The effect of dietary nitrate and vitamin C on endothelial function, oxidative stress and blood lipids in untreated hypercholesterolemic subjects: A randomized double-blind crossover study

Abstract

Background: Vitamin C may enhance nitric oxide (NO) production through stepwise reduction of dietary nitrate (NO 3) to nitrite (NO 2) to NO. The combined effect of vitamin C and NO 3 supplementation is relatively unexplored in untreated hypercholesterolemia. Aims: We aimed to examine whether co-administration of vitamin C and nitrate for 4-weeks would improve endothelial function (primary outcome), plasma NO metabolites, oxidative stress, and blood lipids (secondary outcomes). Methods: Subjects 50e70 years of age with low density lipoprotein (LDL) > 130 mg/dL and RHI 2 were enrolled in this randomized double-blind crossover study. Subjects were assigned to two 4-week supplementation treatments starting with 70 ml of concentrated beetroot juice (CBJ) with 1000 mg of vitamin C (NC) or CBJ with matched placebo (N), then switched to alternate treatment following 2-week washout. The change in reactive hyperemia index (RHI), sum of plasma NO metabolites (NO 2 þ NO 3 (NOx)), oxidized LDL (oxLDL), and serum lipids were assessed at baseline and at 4-weeks of each treatment period. Results: Eighteen subjects (11M:7F) completed all study visits. No significant treatment differences were observed in RHI change (N: 0.21 ± 0.12; NC: 0.20 ± 0.17; p ¼ 0.99). Secondary analysis revealed that a subgroup of NC subjects who started with a baseline RHI of <1.67 (threshold value for ED) had greater improvements in RHI compared to subjects with RHI >1.67 (1.23 ± 0.15 to 1.96 ± 0.19; n ¼ 8 vs. 1.75 ± 0.11 to 1.43 ± 0.10; n ¼ 8; p ¼ 0.02). Compared to N, NC experienced a significant increase in plasma NOx (N: 94.2 ± 15.5 mmol/L; NC: 128.7 ± 29.1 mmol/L; p ¼ 0.01). Although there was no significant difference in oxLDL change between treatments (N: À1.08 ± 9.8 U/L; NC: À6.07 ± 9.14 U/L; p ¼ 0.19), NC elicited significant reductions in LDL (N: 2.2 ± 2; NC: À10.7 ± 23; p ¼ 0.049), triglycerides (N: 14.6 ± 43; NC: À43.7 ± 45; p ¼ 0.03), and no change in serum high density lipoprotein. Within treatment group comparisons showed that only NC reduced oxLDL significantly from baseline to 4 weeks (p ¼ 0.01). Conclusions: No between intervention differences were observed in RHI. RHI only improved in NC subjects with ED at intervention baseline. Four weeks of NC enriched the NO pool and promoted reduction of blood lipids and oxidative stress in subjects with hypercholesterolemia. These preliminary findings highlight a supplementation strategy that may reduce the progression of atherosclerotic disease and deserves further attention in studies using flow mediated dilation methods. Clinical trial registration: www.clinicaltrials.gov (NCT04283630).

The Effect of Nitrate-Rich Beetroot Juice on Markers of Exercise-Induced Muscle Damage: A Systematic Review and Meta-Analysis of Human Intervention Trials

“…Animal studies have shown that the amount of nitrate required to attenuate oxidative stress is equivalent to a high dietary intake of nitrate-rich vegetables in humans (e.g., 100-300 g per day) (Carlstr€ om et al 2011;Gheibi et al 2018). Of the available human studies, which are often in clinical populations (Ashor et al 2016;Basaqr et al 2021), the nitrate dose shown to modulate markers of oxidative stress is broadly comparable to the doses used in the studies in the present meta-analysis ($6.4-11.0 mmol/day). In the present review, markers of oxidative stress were only measured by four studies and none of them used the same markers, meaning a meta-analysis was deemed inappropriate.…”

Reactive Oxygen Species (ROS) and Antioxidants as Immunomodulators in Exercise: Implications for Heme Oxygenase and Bilirubin

“…In addition to these observations, it seems reasonable that dietary nitrate may also work alongside exercise to preserve endothelial function [ 196 ]. Basaqr et al recently found that four weeks of concentrated beetroot juice combined with vitamin C improved endothelial function and the lipid profile of overweight subjects with evidence of endothelial dysfunction [ 197 ]. The exact mechanism of action to explain these findings is unknown but is partially explained by the combined antioxidant effects of vitamin C and concentrated dietary nitrate supplements to decrease oxidative stress [ 198 , 199 , 200 , 201 ].…”

Effect of Vitamin C and Protein Supplementation on Plasma Nitrate and Nitrite Response following Consumption of Beetroot Juice

“…Our study did not measure any physiological effects, such as blood pressure, from the treatments; therefore, it is not known if vitamin C plus beetroot juice in our study led to enhanced physiological actions. In contrast, others found greater change in fasting levels of plasma nitrate from baseline to 4 weeks of supplementation with co-administration of 1000 mg of vitamin C and a 6.4 mmol dose of nitrate in beetroot juice compared to nitrate alone [52]. Surprisingly, the change Basaqr et al found in fasting plasma nitrite was greater for the nitrate supplement alone.…”