Dose-response effect of dietary nitrate on muscle contractility and blood pressure in older subjects

Purpose: We have recently demonstrated that dietary nitrate, a source of nitric oxide via the enterosalivary pathway, can improve muscle contractile function in healthy older men and women. Nitrate ingestion has also been shown to reduce blood pressure in some, but not all, studies of older individuals. However, the optimal dose for eliciting these beneficial effects is unknown. Methods: A randomized, double-blind, placebo-controlled crossover study was performed to determine the effects of ingesting 3.3 mL/kg of concentrated beetroot juice (BRJ) containing 0, 200, or 400 umol/kg of nitrate in nine healthy older subjects (one man, eight women; mean age 70+/-1 y). Maximal knee extensor power (Pmax) and speed (Vmax) were measured 2 h after BRJ ingestion using isokinetic dynamometry. Blood pressure was monitored periodically throughout each study. Results: Pmax (in W/kg) was higher (P<0.05) after the low dose (3.9+/-0.4) compared to the placebo (3.7+/-0.4) or high dose (3.7+/-0.4). Vmax (in rad/s) also tended higher (P=0.08) in the low (11.9+/-0.7) compared to the placebo (10.8+/-0.8) or high dose (11.20+/-.8) trials. Eight out of nine subjects achieved a higher Pmax and Vmax after the lower vs. the higher dose. These dose-related changes in muscle contractility paralleled changes in nitric oxide (NO) bioavailability, as reflected by changes in breath NO and plasma 3 nitrotyrosine levels. No significant changes were found in systolic, diastolic, or mean arterial pressure. Conclusions: Varying doses of dietary nitrate have differential effects on muscle function and blood pressure in older individuals. A moderate dose of nitrate increases muscle speed and power, but these improvements are lost at a higher dose. Blood pressure, on the other hand, is not reduced even with a higher dose.