The use of nutritional supplements as ergogenic resources has been shown to be efficient, as it delays the emergence of fatigue and increases the contractility of the skeletal muscle, improving the ability to perform physical activities and consequently improving sports performance [1,2]. Fatigue is defined as any reduction in the ability to produce strength or power in a muscle or a group of muscles induced by exercise [3]. Although its precise etiology remains controversial, the accumulation of H⁺ and concomitant reduction in sarcoplasmic pH are considered to be the main causes of fatigue caused by training [4,5].

Taking into consideration the buffering capacity for the removal of H⁺ ions and the delay in the inhibition of glycolytic enzymes (especially phosphofructokinase), the use of nutritional supplements is a relevant approach to improving performance [1,6]. According to the American College of Sports Medicine, many substances are potentially effective in improving buffering capacity, including sodium bicarbonate (NaHCO₃) [7]. Consequently, it is believed that supplementation with alkalizing agents before exercise can reduce the metabolic acidosis that occurs during heavy exercise, increasing the buffering capacity, and delaying the start of acidosis and fatigue [4,8].

McNaughton et al. [9] showed that NaHCO₃ supplementation is efficient in athletes who practice a variety of exercises, such as supramaximal exercise and intermittent high-intensity activity. Previously published studies have demonstrated an improvement in acute muscle resistance in strength training [10], delayed ventilatory threshold at Crossfit [11], improvement in lactate buffering in cyclists [12], greater glycolytic contribution in taekwondo [13], and increased time to fatigue in long-distance road running [14].

Although these studies have explored the effects of acute NaHCO₃ intake, according to various aspects of performance, there are still gaps in the literature regarding acute and/or chronic supplementation and its effects on time trial performance and time to exhaustion. Therefore, the objective of this systematic review and meta-analysis was to evaluate the effects of sodium bicarbonate supplementation on time trial performance and time to exhaustion in athletes and sports practitioners.

The present systematic review and meta-analysis evaluated randomized clinical trials of NaHCO₃ supplementation and its effects on sports performance in athletes and sports-people. Our findings show that NaHCO₃ has an ergogenic effect on time to exhaustion during exercise, but not on time trial performance. In practice, supplementation can increase the number of repetitions with a certain load or over a certain time to maintain power training at a higher intensity. These findings are based on studies that made use of a dosage of 0.3 g/kg 3 days, 240, 150, and 90 minutes before exercise [8,12,13,21-23].

Previous studies corroborate the results of our meta-analysis when demonstrating the ergogenic effect of supplementation on time to exhaustion as a measure of performance [12,22,23]. On the other hand, some studies [8,13,21] did not find a beneficial effect in time to exhaustion but found beneficial effects on metabolic markers, such as increased blood lactate concentration and glycolysis capacity during exercise. Danaher et al. [8] used a protocol that consisted of initial supramaximal efforts, which may have contributed to the low beneficial effect of NaHCO₃ in the maintenance of participants’ physical effort.

Interestingly, Hadzic et al. [24] in their systematic review of randomized and controlled studies, analyzed the effects of NaHCO₃ supplementation on performance during <4 mins and >4 mins exercise, so that they could contemplate diverse exercise and modalities, including cycling, swimming, running, boxing, tennis, and taekwondo, and their metabolic specificities. According to the authors, the duration of exercise is the main factor explaining the possible beneficial effects of NaHCO₃. Our study showed that out of the six studies that demonstrated an ergogenic effect on time to exhaustion, four studies had >4 min exercise duration, with a maximum of 49.5 min [23]. Thus, it seems that factors other than the duration of exercise are associated with the outcomes of NaHCO₃ supplementation.

These factors include training level (e.g., amateur, elite), and sex. Trained athletes have higher tamponade capacity and are, consequently, less responsive to supplementation [25]. On the other hand, NaHCO₃ is efficient in athletes who are trained using diverse exercises, such as supramaximal exercises and intermittent high-intensity sports activities, based on their abilities [9]. The lack of consensus in the literature demonstrates that the relationship between conditioning level and responsivity to NaHCO₃ remains inconclusive.

The reasons for the contrasting results between the studies are worth discussion, as the methodological limitations of the studies, make it possible to understand these discrepancies and highlight questions to be answered in further research. We observed that the studies had heterogeneous small sample sizes (the largest sample size in our review was 42 individuals), including individuals with varying levels of conditioning and sex, performing different exercise modalities. Regarding sex, it is important to highlight that the menstrual cycle can alter performance, and this is usually ignored. Furthermore, women showed more time to exhaustion in repetitive sprint exercises [26]. Factors associated with oxidative and glycolytic capacity, as well as neural factors, seem to be involved. Another factor that is associated with misunderstandings in the literature is the presence of “responsive” and “non-responsive,” [27] so that mechanisms that explain these different phenomena are unknown.

However, in this context, most included studies were double-blinded crossover studies, which are the “gold standard.” Nevertheless, none of these studies explained the efficacy of blindness by asking the participants to state which of the supplements was NaHCO₃ and which was placebo. Once NaHCO₃ is associated with side effects, such as gastrointestinal problems, it is possible that some participants may have identified the experimental treatment and thus influenced the results.

The dosages used and NaHCO₃ preparation varied significantly. Most studies used 0.3 g of NaHCO₃ per kg of body weight acutely (up to 240 minutes before the exercise). In addition, the pharmaceutical type used varied, including jelly capsules, tablets, and fluid solutions [8,12,13,16,20,21,23]. On the other hand, the effects of type (fluid solution or capsule) and time of consumption are also not clear, because the results of acute (0.5-4 h before exercise) or chronic intake of NaHCO₃ (three days before exercise) led to similar results [17,19]. This usual dosage is associated with gastrointestinal disorders, which may reduce athletes’ acceptance of the use of NaHCO₃ [6,7]. A method of avoiding this discomfort referred to as “loading,” has been discussed in the literature [28]. The accumulation of CO₂ in the intestine, due to supplementation, may cause abdominal distension and pain, nausea, and vomiting, which are serious and diverse types of side effects [9].

Only two studies performed loading [17,19]. Loading is the intake of small dosages of bicarbonate throughout the day, usually around three times (together with main meals). Theoretically, this protocol can be used in preparation for many events on successive days or to avoid periods of a high risk of intestinal disorders (close to physical exercise). Freis et al. noted that chronic intake of NaHCO₃ can increase the risk of cardiac dysrhythmia due to alterations in the concentration of potassium in the blood and urolithiasis due to the alkalization of the urine and sodium content [13]. Some authors, however, suggest that chronic use may be better than acute use [9,29]. After loading, plasma concentrations of bicarbonate are maintained even after the interruption of supplementation, so that blood can store extra NaHCO₃ and use it.

The works that had been included in the present systematic review are, in general, of good quality. An inherent but inevitable problem in studies of supplementation is the high risk of bias, as multiple factors can influence the outcomes. As previously mentioned, sample blinding has not been fully explained in detail. In addition, food intake during the experimental protocols has not been reported.

The results presented in this review confirm the potential of NaHCO₃ to increase time to exhaustion. Future studies with long-term supplementation are needed to determine whether chronic supplementation improves adaptation to physical training. Moreover, it is suggested that studies that consider the guarantee the efficacy of the blinding for the placebo and NaHCO₃ conditions and compare the results between individuals that can identify NaHCO₃ from those who cannot. Another point that needs attention is the shortage of studies that investigated supplementation in women and older people, so future studies should address this gap in the current literature.

The limitations of our study were the methodological classification of articles after filtering based on eligibility criteria. The classification of risk of bias was a determinant for the inclusion of studies in the meta-analysis [15]; therefore, some studies with relevant results may have been excluded due to lack of data or their partial provision. It is important to note that the variables associated with performance and fatigue are multifactorial, such as the time of intake of the supplement and the level of training of sports activity practitioners. However, specifically investigated the use of NaHCO₃ supplementation on the time trial performance and resistance to fatigue in general. Thus, future investigations should consider the influence of different intervening factors on performance indicators to better understand this phenomenon.

In conclusion, supplementation with NaHCO₃ has been used by coaches, physical trainers, and nutritionists in sports. The findings of this investigation indicate that supplementation with NaHCO₃ may improve the sports performance of athletes and practitioners of sports, so that time to exhaustion may be the most beneficial performance marker of this supplement, while time trial performance seems not to be influenced by NaHCO₃ supplementation. The findings of this review provide relevant information for sports professionals to make strategic decisions aimed at increasing their performance of athletes and sports practitioners in a specific way.