Increasingly, deterioration in the nitric oxide levels of vascular tissue is being seen as a pivotal early change in the aetiology of cardiovascular disease. Reductions in nitric oxide levels decrease the ability of the blood vessels to relax in response to flow (flow mediated vasodilation) and this in turn can lead to high blood pressure and arterial damage. Identification of changes in the flow of blood through arteries, such as artery flow mediated dilation and aortic pulse wave velocity, are therefore useful predictors of future cardiovascular risk for this reason. Such changes to the arteries, termed endothelial dysfunction, may be the cause of atherosclerosis. Endothelial dysfunction is thought to relate to low levels of nitric oxide synthase, the enzyme responsible for synthesising the nitric oxide in vascular tissue. In turn, the low levels of nitric oxide synthase are thought to come about due to high levels of oxidative stress, something which inhibits the enzyme and decreases production of nitric oxide.
Dietary components that reduce oxidative stress may therefore be protective of cardiovascular disease. Fruits and vegetables have been shown to have protective effects against the development of cardiovascular disease. It is not entirely clear why fruits and vegetables protect from cardiovascular disease, and a number of suggestions have been made, including the fibre, vitamin, mineral and phytonutrient content. Some of the components of fruits and vegetables are antioxidants, and these may improve endothelial dysfunction through an inhibitory effect of the oxidative stress that affects the nitric oxide synthase enzyme. However, more recently the dietary nitrate content of fruit and vegetables has been suggested as a possible factor in the protection they provide from cardiovascular disease. Around 80 % of dietary nitrate levels come from vegetables, and so plant foods comprise a significant source of dietary nitrates. These nitrates may be cardioprotective through a protective effect on vascular tissue.
Dietary nitrate is reduced to nitrite by bacterial in the mouth and this nitrite can be absorbed from the gut. The adsorbed nitrite can then be converted to nitric oxide via the nitrite reductase enzyme. Fruits and vegetables may therefore improve vascular function through the provision of a sustained intake of dietary nitrate. Researchers have investigated the effects of high nitrate beetroot juice on endothelial function in hypercholesterolaemic individuals1. Consuming the beetroot juice for 6 weeks resulted in a 1.1 % improvement in flow mediated dilation and small improvements in aortic pulse wave velocity (although the latter was not significantly different from the placebo group). In addition, the researchers reported that the composition of the bacterial genes in the mouth (the microbiome) was significantly different following consumption of the beetroot, but not after consuming the placebo. These results therefore suggest that dietary nitrates may confer health benefits against the development of endothelial dysfunction.
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