Dietary Nitrate Supplementation and Thermoregulation

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.

Blood flow and thermoregulatory response Increases in activity and oxygen demand of working
skeletal muscle necessitate increases in blood flow. In PAD patients, increasing blood flow
to working muscle becomes difficult due to atherosclerotic occlusions in the lower extremity
arteries. Research in healthy populations showed significant increases in forearm blood flow
during hand grip exercise in hypoxic conditions following an acute dosage of BRJ in
comparison to placebo (BRJ 373 ± 38 mL/min; placebo 343 ± 32 mL/min).

Nitric oxide signals smooth muscle within the blood vessels (endothelium) to relax, which in
turn increases blood flow to the localized area of vasodilation. In healthy populations,
dietary nitrate supplementation increases vasodilation near the surface of the skin [7]. This
increase in vasodilatory capacity and blood flow would create a stronger temperature gradient
at the level of the skin, which would facilitate more efficient heat exchange as blood is
cooled at the level of the skin (sweat evaporative, conductive, and convective cooling),
causing decreased strain on the body.

In PAD patients, nitrite-related nitric oxide signaling showed to increase peripheral blood
flow to hypoxic tissue, which is supported by a decrease in gastrocnemius deoxygenation and a
decrease in blood pressure [5]. However, during brachial artery flow mediated dilation, peak
dilation did not significantly change (BRJ 42.6 ± 10.6 seconds; placebo 41.0 ± 10.39
seconds), which suggested that endothelial production of nitric oxide did not change [5]. The
unchanging vasodilatory response is likely due to the study not examining a BRJ dose-response
relationship and its effects on vasodilation.

Exercise tolerance and oxygen cost BRJ supplementation has been shown to decrease oxygen cost
during low-intensity and moderate-intensity exercise. In trained populations supplementing
with BRJ, significant decreases in oxygen cost during the beginning stages of exercise have
been detected [8, 9]. Oxygen cost during walking was shown to decrease by approximately 12%
following BRJ supplementation [8]. BRJ supplementation showed a 20% decrease in oxygen cost
during moderate-intensity cycling in recreationally trained men [10]. This reduction in
oxygen cost implies an increase in exercise efficiency in light-to-moderate level exercise,
which in turn increases exercise tolerance. In high-intensity exercise,
time-to-exercise-failure increased by 15% following BRJ supplementation, which is also
suggestive of an increase in exercise tolerance [8].

BRJ supplementation in clinical populations has also shown to improve exercise tolerance. In
heart failure with preserved ejection fraction patients, submaximal endurance exercise
improved by 24% following BRJ supplementation in comparison to placebo conditions (BRJ 449 ±
180 seconds; placebo 363 ± 125 seconds) [3]. In patients with COPD, walking distance
increased by 11% and time to exercise fatigue increased by 6% [11]. In PAD patients, BRJ
supplementation showed an 18% (32 second) increase to onset claudication pain and a 17% (65
seconds) increase peak walking time in comparison to placebo conditions [5]. These results in
PAD patients show a substantial acute response to BRJ supplementation (~2 hours of
consumption) during exercise testing.

In various populations, BRJ supplementation has shown to decrease blood pressure, improve
blood flow, increase muscle tissue oxygenation, safely maintain core temperature, and
increase exercise tolerance. The effects of BRJ specifically in PAD patients showed decreases
in blood pressure, increases in time to onset claudication and exercise tolerance, but did
not show a significant effect on endothelial function. Examining a dose-response relationship
is necessary to determine the responses to BRJ supplementation (endothelial function, leg
function, and blood vessel oxygen carrying capacity) in PAD patients.In this study, a higher
dose of nitrate (280 mL, 16.8 mmol nitrates) will be examined [12]. BRJ supplementation has
shown reductions in blood pressure, increased muscle tissue oxygenation, blood flow, and
thermoregulatory response. These mechanisms all contribute to improving overall
cardiovascular function. If these results are observed during this study, PAD patients may
experience less claudication pain as well as better tolerance to daily physical activities
and exercise.

Inclusion Criteria:

1. be able to give written, informed consent

2. demonstrate positive history of chronic claudication

3. have a history of exercise-limiting claudication

4. have an ankle/brachial index < 0.90 at rest

5. have a stable blood pressure regimen, stable lipid regimen, stable diabetes regimen
and risk factor control for 6 weeks.

6. be between 50-85 years old

Exclusion Criteria:

1. pain at rest (severe claudication) and/or tissue loss due to PAD (Fontaine stage III
and IV)

2. acute lower extremity ischemic event secondary to thromboembolic disease or acute
trauma

3. limited walking capacity due to other conditions other than PAD

4. a form of nitrate supplementation already included in their diet/regimen

5. an allergy to beetroot juice