Dietary nitrate supplementation enhances exercise performance in peripheral arterial disease


Journal Title (Medline/Pubmed accepted abbreviation): J. Appl. Physiol.
Year: 2011
Volume: 110
Volume: 6
Page numbers:1582-1591
doi (if applicable): 10.1152/japplphysiol.00071.2011

Summary of background and research design

Background: Nitric oxide (NO) is a compound that is produced in the body from circulating nitrite (NO2-), which is in dynamic equilibrium with nitrate (NO3-).  NO is thought to increase blood flow, especially when oxygen is scarce.  Beetroot juice has high amounts of nitrate, and is therefore hypothesized to increase NO, increase blood flow to muscles, and increase the ability of the muscles to perform during physical activity.

In this study, the participants were patients with peripheral artery disease (PAD), a condition in which blood flow is poor, especially in the legs, causing walking more than a few minutes to be painful.

Hypothesis: Beetroot juice, because its high level of nitrate, is hypothesized to increase the production of NO, increase blood flow in people with PAD, and therefore allow them to walk greater distances without pain.

Subjects: PAD patients, 4 male and 4 female, age 67 ± 13 y

Experimental design: randomized, cross over

Treatments: 500 mL of beetroot juice or a placebo (orange juice)

Protocol: The participants arrived at the laboratory fasted and they sat for 10 min. They then had resting blood pressure assessed and a 5 mL blood draw. They consumed the supplement and then sat 120 min (2 hrs). Subsequently, blood pressure was measured, a second blood sample was acquired, and further vasculature tests were undergone. The subjects had a low nitrate snack with any of their blood pressure medications then sat for 30 min more. About 180 min (3 hrs) after consuming the beverage, the participants underwent cardiopulmonary exercise testing; they walked on a treadmill at 2 mph with a 2% increase in grade every 2 min. Meanwhile, their gastrocnemius (calf) muscle was being monitored for oxygenation by near infared spectroscopy (NIRS), their blood pressure was checked every time the grade was increased, and respiratory gases were monitored at 15 sec intervals. The onset of claudication (pain due to lack of blood flow) was determined and the participants walked until exhaustion (“peak walking time”). The subjects repeated the protocol 7-14 days later with the other beverage.
 
Summary of research findings
  • Plasma nitrate levels were significantly greater after the beetroot juice vs. the orange juice (about 400-500 µM compared to <100 µM for the placebo).  Plasma nitrite levels were elevated as well.
  • The beetroot juice allowed subjects to walk an average of 32 sec longer (18%) before onset of claudication pain (P < 0.01) and 65 sec longer (17%) until exhaustion (P < 0.05).  The beetroot juice helped 7/8 of the patients for both metrics and 8/8 patients for at least one metric.
  • According to NIRS, oxygenation of the calf muscle was greater following ingestion of beetroot juice vs. the placebo.
  • The beetroot juice significantly reduced the participants’ diastolic blood pressure 3 hrs after juice ingestion and through the first part of treadmill exercise.
  • Artery dilation was not noted after consumption of either beverage, though the arteries would have been expected to be dilated if NO production was increased.

Interpretation of findings/Key practice applications

An acute dose of beetroot juice increased the time of pain-free walking and the time to exhaustion to 1/3 the extent of a 3 month exercise intervention in PAD participants.  This shows that beetroot juice provides a significant benefit to this population during physical activity and could increase the ability of PAD patients to complete routine physical activities with less pain.  It is believed that the nitrate content of the juice (maybe converted to NO within the muscle) causes blood to flow into the muscles, reduce pain from too little blood flow, and reduces other feelings of discomfort during exercise. Another potential explanation for the observed results is that nitrate could improve the efficiency of  oxygen utilization by the gastrocnemius muscles.  This would account for the improved walking performance despite  30-48% reductions in fractional oxygen extraction that were observed.  Similar results regarding the ability of nitrate to improve the efficiency of oxygen utilization by muscle tissue have been reported in other studies.

Limitations

Because these people have poor circulation, the results of this study should not be directly translated to people that have healthy circulation. However, there are several studies showing positive effects of either beetroot juice or sodium nitrate supplementation on exercise performance and reduction of the oxygen cost of exercise in healthy individuals.

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