Hypertension, defined as systolic blood pressure (SBP) greater than or equal to 140 mmHg, or diastolic blood pressure (DBP) of 90 mmHg or greater, is a prevalent risk factor for cardiovascular disease that affects more than 75 million people in the United States. It is also estimated that 1 out of 3 Americans have prehypertension, which is classified as SBP ranging from 120-139 mmHg, or DBP from 80-89 mmHg. As aging occurs, there is a significant increase in the risk for development of elevated blood pressures (BP) at rest or during stress related activities. Measurements of BP response to stress are used to predict individuals at greater risk of developing prehypertension or hypertension. There are many theories as to why BP increases with age such as baroreflex dysfunction, increased sympathetic nervous system activity, and vascular dysfunction. Additionally with aging, there is a decrease in the important molecule nitric oxide (NO), which plays a critical role in the regulation of vasomotor tone as well as in the tonic restraint of central sympathetic outflow, both of which are integral for BP regulation. Consumption of exogenous nitrate/nitrite can increase NO bioavailability resulting in beneficial attenuating effects on BP at rest. However, to our knowledge, no data exists as to whether improvements in spontaneous cardiovagal baroreceptor sensitivity (BRS) occur following nitrate supplementation, and if this contributes to improvements in resting BP. Furthermore, the beneficial effects of dietary nitrate supplementation on BP responsiveness to stress in older adults are unclear. Therefore, we examined whether baroreflex sensitivity is improved in older adults following 4-weeks of dietary nitrate supplementation and whether this improvement is associated with reductions in resting BP. Additionally, we investigated whether dietary nitrate supplementation attenuates BP responses to stress in older adults.
Ten young (26 ± 2 years; 5 male, 5 female) and ten older (67 ± 1 years; 6 male, 4 female) adults volunteered to participate in this study. Young subjects served as a control study group to assess age-related changes in all outcome measures and did not participate in the intervention portion (dietary nitrate supplementation) of the study. Older subjects participated in a randomized placebo controlled cross-over study over a 3 month period consisting of four separate visits (each separated by 4 weeks) comprised of identical testing procedures. Each older subject was randomized to receive either a beetroot powder containing nitrate and nitrite (active) or beetroot powder devoid of nitrate and nitrite (placebo) for 4 weeks. During each study visit, subjects underwent the same battery of tests to assess spontaneous cardiovagal baroreflex sensitivity (BRS), the metaboreflex arm of the exercise pressor reflex and blood pressure responsiveness to cold induced stress (cold pressor testing, CPT). Venous blood samples were obtained each visit to measure plasma levels of nitrate and nitrite.
Despite similar resting BP values between age groups, older adults demonstrated decreases in overall, up, and down spontaneous cardiovagal BRS (64%, 66%, and 61% respectively; P< 0.01 for all) compared to the young adults. During metaboreflex testing, older adults demonstrated a greater increase in SBP than the young during post exercise ischemia (PEI) (P< 0.05). SBP and mean arterial pressure (MAP) responses were similar between young and older adults during CPT, however, young adults demonstrated a greater increase in DBP vs. older adults (P< 0.05). In the older adults, nitrate supplementation increased plasma levels of nitrate and nitrite (P< 0.01 for both) compared to pre-supplementation as well as in comparison to placebo (P< 0.05 for both). Additionally, resting SBP and MAP were reduced following nitrate supplementation (P< 0.05 for both). However, changes in resting BP were not associated with improvements in spontaneous cardiovagal BRS. The magnitudes of change in DBP, SBP, and MAP in response to isometric handgrip exercise (IHG) did not change following nitrate supplementation. However, during the PEI portion of the metaboreflex the magnitudes of change in DBP (P< 0.05), SBP (P< 0.01), and MAP (P< 0.01) were attenuated following active supplementation but not for the placebo group (P=0.25, P=0.99, P=0.45 respectively). In response to cold induced stress, the magnitudes of change in in DBP (P< 0.05), SBP (P< 0.01), and MAP (P< 0.01) were reduced following active supplementation versus no change in the placebo group (P=0.13, P= 0.97, P=0.28 respectively). Our results suggest that there is a beneficial effect of dietary nitrate supplementation on the metaboreflex-mediated increases in BP during exercise in older adults. Additionally, dietary nitrate reduces BP responsiveness to cold induced stress (via CPT) in older adults. In conclusion, we suggest that dietary nitrate supplementation has beneficial antihypertensive properties, and its consideration as an alternative to pharmacological intervention could prove to be therapeutic and cost effective.
| Identifer | oai:union.ndltd.org:uiowa.edu/oai:ir.uiowa.edu:etd-7323 |
| Date | 01 August 2017 |
| Creators | Schneider, Aaron C. |
| Contributors | Casey, Darren P., Pierce, Gary L. |
| Publisher | University of Iowa |
| Source Sets | University of Iowa |
| Language | English |
| Detected Language | English |
| Type | thesis |
| Format | application/pdf |
| Source | Theses and Dissertations |
| Rights | Copyright © 2017 Aaron C. Schneider |
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