In the present study we investigated the influence of a 4-day alkalizing versus acidizing diet on 400-m sprint performance and associated physiological markers in moderately trained young participants. Our major finding is that the alkalizing diet results in an improved 400-m sprint time, higher blood lactate, but unchanged blood pH values compared with the acidizing diet.
In the present investigation, we found significantly higher urine pH values for the BASE trial (7.0 ± 0.7) compared with the ACID trial (5.5 ± 0.7). Thus, we assume that the dietary intervention was conducted successfully because a urine pH of ≥7.0 is expected for successful low-PRAL diets and ≤ 6.0 for high-PRAL diets [7, 25].
To the best of our knowledge, this is the first study to estimate the influence of acid- and alkaline-forming nutrition on anaerobic exercise performance with high applicability for a sport discipline. There are a number of studies estimating the effects of a dietary acid load on anaerobic exercise performance using exercise tests exclusive to cycling or treadmill running [2, 6, 7, 18, 21, 22]. However in a recent review, Applegate et al.  postulated a lack of studies examining different exercise intensities and measures of performance regarding an alkalizing diet. Additionally, Caciano et al.  recommended dietary manipulation of PRAL for sporting events where performance is limited as result of acidosis, like 100–200-m swimming or 400–800-m running events. Sprint performance for 400-m trials has already been suggested to improve after ingestion of NaHCO3 [16, 23], but this has not been investigated for an alkalizing diet. Therefore, based on the presumption that an alkalizing, low-PRAL diet increases systemic alkalinity and blood buffer capacity, we hypothesized that an alkalizing diet also increases 400-m sprint performance [7, 21]. Indeed, in the recent study, 400-m sprint performance time was significantly lower for the BASE trial compared with the ACID trial, indicating that sprint performance was enhanced after consuming mainly low-PRAL nutrients for 4 days prior to the sprint test. However, the sprint performance enhancement was only 2.3% in our study and less pronounced compared with the 21% increase of exercise performance in the recent literature . We consider the difference in the performance tests as the main reason for this incongruence. Whereas we estimated performance as the run time for a fixed distance (time-trial test), Caciano et al.  assessed anaerobic performance as time-to-exhaustion while running on a treadmill with an individually defined and fixed speed. Open-ended protocols with time-to-exhaustion introduce larger variability in performance output than distance-based performance tests, mainly because of motivational and mental aspects [23, 26]. Therefore, we assume that a lower but more constant performance improvement is to be expected for time-trial tests, such as a 400-m sprint trial, compared with time-to-exhaustion tests after an alkalizing low-PRAL diet . Thus, 400-m sprint performance time was enhanced after the low-PRAL diet, though the use of hand timing to measure the 400-m time trial is one of the limitations of this study. The most precise and preferred method of timing is by electronic methods because of the absolute errors associated with hand timing [27, 28]. For example, variations among hand timers are likely to occur . Additionally, hand timing produces a faster sprint time than electronic timing [28, 29], and a correction factor of 0.2 s has traditionally been used for hand timing . On the other hand, small mean errors (0.04–0.05 s) and very high correlation values (ICC 0 0.99) have been observed between hand timing and electronic timing, which indicates that hand timing produces consistent sprint times for the same hand timer [28, 31]. Hand timing was the only method available to be used in evaluating sprint times in this investigation. Therefore, we decided to apply several measurement strategies supposed by Mayhew et al.  in order to minimize problems with this method. We used the same timer for each participant to attempt a higher interrater reliability of the hand-timing method, and timers were positioned in consistent timing positions perpendicular to the finish line. Each timer was proficient in the use of a stopwatch and spent time learning the characteristics of the stopwatch used in this study. Furthermore, we asked the tester to initiate the timing with the index finger and not with the thumb, as it was previously reported that the most reliable and objective handheld stopwatch times are achieved when the timer uses the index finger to operate the stopwatch [28, 32]. We think that these strategies reduced the errors associated with hand timing and resulted in consistent sprint times within the present study.
Blood lactate and blood gas analysis
We found significantly lower values for the blood gas parameters pH, [HCO3−], and BE post-exercise compared with pre-exercise for both dietary interventions (BASE and ACID). This indicates a profound exercise-induced metabolic acidosis after 400-m sprint trials for both conditions.
Further, we found higher maximum post-exercise lactate concentrations after 400-m sprint performance during the BASE trial compared with the ACID trial. Robergs et al.  state lactate production during intense exercise more as a consequence rather than a cause of cellular conditions that cause acidosis. However, these authors conclude that lactate is still a good indirect marker for cellular metabolic conditions that induce metabolic acidosis because increased lactate production coincides with acidosis . Therefore, higher blood lactate values during the BASE trial within the recent study in combination with the improved 400-m sprint time (i.e., more energy demand per time unit) might indicate a higher efflux of H+ ions from the muscle cell across the interstitial space and into the venous circulation, creating a more severe metabolic acidosis. However, we found no differences in blood pH between BASE and ACID within the recent study. The lack of differences in blood pH between both dietary interventions is probably a result of the higher blood buffer capacity because of high [HCO3−] concentrations associated with an alkalizing diet .
An augmentation of the [HCO3−] concentration as well as an increased blood pH can both be found after sodium bicarbonate supplementation [15, 16, 24, 35]. Unfortunately, the alkalizing or acidizing dietary intervention did not result in significant differences for any of the blood gas parameters within this study (Table 1). However, we found a slight tendency towards higher [HCO3−] values following a low-PRAL diet for 4 days (Table 1). It has been suggested in recent literature that alkalizing diets are unlikely to produce the same changes in buffer capacity compared with alkalizing ergogenic aids and that consumption of low-PRAL diets produces only a slight, but insufficient alkaline environment to enhance buffer capacity [2, 13]. Our study, however, clearly indicates that total buffer capacity must have been increased after a 4-day alkalizing diet because we did not find changes in blood pH but increased blood lactate concentrations and faster 400-m sprint times. Therefore, we assume that the non-significant tendencies towards [HCO3−] and BE values (Table 1) indicate a higher buffer capacity after an alkalizing diet and might be more apparent when testing a larger sample size or longer duration of the dietary intervention.
First, a large inter-subject variation in PRAL from normal Western diets exists among athletes [6, 12]. Considering this individual variability, sprint athletes and coaches should be encouraged to undergo a dietary assessment, including urine pH measurements, before an alkalizing diet is applied. Fasted morning urine pH can be monitored for assessment and during the low-PRAL dietary intervention to confirm that the diet adequately alters dietary acid load . Urinary pH values of ≥7.0 may be interpreted as a successful low-PRAL diet and values of ≤6.0 as high-PRAL diets [7, 25]. However, individual variability must be considered when interpreting urine pH values.
Second, when consuming alkalizing diets, it is often suggested to obtain the PRAL by increasing consumption of fruits and vegetables and minimizing consumption of meats and grains . Based on this advice, a caloric deficit during consuming alkalizing diets is reported . Conclusively, especially for sprint athletes, the higher energy demands and needs for dietary protein and carbohydrate sources, of which increase the PRAL, may make it difficult to realize an alkalizing diet [6, 12, 13]. Regarding this problem, we highly advise the additional use of mineral waters rich in bicarbonate to simplify the realization of an alkalizing diet [13, 36, 37]. Additionally, consumption of carbohydrate-rich fruits and vegetables, such as fresh and dried fruits, fruit juices, and potatoes, should be encouraged . A food diary might be used to control the amount of foods eaten during a low-PRAL diet period. Food diaries can be analyzed for energy and macronutrient intake as well as for calculation of the overall PRAL per day. The lack of food diaries as well as analyses of PRAL values, energy intake and macronutrient content is another limitation of the present study. We asked our participants to report the foods eaten within each day of the dietary interventions, however, we did not collect amount of foods. Thus, we assume that the dietary interventions had been conducted successfully, because diaries mainly contained of vegetables and fruits during the low-PRAL diet and of grain and dairy products during the high-PRAL diet. However, we were not able to analyze energy intake or macro- and micronutrient content of the foods. Therewith we cannot report about an influence of carbohydrate (CHO) content on sprint performance, which has already been investigated [38, 39]. Couto et al.  showed that a high CHO diet induced higher CHO oxidation rates and increased running speed in 400-m sprints. Although, we do not think that high CHO intake might have influenced the 400-m sprint performance for the low-PRAL trial positively in this study. We presume low-PRAL dietary recommendations for that, because recommendations limit the use of carbohydrate sources (grains, e.g. bread or pasta) as they increase the PRAL. Therewith, these dietary recommendations lead more to caloric deficits during consuming alkalizing diets than to CHO loading .
In addition, some authors suggest a responder/non-responder phenomenon to the ergogenic potential of bicarbonate supplementation, with a tendency for highly trained athletes to show higher effects than untrained individuals [15, 40]. Gastrointestinal (GI) discomfort is dosage-dependent for NaHCO3 ingestion and GI discomfort may negatively affect sprint performance [15, 41]. Neither has been reported for a low-PRAL diet so far; however, we highly recommend a test phase for each athlete during a non-competitive training period before changing the usual diet during competitions to inhibit discomfort from the dietary intervention.
Moreover, alkalizing low-PRAL diets lead to a chronic alkalotic state and, therefore, might be compared with chronic use of NaHCO3 in some respects. There is evidence that despite the acute effects of bicarbonate ingestion on anaerobic performance in competitive situations, chronic use of NaHCO3 in combination with specific training may lead to aerobic adaptions. Chronic NaHCO3 ingestion coupled with high intensity training may further influence mechanisms associated with muscle force production or rapid force-generating capacity . The authors conclude that there is a lack of investigation into the possible effects of chronic adaptions to training in an alkalotic state. Regarding alkalizing dietary recommendations for sprint athletes, which are mainly chronic interventions, further research in this field is needed to clarify these training effects in a chronic alkalotic state.
Limitations of the study
A limitation of our study was the use of hand timing, which introduces a certain level of inaccuracy in measuring 400-m sprint performance. Previous studies have shown that electronic timing is the more precise and preferred method of timing [27, 28]. To reduce the potential errors associated with hand timing, we applied several measurement strategies, including using the same timer for each participant, and consistent positioning of the timers perpendicular to the finish line . However, future studies should consider electronic timing when investigating 400-m sprint performance in a small sample size. Another limitation of the study is the lack of quantitative information on food intake to allow for detailed analyses of PRAL values, energy intake, and macronutrient content. The participants were asked to provide daily qualitative food reports. However, the total amount and dietary composition were not controlled. The food reports conducted in our study mainly contained vegetables and fruits during the low-PRAL diet, and grain and dairy products during the high-PRAL diet. The application of extensive nutritional analyses in future studies is required to support the validity of our findings. Finally, there was a small sample size (n = 11) in our study, which resulted in wide confidence intervals and high p-values. Nevertheless, despite this limitation, a significant effect of dietary intervention was observed.