Different aerobic and anaerobic training modalities have been shown to enhance performance, including 5- to 30-second repeated sprints (9,10,15,29), 1- to 2-minute intervals (15,39,40,51), and 2- to 4-minute intervals (34,47). Recent literature has termed this type of training, high-intensity interval training (HIIT). Not surprisingly, HIIT has been hypothesized to improve both oxygen uptake and hydrogen-ion buffering capacity. Exercise-induced decreases in intramuscular pH (i.e., metabolic acidosis) effect excitation-contraction coupling by disturbing calcium regulation (31) and myofilament binding (17,30) thereby causing an inability to maintain a consistent work output. Consequently, the exercise intensity at which performance decrements rapidly ensue has been considered the onset of fatigue (3,5,7,36) or loosely termed the anaerobic threshold (AT). Previous studies have investigated the relationship between the ventilatory threshold (VT) and the lactate threshold (LT), and although they represent different metabolic mechanisms, they appear to be highly correlated and representative of the AT (35,37,50). In fact, several studies have reported improvements in the VT and LT intensities after short-term (2-6 weeks) HIIT (15,28,35). Intermittently exercising at intensities above the AT has also been shown to enhance maximal oxygen uptake (o2max), with HIIT being as effective as traditional endurance training (19,28,35). Therefore, HIIT may be an effective, time-efficient training strategy for improving cardiovascular fitness and the AT, which is particularly important considering that one of the leading barriers to exercise is the time commitment (21). Thus, in theory, the value of HIIT is that the total exercise time is less than most traditional endurance-training techniques to achieve similar fitness and performance conditioning outcomes.
Nutritional supplementation with β-alanine alone has also been shown to improve the VT (43,53). β-Alanine, a nonproteogenic amino acid, is 1 of the 2 constituents of carnosine (histidine and β-alanine), and is considered the rate-limiting step in carnosine syntheses (2,12). Harris and colleagues (22,25) have shown that β-alanine supplementation may improve muscle-buffering capacity by increasing carnosine concentrations. Carnosine is a histidine-containing dipeptide (β-alanyl-l-histidine) and is considered a physiochemical buffer of H+ (24,25). More specifically, the pKa of carnosine is ideal for buffering protons within the physiological pH range (32,41). Therefore, increases in carnosine concentration through regular supplementation with β-alanine may improve muscle-buffering capacity and delay the onset of fatigue during intense exercise (24,44). In fact, Parkhouse et al. (33) suggested that anaerobic athletes may be more efficient at buffering metabolites from high-intensity exercise than endurance athletes or untrained individuals partly because of higher carnosine concentrations (33). Suzuki et al. (44) supported this hypothesis and reported a direct relationship between muscle carnosine concentrations and anaerobic exercise performance. Therefore, it is possible that a combination of HIIT and β-alanine supplementation may result in profound improvements in the AT that otherwise could not have been achieved by HIIT or β-alanine alone.
Most recent studies involving HIIT or β-alanine supplementation have displayed promising results, though the majority of the studies have been performed on men (25,42). However, Edge et al. (14) evaluated the effects of different training intensities on the muscle-buffering capacity in women and reported the greatest improvements occurred with HIIT compared with continuous, moderate-intensity training. In addition, Stout et al. (43) reported a significant delay in fatigue during incremental exercise after 4 weeks of β-alanine supplementation in women. However, there have been no previous studies to examine the combination of β-alanine supplementation and HIIT on cardiovascular fitness and the AT in women. Therefore, the purpose of the present study was to evaluate the effects of cycle ergometry HIIT with and without β-alanine supplementation on maximal oxygen consumption rate (o2peak), VT, and body composition in recreationally trained women.
Experimental Approach to the Problem
This study was conducted with a randomized, double-blind, placebo-controlled, parallel design. After baseline testing (week 0), all participants were randomly assigned to 1 of 3 treatment groups: β-alanine (BA, n = 14), placebo (PL, n = 19), or control (CON, n = 11). The BA and PL groups were given supplements, but the CON group did not engage in exercise training or ingest any supplements for the duration of the study. The BA and PL groups performed the HIIT 3 times per week for 6 weeks, and subsequent testing was conducted for all groups at weeks 4 and 8. A timeline for this study is presented in Figure 1.
Forty-four healthy, recreationally active (1-5 hours of exercise per week) women volunteered for this investigation in mid-August. Table 1 presents the participants' characteristics by group. This study was approved by the University of Oklahoma Institutional Review Board for the protection of human subjects, and written informed consent was obtained from each participant before any testing. Supplement history was also recorded, and none of the participants had taken any nutritional supplements within 9 weeks of their initial testing date.
Body Composition Assessment
Body composition was assessed by a company-certified investigator using air displacement plesmythography (BOD POD®, Life Measurements, Inc., Concord, CA). Before each test, the device was calibrated according to the manufacturer's instructions with the chamber empty using a cylinder of known volume (49.558 L). The participant, wearing a swimming cap and a tight-fitting bathing suit or compression shorts and sports bra, was weighed before being seated in the fiberglass chamber. The device was sealed, and the participant breathed normally for 20 seconds while body volume was measured. The participant was then connected to a breathing tube internal to the system to measure and correct for thoracic gas volume. Percent body fat was calculated from the corrected body volume using the 2-compartment equations of Siri (38) and Brozek et al. (8). Previous test-retest reliability data for this body composition assessment in our laboratory indicated that, for 14 young adults (24 ± 3 years) measured on separate days, the intraclass correlation coefficient (ICC) was 0.99 with a standard error of measurement of 0.47% body fat. Similar ICCs for the same procedure have been reported by Fields et al. (18) (ICC = 0.98).
Graded Exercise Test Protocol
The graded exercise tests were completed on an electronically braked cycle ergometer (Lode, Groningen, Netherlands). Before any bike tests, participants' seat height was measured and recorded for consistency between trials. Seat height was adjusted to ensure a 5° flexion of the knee joint at bottom of the pedal stroke. Manufacturer-provided straps were used to secure the feet to the pedals. After a 5-minute warm-up at 50 W, the workload increased 25 W every 2 minutes. Strong verbal encouragement from the investigators was provided to the participants to maintain 70 rpm, but the test was terminated when the participant could no longer maintain 60 rpm (volitional exhaustion). Each participant's rating of perceived exertion was also recorded during every stage using a standard Borg scale (6). A true o2peak was determined if 3 of the 5 indicators according to the American College of Sports Medicine Guidelines (52) were met.
During the graded exercise tests, respiratory gases were collected and monitored using a metabolic cart (Parvo Medics TrueOne® 2400 Metabolic Measurement System, Sandy, UT, USA). The metabolic cart was calibrated before each test with room air and standard gases of known volume and concentration for the O2 and CO2 analyzers. Flowmeter calibration was also performed before each test. Gases were collected using a 2-way rebreathing valve (Hans-Rudolph Inc., Shawnee, KS, USA) and mouthpiece attached to headgear that supported the weight of the valve and mouthpiece. Participants wore a nose clip to ensure that breathing occurred only through the mouth. O2 and CO2 concentrations were analyzed from sampled gasses that had passed through a heated pneumotach and mixing chamber. The metabolic cart software reported the values as ventilated oxygen and carbon dioxide (o2 and Co2, respectively). o2peak and VT were automatically determined by the manufacturer's software that controlled and analyzed the metabolic data. Ventilatory threshold was represented as the workload (W) at which the VT occurred (i.e., VTW).
After the baseline testing (week 0), participants were required to visit the laboratory on 3 nonconsecutive days per week for 3 weeks to perform the HIIT. After the first 3 weeks of HIIT, participants were retested (week 4) for body composition, o2peak, and VTW, followed by another 3 weeks of HIIT training with an increased volume and intensity from the initial training period. Posttesting occurred during the last week (week 8). All HIIT was performed on the same cycle ergometer adjusted to the same seat height that was used for the graded exercise tests. Participants warmed up at 50 W for 5 minutes followed by 5 sets of 2-minute exercise bouts at a predetermined percentage of their o2peak workload using a fractal periodization scheme (Figure 2). One minute of passive recovery was allowed between each set. Eighteen training sessions were completed overall: 9 sessions before midtesting (week 4) and 9 sessions after week 4, before the posttesting (week 8). Participants consumed their respective supplement powder mixed in 4-8 oz of water 30 minutes before and immediately after each training session.
After the week 0 testing, participants were randomly assigned to either the β-alanine (BA) or placebo (PL) group. The supplements were administered 4 times per day for 21 days. The BA group took a flavored powder blend of 1.5 g β-alanine and 15 g dextrose in 4-8 oz of water, whereas the PL group consumed an identically flavored powder with 16.5 g dextrose. On training days, participants consumed 2 doses in the laboratory: 30 minutes before and immediately after each training session. The remaining 2 doses were taken outside the laboratory later that day at the participant's leisure. On nontraining days, participants were asked to mix and consume their supplements on their own 4 times per day. For the first 3 weeks, 4 packets of the mixture were consumed (loading) After the midtesting (week 4), only 2 servings of the supplements (BA or PL) were consumed per day during the final 3 weeks (maintenance).
Subjects were instructed to continue their normal dietary and physical activity routines throughout the duration of this study. Three-day dietary recalls were administered and evaluated for macronutrient intake during the testing weeks (weeks 0, 4, and 8). Participants were asked to record the specific food-type, amount, and time of consumption for 2 nonconsecutive weekdays and a weekend day. Diet Analysis Plus (DA+ Version 7.0, Thompson Learning 2005) was used to determine total kilocalorie intake (kcal) and total grams of protein and percentage of kcals from protein.
Eight separate 2-way (3 × 3) mixed factorial analyses of variance (ANOVAs) (time [week 0 vs. week 4 vs. week 8] × treatment [BA vs. PL vs. CON]) with repeated measures on the time factor were used to analyze body mass (BM), %fat, FFM, o2peak, VTW, total kcal intake, and grams of protein, and percentage of kcals from protein. Follow-up analyses included dependent-sample t-tests and 1-way ANOVAs. Before all statistical analyses, the alpha level was set to p ≤ 0.05 to determine statistical significance. Data were analyzed using SPSS for Windows version 14.0 (SPSS Inc., Chicago, IL, USA).
Table 2 provides the means and SEs for BM, %fat, FFM, o2peak, VTW, total kcal intake, and total grams of protein and percentage of kcals from protein at baseline (week 0), mid (week 4), and post (week 8) treatment for all groups (PL, BA, and CON). Individual response scores relative to treatment are displayed in Figures 3A-C for BM, o2peak, and VTW, respectively.
There was a 2-way interaction (time × treatment) for BM (p ≤ 0.01, effect size (ES) = 0.27; Figure 4). Body mass increased for the BA group from weeks 0 to 4 (p ≤ 0.01, ES = 0.08), weeks 4 to 8 (p ≤ 0.05, ES = 0.04), and weeks 0 to 8 (p ≤ 0.01, ES = 0.12). However, there were no changes in BM for the PL or CON groups. There were no differences among the groups (BA, PL, or CON) at week 0 (p > 0.05, ES = 0.03), week 4 (p > 0.05, ES = 0.03), or week 8 (p > 0.05, ES = 0.02).
There was no 2-way interaction (p > 0.05, ES = 0.07), no main effect for treatment (p > 0.05, ES = 0.03), but there was a main effect for time (p ≤ 0.01, ES = 0.01). Percent fat decreased from weeks 0 to 4 (p ≤ 0.05, ES = 0.10) and weeks 0 to 8 (p ≤ 0.05, ES = 0.13) for all groups (BA, PL, and CON). There were no changes in %fat from weeks 4 to 8 (p > 0.05, ES = 0.03).
There was no 2-way interaction for FFM (p > 0.05, ES = 0.07), no main effect for treatment (p > 0.05, ES = 0.09), but there was a main effect for time (p ≤ 0.05, ES = 0.09). Fat-free mass increased from weeks 0 to 8 (p ≤ 0.05, ES = 0.28) for all groups (BA, PL, and CON). There were no changes in FFM from weeks 0 to 4 (p > 0.05, ES = 0.19) or weeks 4 to 8 (p > 0.05, ES = 0.04).
Maximal Oxygen Consumption Rate
There was a 2-way interaction (time × treatment) for o2peak (p ≤ 0.01, ES = 0.25; Figure 5A). o2peak increased for the BA and PL groups from weeks 0 to 4 (p ≤ 0.01, ES = 0.61 and p ≤ 0.01, ES = 0.71, respectively), weeks 4 to 8 (p ≤ 0.05, ES = 0.08 and p ≤ 0.01, ES = 0.23), and weeks 0 to 8 (p ≤ 0.01, ES = 1.03 and p ≤ 0.01, ES = 0.74). However, there was no change for the CON group at any time point (p > 0.05, ES = 0.21). There were no differences among the groups (BA, PL, or CON) at week 0 (p > 0.05, ES = 0.27), week 4 (p > 0.05, ES = 0.92), or week 8 (p > 0.05, ES = 1.18).
Power Output at Ventilatory Threshold
There was no 2-way interaction (p > 0.05, ES = 0.05) for VTW (Figure 5B) and no main effect for treatment (p > 0.05, ES = 0.10), but there was a main effect for time (p ≤ 0.01, ES = 0.17). VTW increased from weeks 0 to 4 (p ≤ 0.05, ES = 0.51) and weeks 0 to 8 (p ≤ 0.01, ES = 0.57) for all groups (BA, PL, and CON). There were no changes in VTW from weeks 4 to 8 (p > 0.05, ES = 0.21).
There were no 2-way interactions (p > 0.05) for total kcal, grams of protein, or percent of kcals from protein (Table 2), no main effects for treatment (p > 0.05), and no main effects for time (p > 0.05). There were no differences among the groups (BA, PL, or CON) at week 0 (p > 0.05, ES = 0.21), week 4 (p > 0.05, ES = 0.18), or week 8 (p > 0.05, ES = 0.15).
The main results of the present study were that 3-6 weeks of HIIT on a cycle ergometer elicited 4-18 and 5-16% increases in o2peak for the BA and PL groups, respectively, with no changes in the CON group. Body mass increased by 1-2% in the BA group, but did not change for the PL or CON groups, and the changes in %fat, FFM, and VTW occurred over time for all 3 groups (BA, PL, and CON). Overall, these findings were consistent with the 4-22% increases in o2peak reported after 3-6 weeks of cycle ergometry HIIT in men (20,27,40) and women (13) after NaHCO3 (13), creatine 20,27), or β-alanine (40) supplementation. However, the unique aspect of the present study was that these results were the first to examine the effects of HIIT and β-alanine supplementation on cardiovascular adaptations in women.
Several previous studies have examined the positive training-induced adaptations with HIIT on a cycle ergometer (10,14,15,16,19,34,47). Edge et al. (14,15) studied the effects of 5 weeks of either HIIT or moderate-intensity cycle ergometry training in recreationally active women, and reported that both training regiments elicited similar increases in o2peak and LT. Talanian et al. (47) supported these findings and showed that only 7 sessions of cycle ergometry HIIT improved o2peak in moderately-active women. Other studies have examined HIIT on trained cyclists and reported 4 - 9% increases in o2max after 3 different HIIT protocols (28). Creer et al. (10) reported 5% increases in o2max for both the HIIT and control groups after 4 weeks, however, training outside of the study was not monitored and the control group experienced higher training volumes than the HIIT group, which may explain the lack of differences observed between the 2 training regiments. Overall, the results of the present study extended the previous findings (10,14,15,28,47) and suggested that HIIT training on a cycle ergometer can be used as a time-efficient training method for improving cardiovascular fitness in novice and trained individuals-compared with the more traditional long slow distance training techniques.
β-Alanine supplementation has also been shown to improve cycle ergometry performance (26,39,42,43,49,53). It has been demonstrated that β-alanine supplementation increases muscle carnosine concentrations (25,26,46). Elevated muscle carnosine may improve the H+ buffering capacity, which in turn, may delay the fatigue process during intense exercise (4,23,44). Stout et al. (43) reported increases in VT after 28 days of β-alanine supplementation in women when compared with a placebo. Hill et al. (26) reported increases in cycling time to exhaustion after 4 and 10 weeks of β-alanine supplementation and found higher intramuscular carnosine concentrations. Van Thienen et al. (49) examined whether β-alanine would benefit trained cyclists during a sprint at the end of a 110-minute endurance trial and a 10 minute time trial, which was intended to simulate the end of a race. The authors found that 8 weeks of β-alanine supplementation resulted in 11% higher peak power output and 5% higher mean power than the placebo during a 30-second sprint. However, it should be noted that none of the aforementioned studies (26,43,49) included exercise training in the research designs, which is important because 2 studies have demonstrated that exercise training alone (HIIT or resistance training) can increase skeletal muscle carnosine concentrations (45,48). Therefore, it is possible that the HIIT in the present study may have increased the muscle carnosine concentrations in both the BA and PL groups such that any additional effects of the β-alanine supplementation may not have been detectible in the o2peak scores. Furthermore, VTW increased for all 3 groups (BA, PL, and CON), which suggested that neither the β-alanine supplementation nor the HIIT training impacted VTW. However, the fact that BM increased in the BA group only, provides tentative evidence that the β-alanine supplementation may have elicited some physiological changes. Future studies should consider examining the combined effects of HIIT and β-alanine supplementation on a wide variety of outcome measures, including anaerobic strength and power output measures in addition to the aerobically-based measures assessed in the present study (o2peak and VTW).
Although intramuscular carnosine was not directly measured, it is assumed that it increased with 3 weeks of loading and 3 weeks of maintenance in view of results of previous studies conducted with the same dosing strategies.(11,22,25,26) However, Baguet et al. (1) recently reported that there may be “high responders” and “low responders” to supplementation resulting in variable increases in intramuscular carnosine concentration. Of 8 subjects, only 3 were considered high responders, defined as “>30% increase postsupplementation.” (1) If less than half of a subject pool are “high responders,” little to no effect would be seen especially when in conjunction with HIIT. This could explain why there was no observable difference between the PL and BA groups in the present study.
A particularly unique finding of the present study was the 1-2% increases in BM observed in the BA group. No previous studies have reported significant changes in BM (−0.4 to 1.7%) after β-alanine supplementation (26,40,43,49). Although not statistically significant, the fact that the average BM increase in the CON group in the present study was 0.5% may warrant caution when interpreting the practical significance of this observation. Nevertheless, future studies are necessary to investigate and carefully delineate the mechanisms for β-alanine-induced increases in BM.
Cardiovascular fitness (o2peak) increased in the BA and PL groups after 6 weeks of cycle ergometry HIIT. At the most practical level, HIIT on a cycle ergometer can be a time-efficient method for increasing cardiovascular fitness compared with the traditional long, slow distance training. The lack of difference observed between the BA and PL groups' o2peak after training and supplementation may be explained by a training-induced increase in intramuscular carnosine content that may have masked the increases in muscle carnosine that are known to occur after β-alanine supplementation (45). Edge et al. (14) suggested that training intensity may be a strong factor when addressing changes in muscle buffering capacity because of an increase in intramuscular carnosine concentrations as long as the training intensity is above the LT. The current study's training intensity varied from 90 to 110% o2peak, when the average VT was 68 and 64% of the o2peak for the BA and PL groups, respectively. Although not directly measured, it is likely that the HIIT in this study was completed well above the LT during the 6-week training period. Therefore, it is possible that the β-alanine supplementation had very little additive effects beyond the HIIT-induced adaptations.
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