Effects of Nitrate Supplementation on Cardiovascular and Autonomic Reactivity in African-American Females

Previous studies have shown that beetroot juice (BJ) decreases systolic blood pressure (SBP) and oxygen demand. This study tests the hypothesis that a beetroot juice (BJ) treatment increases heart rate variability (HRV) measured by the average standard deviation of normal-normal electrocardiogram RR intervals (SDNN) and the low frequency (LF), mainly sympathetic, fast Fourier transform spectral index of HRV. The subjects were 13 healthy young adult African-American females. Placebo control orange juice (OJ) and BJ treatments were given on separate days. Blood nitric oxide [NO], SBP and RR intervals were measured at rest and at constant workloads set to 40% and 80% of the predetermined . Two hours after ingestion the BJ treatment increased [NO] and decreased SBP. BJ also increased SDNN at rest and at the 40% workload, without significant effects on LF. SDNN was significantly greater after the BJ than after the OJ treatment, across the two physical activity conditions and SDNN was (negatively) correlated with SBP. These results suggest that BJ decreases SBP and increases HRV at rest and during aerobic exercise. Similar results in subjects with prehypertension or hypertension could translate to a dietary nitrate treatment for hypertension. 1. Introduction Studies have consistently reported a greater prevalence of hypertension in African Americans than in other ethnic groups [1]. Adolescent and young adult African Americans consistently have exhibited greater pressor responses to laboratory mental and physical stressors than Caucasian Americans [2–8]. Because it characterizes the early stages of hypertension, it is speculated that such heightened pressor responsiveness to stress may be a key factor in the pathogenesis of hypertension [9–11]. Blood pressure reactivity during laboratory stressors has also been found to be more predictive of increased ambulatory blood pressure measured three years later, in African Americans compared to Caucasians [12]. In examining blood pressure responses to the physical stress of cycle ergometry in normotensive individuals, 82% of blood pressure hyperresponders developed hypertension after a two-year follow-up [13]. Nitric oxide (NO) is a potent endogenous vasodilator [14] that plays an important role in regulating blood pressure. An ethnicity-specific difference in endothelial NO with less NO production in blacks than whites is reported [15]. There is substantial evidence that the inorganic nitrate anion, either generated in vivo as an oxidative metabolite of NO itself [16] or ingested through the diet, main dietary