Factors influencing dietary nitrate metabolism in humans
Thesis or dissertation
University of Exeter
Dietary nitrate supplementation, with nitrate-rich beetroot juice (BR), increases nitric oxide (NO) bioavailability by the stepwise reduction of nitrate to nitrite and NO. This has been associated with a number of beneficial physiological and exercise performance effects, but understanding of nitrate metabolism is incomplete. The purpose of this thesis was to investigate some of the factors purported to influence dietary nitrate metabolism in humans. Specifically, the influence of oral cavity temperature and pH on dietary nitrate metabolism, and the influence of muscle oxygenation on exercise economy and tolerance were investigated following BR supplementation. Chapter 3: Salivary and plasma [nitrate] and [nitrite] were assessed at a neutral oral pH with a low (TempLo-pHNorm), intermediate (TempMid-pHNorm) and high (TempHi-pHNorm) oral temperature, and at an alkaline oral pH with a low (TempLo-pHHi), intermediate (TempMid-pHHi) and high (TempHi-pHHi) oral temperature. Compared to the TempMid-pHNorm trial (976 ± 388 µM), mean salivary [nitrite] over the protocol was higher in the TempMid-pHHi (1855 ± 423 µM), TempHi-pHNorm (1371 ± 653 µM), TempHi-pHHi (1792 ± 741 µM), TempLo-pHNorm (1495 ± 502 µM) and TempLo-pHHi (2013 ± 662 µM) conditions, with salivary [nitrite] also higher at a given oral temperature when oral pH was increased (P<0.05). The increase in mean salivary [nitrite] was positively correlated with the increase in salivary flow rate when all data were combined (r = 0.48, P<0.01). Plasma [nitrite] was higher 3 hours post BR consumption in the TempMid-pHHi, TempHi-pHHi and TempLo-pHHi trials, but not the TempLo-pHNorm and TempHi-pHNorm trials, compared to the TempMid-pHNorm trial (P<0.05). Chapter 4: Work-to-work step cycle tests were completed to exhaustion (Tlim) in normobaric hypoxia (fraction of inspired oxygen; FiO2: 15%), normoxia (FiO2: 21%) and hyperoxia (FiO2: 40%). Plasma [nitrite] was higher in all BR compared to all PL trials (P<0.05). Quadriceps tissue oxygenation index (TOI) was higher in normoxia compared to hypoxia (P<0.05) and higher in the hyperoxia compared to hypoxia and normoxia (P<0.05). Tlim was improved after BR compared to PL consumption (250 ± 44 vs. 231 ± 41 sec), with the magnitude of improvement being negatively correlated with quadriceps TOI at exhaustion (r = -0.78), in hypoxia (P<0.05). Tlim tended to be improved with BR in normoxia (BR: 364 ± 98 vs. PL: 344 ± 78 sec; P=0.087), but was not improved in hyperoxia (BR: 492 ± 212 vs. PL: 472 ± 196 sec; P>0.05). BR consumption increased peak oxygen uptake in hypoxia (P<0.05), but not normoxia or hyperoxia (P>0.05). Collectively, these findings improve our understanding of dietary nitrate metabolism and might help guide future studies assessing the efficacy of dietary nitrate supplementation on human health and performance.
MbyRes in Sport and Health Sciences