominant effect of sodium bicarbonate on high-intensity intermittent exercise. According to a review by Carr et al. [
15], sodium bicarbonate has a greater effect on trained individuals, so differences in training status may also partly explain the discrepancy between findings in different studies. In the present study, the subjects were well trained, having a maximal oxygen uptake comparable to top-class team-sport athletes and a Yo-Yo IR2 performance slightly below top-class central defenders in football [
21]. Moreover, they were all involved in high-intensity sport, and the results indicate that this athlete population may benefit from sodium bicarbonate intake.
In the present study, we did not apply a placebo, since the gastrointestinal effects of high doses of bicarbonate are usually easy to trace [
15]. A potential placebo effect may therefore have had an impact on the results, which is a limitation of the study design. However, in the present study a 14 % improvement was seen in the sodium bicarbonate trial, with nine of the participants displaying an improvement and only one having a decline in performance. In comparison, the placebo effect on high-intensity exercise performance reported in the literature is 1–3 % [
26]. There are two subjects that display a very large improvement in performance (~50 and ~75 %; Fig.
1) and are likely to be high-responders to the sodium bicarbonate, which may have affected the results. However, even when these two outliers are omitted from the statistical analysis, there is still a significant 6 % increase in Yo-Yo IR2 performance after sodium bicarbonate intake.
Part of the improvement in intermittent exercise performance may be explained by an elevated bicarbonate-induced buffer capacity in the blood, which will increase the muscle-to-blood H
+ gradient [
27]. Before the start of the Yo-Yo IR2 test, blood bicarbonate was elevated by 23 % in the sodium bicarbonate trial compared to control, with a significantly increased blood pH value. Thus, the blood buffer capacity was increased, which also resulted in higher blood pH and bicarbonate concentrations at exhaustions, despite a markedly longer exercise time in the intervention trial. The anaerobic contribution to the energy yield also appeared to be higher after the treatment with sodium bicarbonate, as reflected by the 15 and 20 % higher peak blood lactate and exhaustion levels, which may also be associated with longer exercise time.
Fatigue development during high-intensity intermittent exercise may be caused by a complex interplay between intra- and extracellular concentrations and gradients of ions such as K
+, Na
+, Cl
− and H
+ [
28,
29]. In the present study, no differences were detected in plasma [K
+] and [Na
+] between the sodium bicarbonate and control trials. However, since the potential fatiguing effect of a homeostatic imbalance in these ions is exerted in the muscle interstitial compared to intracellular environment [
28‐
32], a sodium bicarbonate induced effect on the kinetics of these ions during exercise cannot be ruled out. This is especially relevant to consider when the venous ion concentrations do not seem to reflect the interstitial values during intense exercise [
30]. In support of a link between alkalosis and improved muscle K
+ regulation, Street et al. [
27] found, using the microdialysis technique in human skeletal muscle, that the interstitial K
+ accumulation rate during intense exercise was attenuated after drug-induced alkalosis.
In the present study, RPE was lowered after 440 m of running in the Yo-Yo IR2 test in the sodium bicarbonate trial compared to the control, despite the fact that the heart rate and blood lactate concentration at this time-point were similar between trials. This may suggest that centrally mediated mechanisms were affected. The participants experienced less exertion late in the Yo-Yo IR2 test in the SBC trial when the heart rate was >95 % of HR
max and blood lactate and plasma K
+ were markedly elevated. Moreover, although further distance was covered before exhaustion, the RPE scores remained the same as in the control trial at the point of fatigue. Thus, a higher performance level was achieved while reporting an equal level of perceived fatigue at exhaustion. Peripheral alterations are likely to lead to modulation of neural strategies, for example via group III and IV muscle afferents, widely distributed through muscle and responsive to a variety of chemical stimuli, including altered H
+ [
33,
34], which was lower in the present study after sodium bicarbonate intake. Thus, part of the improved performance after sodium bicarbonate treatment may relate to less negative feedback from the muscle and, thereby, less effect on the descending drive to the motoneurons [
33,
35].
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Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
PK and MM developed the design of the study and carried out the data collection in cooperation with GE, who performed all blood analyses. PK, GE and MM performed the data treatment, including statistical analyses. PK and MM drafted the manuscript with the help of GE. All authors read and approved the manuscript.