Exploring the Effect of Inorganic Nitrates on the Human Microvascular Physiology - a Pilot Study.

Cardiovascular disease (CVD) is the largest cause of global mortality accounting for over twenty-nine percent of all deaths with coronary heart disease and stroke projected to become the global leading causes of mortality by 2020. Increased mortality represents only one side of the problem since the clinical care of CVD is costly and prolonged and potentially diverts family and societal resources towards life-saving intensive medical treatments. Therefore, it is crucial to develop preventative and therapeutic strategies that will reduce the financial and social burden of CVD.

Ageing is a major risk factor for chronic diseases such as cancer, diabetes neurodegeneration and CVD. For example, the prevalence and incidence of Chronic Venous Insufficiency (CVI) is closely linked to an age-related dysfunction of micro-circulation control. Similarly, ageing is associated with increased risk of atherosclerosis and formation of macroscopic lesions in large arteries leading to obstruction of blood flow and onset of ischaemic events . The rapid changes in worldwide trends of age-demographics are therefore closely correlated with the rising burden of CVD.

Hypertension is an important risk factor for CVD. Raised blood pressure (BP) is primarily caused by increased vascular resistance, mainly due to arterial wall thickening, autonomic dysregulation and endothelial dysfunction . Endothelium-derived nitric oxide (NO) is a potent vasodilator that plays a pivotal role in the moment-to-moment control of vascular tone . Therefore, it is no surprise that a key aspect of endothelial dysfunction is a reduced NO production via the L-arginine-dependent enzymatic pathway. NO also decreases platelet aggregation, improves mitochondrial function, and diminishes cellular vascular adhesion . Hence a reduction in NO bioavailability is causally related to an increase in BP and atherosclerotic risk.

Until recently it was believed that dietary nitrate (NO3-) and nitrite (NO2-) offered limited physiological benefits, despite them being produced endogenously as part of the NO metabolic pathway . The discovery of the entero-salivary circulation and non-enzymatic reduction of NO3- into NO2- and NO provides an important alternative source of NO, particularly in environment with reduced pH and oxygen tension . The major sources of dietary NO3- are green leafy vegetables such as rocket or spinach or tuberous product such as beetroot. The typical nitrate intake of a western diet provides between 50-100mg/NO3-/day whereas diets rich in fruit- and vegetables such as the Mediterranean or Japanese diets could provide up to 1000mg/NO3-/day. Increasing evidence seems to support the association between the high NO3- content of these dietary patterns with protective cardiovascular effects.

The cardiovascular benefits of NO3- are thought to be derived by an improvement of endothelial function achieved by improved NO bioavailability. Other mechanisms may include a direct effect of NO2- on calcium-channels in vascular smooth muscle cells (SMVCs) in resistance vessels . The conversion of NO3- into NO2- by the oral micro-environment is a key mechanism of the entero-salivary NO3- circulation and non-enzymatic synthesis of NO. The oral microbiota is critically involved in the first reducing step (NO3- into NO2-) and an alteration of the reducing capacity of the oral microbiota (i.e., spitting, antiseptic mouthwash) interrupts the beneficial effects of inorganic NO3- on vascular function. Siervo et al has recently demonstrated a reduced efficacy of concentrated beetroot juice supplementation in older overweight and obese subjects. Ageing appears to be linked to changes in the oral micro-environment as well as a reduced responsiveness of the endothelium and SMVCs to NO.

However, there is limited knowledge on the influence of the ageing process on factors that affect NO3- conversion starting from the NO3- -reducing potential in the oral environment. Therefore, a better understanding of the age-related mechanisms linked to endothelial dysfunction could lead to the development of effective and targeted dietary strategies for the early prevention of hypertension, arterial stiffness and CVD across the life-course.