Ageing, dietary nitrate and whole beetroot consumption :acute and long-term effects on metabolic, vascular and cognitive function

Abstract

Dietary nitrate is a natural component of vegetables that can have profound effects on physiological functions in the body. One vegetable in particular, beetroot (Beta Vulgaris), has received attention in the last decade as a source of nitrate and more importantly, nitric oxide (NO). NO is a potent vasodilator that is produced endogenously from L-arginine but can also be produced from the sequential reduction of dietary nitrate to nitrite in the oral cavity and further to NO in the acidic environment of the stomach. The production of NO is essential in maintaining a healthy vascular system and low availability of NO is implicated in vascular dysfunction and thus the onset of cardiovascular and cerebrovascular diseases. Low endogenous NO production is closely associated with the ageing process and could contribute to the increase in high blood pressure and impairment of endothelial and cognitive function seen in older adults. Moreover, it is postulated that age attenuates the production of NO from exogenous nitrate, further contributing to the symptoms of low NO availability. Thus, ways to increase NO availability in an ageing population are imperative. Although highly concentrated beetroot juice has been used as an effective vehicle of dietary nitrate in several studies, there is a significant gap in the research regarding longer-term intervention, for which whole beetroot may be more applicable. Processed forms of beetroot are more costly, contain more sugar and lack the dietary fibre present in the whole vegetable. Fibre, alongside the range of phenolic compounds, antioxidants and vitamins present in beetroot, may have further health benefits. Therefore, the overarching aim of this thesis was to investigate the effects of whole beetroot consumption in both acute and prolonged studies, particularly in an older population. A number of noteworthy findings emerged from the studies conducted during this research. Firstly, although whole beetroot, providing between 272mg and 816mg nitrate, was found to effectively raise plasma nitrate and nitrite levels in older participants, this did not translate to beneficial physiological outcomes such as blood pressure reduction and improved blood flow, which were evident in a younger population. Larger nitrate doses (1000mg) in the form of potassium nitrate did, however, lower blood pressure in an older population, reducing the disparity between the age groups. Secondly, more prolonged and consistent consumption of iii whole beetroot over an eight-week period was found to reduce blood pressure in an older population and led to improvements in gut bacterial diversity, short-chain fatty acid (SCFA) production, cognitive reaction time and memory retrieval speed. This suggests additive effects of dietary nitrate beyond those seen in an acute setting and highlights the benefits of the whole vegetable as a supplement to the diet. In summary, whole beetroot was found to be an acceptable dietary source of nitrate and one that had acute physiological benefits in a young population. Ageing, however, reduced the activity of dietary nitrate in this acute setting. Despite this, the prolonged consumption of whole beetroot has the potential to lower blood pressure, improve cognitive function and potentially impact on gut health in an ageing population