Dietary Nitrate Supplementation and Thermoregulation During Exercise

Background and rationale The vast majority of the research involving dietary nitrate supplementation, namely beetroot juice, has been performed mostly on samples of healthy populations. A few clinical populations have been studied, including heart failure with preserved ejection fraction, chronic obstructive pulmonary disease (COPD), and peripheral artery disease (PAD). In various populations, beetroot juice (BRJ) has been shown to decrease overall blood pressure, increase blood flow, increase muscle tissue oxygenation, improve body core temperature regulation, and increase exercise tolerance while decreasing the oxygen cost of exercise.

Blood pressure BRJ supplementation has been shown to have an immediate effect on reducing blood pressure, with the most substantial decreases recorded within approximately 2.5-3 hours following supplement consumption [1, 2]. Systolic, diastolic, and mean arterial blood pressures showed decreases of 10.4±3 mmHg, 8.1±2.1 mmHg, and 8.0±2.1 mmHg, respectively, in healthy volunteers [2]. During exercise in healthy individuals, systolic blood pressure remained lower throughout varying exercise intensities [1]. Blood pressure was also reduced in heart failure with preserved ejection fraction patients. After an acute dose of BRJ, resting systolic blood pressure significantly decreased when compared to placebo conditions (BRJ 127 ± 14 mmHg; placebo134 ± 14 mmHg) [3]. A slight reduction in systolic blood pressure during exercise may be beneficial to PAD patients. Hypertension, or high blood pressure, is a major risk factor for the development of PAD [4]. Decreasing overall blood pressure and slightly decreasing systolic blood pressure in response to light activity would result in a decrease in unnecessary stress placed on the cardiovascular system at rest and during exercise.

In a study involving PAD patients where 7 of the 8 participants were classified as either hypertensive or prehypertensive, BRJ consumption caused a significant reduction in diastolic blood pressure during rest that was maintained during exercise testing [5]. The study concludes that their data suggest that BRJ supplementation reduces blood pressure in PAD patients and this effect is maintained throughout exercise [5].

Oxygen delivery and muscle tissue oxygenation Oxygen demand of working muscle increases as activity level increases the efficiency of oxygen delivery and oxygen utilization is crucial to muscle function, as well as a necessary increase in blood flow. This is especially important in PAD patients; increasing oxygenation to areas of skeletal muscle ischemia may increase physical function. By using near-infrared spectroscopy, muscle tissue oxygenation can be examined. During cycling in healthy males supplementing with BRJ, the right vastus lateralis muscle oxyhemoglobin concentrations were greater than that of the placebo condition [6]. While cycling at a moderate intensity, the male cyclists had a 13% reduction in deoxyhemoglobin concentration amplitude post-supplementation, which indicated a reduction in fractional oxygen extraction in the right vastus lateralis muscle [6]. These results suggest that BRJ supplementation may promote a better balance between localized oxygen delivery and utilization as an index of muscle fractional oxygen extraction [6]. Overall, BRJ supplementation has been shown to increase working muscle tissue oxygenation during exercise.

In PAD patients, the oxygenation of the gastrocnemius with the worst PAD symptoms was monitored during a walking cardiopulmonary exercise test. Subjects showed a 48% reduction in deoxyhemoglobin concentration amplitude following BRJ supplementation, which indicated that fractional oxygen extraction was reduced [5]. During the exercise protocol, deoxyhemoglobin amplitude measures for the BRJ supplementation group at 100 and 200 seconds into exercise were reduced by 44% and 53%, respectively [5]. This response implies that BRJ supplementation in PAD patients improves a balance between local oxygen delivery and utilization as an index of muscle fractional oxygenation extraction by the working muscle. Increasing oxygenation to areas of skeletal muscle ischemia in PAD patients may increase physical function, but in order for this to be as efficient as possible, a dose-response relationship must be determined.