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Diabetes  |  HIV

Antioxidants: Hypertension

Introduction

There is a growing body of evidence supporting the influence of various dietary components, including the antioxidant vitamins, on blood pressure.  The strongest effect of antioxidants on blood pressure relates to vitamin C.  Vitamins C and E may affect the vasoconstriction of blood vessels, while vitamin C alone appears to have a direct effect on vascular endothelial function regulating vasomotor tone. 

Studies using supraphysiological doses of vitamin C infused intravenously have shown that ascorbic acid can improve blood flow and endothelium-derived nitric oxide (EDNO)-mediated vasodilation of forearm vessels in hypertensive patients.[1]  Evidence from these and other related studies support a mechanism involving the reduction of oxidative stress.[2]

Oxidative Stress and Hypertension

Biomarkers of elevated oxidative stress have been detected in hypertensive animals and humans.  Compared with normal rats, spontaneously hypertensive rats were found to have significantly lower total antioxidant status and plasma superoxide dismutase (SOD) activity, and higher lipid peroxide levels in plasma and the thoracic aorta.[3]  Similarly, in humans the total antioxidant power (assayed with the ferric reducing activity of plasma or FRAP assay), SOD, glutathione peroxidase, total glutathione, and levels of vitamins A and C were significantly lower in essential hypertensives compared with normotensive adults.[4]

Observational Studies

Evidence from observational studies suggests that dietary intake and plasma antioxidant levels may be associated with blood pressure.  In multivariate regression analyses using data from NHANES III (n = 15,317), serum vitamins A and E were positively associated with both systolic and diastolic blood pressure, while β-carotene was inversely associated with systolic and vitamin C with diastolic blood pressure.[5]  Lower β-carotene levels have also been found among free-living hypertensive adults in Australia.[6]  In the Kuopio Ischaemic Heart Disease Risk Factor Study, both plasmaascorbicacid and serum selenium concentrations had a moderate, independent inverse association with mean resting bloodpressure in Finnish men.[7]  Significant inverse correlations between plasma ascorbic acid and both systolic and diastolic blood pressures have also been found in African-American8 and older adult populations.[9]   

Intervention Studies

High dietary intake (≥ 5 servings/d) of antioxidant-rich fruit and vegetables can significantly increase plasma levels of carotenoids, including β-carotene, and vitamin C and lower both systolic and diastolic blood pressure levels.[10]  Human clinical trials of vitamin C supplementation at 500 mg/d for ≥ 1 month have shown significant blood pressure reductions among hypertensives[11] and older adults,[12] and the reversal of endothelial vasomotor dysfunction in CAD patients.[13]  Vitamin E supplementation at 400 IU/d for 2 month improved the ex vivo resistance of LDL to oxidation among hypertensives to levels similar to those of normotensives.[14]   Prior to supplementation, the hypertensive subjects had a higher susceptibility to LDL oxidation.  The SU.VI.MAX trial of combined antioxidant micronutrient supplementation, however, showed no effect on the risk of developing hypertension after 6.5 y.[15] 

Critical Thinking Questions:

1. Should vitamin C supplements be recommended to all patients presenting with high blood pressure?

1. Should vitamin C supplements be recommended to prehypertensives and other patients at high risk of developing hypertension?

1. Do blood pressure medications affect micronutrient status?

1. Are hypertensive patients who take vitamin C supplements at lower risk of developing stroke?

1. Can antioxidant supplements prevent the development of atherosclerosis in hypertensive patients?

1. What are the risks of recommending antioxidant supplements to hypertensive patients?

1. Is there an ideal or appropriate combination of antioxidant supplements for preventing/treating hypertension?

1. What specific antioxidants (nutrient and non-nutrient) are provided in the DASH diet?   

Professional Societies, Agencies, and Government Organizations:

• National Heart, Lung, and Blood Institute (NHLBI) Workshop on Oxidative Stress/Inflammation:
  http://www.nhlbi.nih.gov/meetings/workshops/oxidative-stress.htm

• The DASH Eating Plan:
  http://www.nhlbi.nih.gov/health/public/heart/hbp/dash/