Abstract
Dietary inorganic nitrate supplementation (probably via conversion to nitrite) increases skeletal muscle metabolic efficiency. In addition, it may also cause hypoxia‐dependent vasodilation and this has the potential to augment oxygen delivery to exercising skeletal muscle. However, direct evidence for the latter with spatial localization to exercising muscle groups does not exist. We employed quantitative functional MRI (fMRI) to characterize skeletal muscle oxygen utilization and replenishment by assessment of tissue oxygenation maximal change and recovery change, respectively. Eleven healthy subjects were enrolled, of whom 9 (age 33.3 ± 4.4 years, five males) completed the study. Each subject took part in three MRI visits, with dietary nitrate (7cl concentrated beetroot juice) consumed before the third visit. During each visit fMRIs were conducted concurrently with plantar flexion exercise at workloads of 15% and 25% maximum voluntary contraction (MVC). No significant changes were found between visits 1 and 2 in the fMRI measures. A decrease in maximal change was found at 15% MVC in soleus between visits 2 and 3 (5.12 ± 2.36 to 2.55 ± 1.42, P = 0.004) and between visits 1 and 3 (4.43 ± 2.12 to 2.55 ± 1.42, P = 0.043), but not at 25% MVC or within gastrocnemius. There was no difference in recovery change between visits. We found that dietary nitrate supplementation reduces tissue oxygenation alterations during physical exercise in skeletal muscle. This effect is more prominent in muscles with predominantly type 1 fibers and at lower workloads. This indicates that in healthy subjects dietary nitrate predominantly affects skeletal muscle energy efficiency with no change in oxygen delivery.
Highlights
Skeletal muscle function is often altered by ischemic vascular diseases, such as peripheral arterial occlusive disease (PAOD) (Casaburi 2001; Criqui MaN 2006)
At 15% maximum voluntary contraction (MVC), tissue oxygenation maximal change showed no significant difference between visits 1 and 2
We have employed a quantitative functional MRI (fMRI) method to examine the effects of dietary nitrate supplementation on tissue oxygenation in skeletal muscle during physical exercise
Summary
Skeletal muscle function is often altered by ischemic vascular diseases, such as peripheral arterial occlusive disease (PAOD) (Casaburi 2001; Criqui MaN 2006). Dietary inorganic nitrate (found in high concentrations in green leafy vegetables and in beetroot) supplementation has been shown to improve skeletal muscle metabolic efficiency during exercise and in hypoxic environments (Engan et al 2012), and has blood pressure lowering and antiplatelet effects (Webb et al 2008). Dietary nitrate is absorbed from the gut, taken up into the salivary glands and concentrated in the saliva (Kapil et al 2013). Physiological Reports published by Wiley Periodicals, Inc. on behalf of the American Physiological Society and The Physiological Society
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