Nitrate-rich beetroot juice and its effects on cardiovascular health and cognition in younger and older adults : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Sport and Exercise Science at Massey University, Auckland, New Zealand

Abstract

Background: Evidence suggests supplementation with nitrate-rich beetroot juice (BR) may improve cardiovascular responses and cognition in male and female, younger and older adults. However, there is still limited research in this area, particularly in older adults and with regards to measures of endothelial and cognitive function where equivocal findings have been observed. In addition, it is unclear whether age-related differences impact on the benefits from dietary nitrate supplementation. Older adults tend to have an increase in blood pressure (BP) and decline in cognition with aging, and thus may respond more favourably to BR supplementation compared to younger adults. Additionally, the effects of dose and duration (acute versus chronic) of supplementation with dietary nitrate are still not well known. Overall aims: To 1) investigate the effects of varying doses of nitrate-rich BR on cardiovascular responses and cognition in older adults, and 2) examine the effects of acute and chronic supplementation with BR on cardiovascular responses and cognition in the two age groups. Methods: The “Dose-response study” (Chapter 4) examined the effects of acute supplementation with varying doses of BR (0.15 (placebo; PL), 2.5, 4.9, and 9.8 mmol nitrate) on cardiovascular and cognitive responses in older adults (50-80 years; n=11) in a randomised, double-blind, crossover designed trial. At each visit, resting blood samples for plasma nitrate and nitrite, BP, mean arterial pressure (MAP), systemic vascular resistance (SVR), heart rate (HR), resting metabolic rate, and cognitive function were completed pre- and 2.25 h post-supplementation. The “Acute study” (Chapter 5) was a randomised, double-blind, crossover study. Twenty-four participants, 13 younger (18-30 years) and 11 older (50-70 years), completed resting blood samples, BP, HR, cognitive function, and mood and perceptual measurements pre- and post-supplementation with nitrate-rich BR (10.5 mmol) or placebo (1 mmol). The “Chronic study” (Chapter 6) was a double-blind, randomised control trial investigating acute, acute + chronic, chronic + acute (exercise performance) and chronic supplementation with either nitrate-rich BR (10.5 mmol) or PL (0.15 mmol) over 28 days in 21 younger (18-30 years) and 22 older (50-80 years) adults. On days 0, 14, and 28 resting blood samples, BP, MAP, SVR, HR, and cognitive performance measures were completed pre- and 2.25 h post-supplementation, while oxygen uptake (V̇O₂) and time trial performance were measured 2.5 h post-supplementation. Results: Ingestion of varying doses of dietary nitrate in the Dose-response study increased plasma nitrate and nitrite concentrations in a dose-dependent manner, with a significant difference shown between each treatment for plasma nitrate (p < 0.001). However, plasma nitrite was only significantly increased following consumption of the 9.8 mmol treatment compared to PL (p = 0.004). Systolic blood pressure (SBP), diastolic blood pressure (DBP), and MAP were reduced 2.25 h post-supplementation with the 9.8 mmol treatment compared to the PL, 2.5, and 4.9 mmol treatments (p < 0.05). Following supplementation with the 9.8 mmol dose, Corsi span (cognitive function) was improved during the Corsi block tapering test (CBT) compared to PL (p = 0.013) and 2.5 mmol (p = 0.004) treatments. In the Acute study, supplementation with nitrate-rich BR also significantly increased plasma nitrate (p < 0.001) and nitrite (p = 0.003) concentrations in younger and older adults relative to PL. Systolic BP was reduced in both younger and older adults (p < 0.001) 2.25 h post-supplementation with BR compared to PL, while DBP was only reduced in older adults (p = 0.013). Older adults had a greater increase in plasma nitrite (p = 0.038) and reduction in DBP (p = 0.005) compared to younger adults. Cognitive performance measures were also improved in younger and older adults following acute supplementation with nitrate-rich BR, with a reduction in reaction time observed during the Stroop test (p = 0.045). The Chronic study showed a significant reduction in SBP, DBP, and MAP following acute supplementation with nitrate-rich BR on day 0 in older adults (p < 0.001) and following an acute dose in the context of chronic supplementation (acute + chronic) on day 28 in both older (p < 0.01) and younger (p < 0.05) adults. Fourteen days’ chronic supplementation with nitrate-rich BR, in the absence of an acute dose, also reduced SBP (p = 0.019), DBP (p = 0.004), and MAP (p = 0.005) in older but not younger adults. Older compared to younger adults also had a greater reduction in SBP, DBP, and MAP following acute supplementation with nitrate-rich BR on day 0 (p < 0.05) and following chronic supplementation on day 14 (p < 0.05). Acute + chronic supplementation with nitrate-rich BR reduced SVR in older adults on days 14 (p = 0.032) and 28 (p = 0.016), with a greater reduction in older compared to younger adults observed on day 14 (p = 0.015) and a trend for greater reduction in older adults observed on day 28 (p = 0.056). Acute (on day 0) and acute + chronic (on day 28) consumption of nitrate-rich BR improved reaction time during the Stroop test in older adults (p = 0.042 and 0.006, respectively), while older (relative to younger) adults also showed a greater improvement in reaction time during the Stroop test following chronic supplementation on days 14 (p = 0.016) and 28 (p = 0.02) despite no effect of treatment on these days. Older adults also showed a greater reduction in V̇O2 at 20% completion of the cycle time trial following acute supplementation with BR on day 0 (p = 0.044), despite no effect of treatment at this time point. Conclusion: The results from this PhD project showed that acute and daily supplementation with nitrate-rich BR can improve BP and cognitive function in younger and older adults, with greater benefits observed in the older cohort. Furthermore, chronic supplementation with nitrate-rich BR in older adults can improve BP independent of an acute dose. These results suggest that daily supplementation with nitrate-rich BR may have a role in clinical settings for helping maintain healthy cardiovascular and cognitive function. Future research should investigate the use of nitrate-rich BR as a potential preventative therapy for cardiovascular and cognitive diseases such as hypertension and dementia.