Dietary Nitrate Influence on Oxygen Uptake Response to Pseudorandom Binary Sequence Cycling Protocols

Abstract

Dietary nitrate supplementation in the form of beetroot juice has been the subject of numerous studies in recent literature with inconclusive results. Beetroot juice (BRJ) has been consistently tested using constant-load or maximal-exertion exercise, but the influence of nitrate on pulmonary oxygen uptake (VO2p) and microvascular oxygenation to continually-changing exercise demands has received less attention. Five healthy males (30 ± 6 yrs, 83.4 ± 20.9 kg, VO2pk, 38.9 ± 5.9 mL/kg/min; mean ± SD) completed a ramp exercise to fatigue for the determination of the lactate threshold (LT) and peak VO2p (VO2pk). Each bout of PRBS exercise transitioned from a baseline of 20 W to a WR corresponding to 90% of LT (MOD, 100 ± 7 W). The PRBS exercise consisted of 30 units of 15 s that alternated between BSL and 90% LT in a pseudorandom pattern; the pattern was repeated 3 times. VO2p was measured breath-by-breath using a metabolic cart and interpolated to 1 s intervals. For each subject and WR, the 3 repetitions for each trial were ensemble averaged yielding a single 450 s response. Changes in hemoglobin oxygenation (deoxy-[Hb+Mb]) and total hemoglobin concentration ([THC]) were measured from the vastus lateralis (VL) using frequency-domain NIRS. Data were collected at 1 Hz and ensemble averaged to yield a single 450 s response. VO2p gain was calculated in the time domain by dividing the ΔVO2p by the ΔWR. VO2p and deoxy-[Hb+Mb] responses were transformed into the frequency domain and analyzed using standard approaches and the mean-normalized gain (MNG). Subjects completed the first trial under control conditions (CON), and then were put on a dosing regimen of 400mg of dietary nitrate (Beet It! Beetroot Juice) 2x/day for 3 days before returning to the lab for the second trial (BRJ). The gain (VO2/WR) was decreased in the BRJ condition versus CON (CON: 5.58 ± 0.65 mL/min/W, BRJ: 4.66 ± 1.03 mL/min/W, p = 0.049). The VO2 harmonics were not different between conditions. The deoxy-[Hb+Mb] harmonics were lower at all frequencies in the BRJ condition versus CON (p < 0.05). MNG is a measure that isolates the temporal characteristics of the investigated variable, using a normalization procedure. MNG for VO2 was not significantly different between conditions. The deoxy-[Hb+Mb] MNG was lower in BRJ (CON: 77.4 ± 26.6 %, BRJ: 42.3 ± 11.8 %, p = 0.011). The results of the current study are consistent with previous studies showing that BRJ supplementation decreases the oxygen cost of exercise for a given work rate. The similarity in the harmonics and the MNG for VO2 suggest that there was no impact of BRJ on the speed of VO2 adaptation, but the significant decrease in the MNG for the deoxy-[Hb+Mb] suggests that the rate of adaptation of deoxy-[Hb+Mb] was slower in the BRJ condition. The PRBS protocol has many on- and off-transitions that may identify influences of dietary nitrate that are not discernable during the rapid adaptation of deoxy-[Hb+Mb] to a single-step transition. Additionally, the results of the current study indicate that further investigation is warranted into the influences of dietary nitrate on the microvascular response to exercise.