The endothelium is a thin membrane that lines the inside of the heart and blood vessels and endothelial cells release substances that control vascular relaxation and contraction.
Postmenopausal women (PMW) may experience endothelial dysfunction associated with arginine (ARG) deficiency relative to asymmetric dimethylarginine (ADMA) caused by oxidative stress. This can develop into arterial stiffness and hypertension in healthy postmenopausal women.
Another mechanism for hypertension in postmenopausal women is elevated sympathetic activity. Evidence indicates that augmented systolic blood pressure (BP) reactivity to a sympathetic stimulus (cold pressor test, CPT) predicts future hypertension and heart failure.
Nitric oxide (NO) is the main vasodilator (meaning it dilates blood vessels) and this is produced from the amino acid L-arginine (ARG) by endothelial NO synthase (eNOS). Dietary strategies to increase circulating ARG may improve vascular function but oral ARG becomes ineffective when there is stimulation of arginase activity (caused by inflammation).
Citrulline (CIT) inhibits arginase activity, and thus, has been found to be more efficient than oral ARG in increasing circulating ARG levels.
The purpose of this study was to investigate the effects of supplementing daily for four weeks with CIT alone and CIT alongside the antioxidant glutathione (GSH) on vascular function, assessed by brachial flow-mediated dilation (FMD, a biomarker for predicting cardiovascular event), arterial stiffness, and BP reactivity to CPT in postmenopausal women. Moreover, they assessed fasting blood glucose (FBG), serum levels of ARG and its metabolites, and markers of oxidative stress.
Their hypothesis was that both interventions would improve responses to sympathetic activation induced by CPT.
They found that CIT+GSH supplementation improved endothelial function through an increase in the ARG/ADMA ratio, as well as attenuating BP responsiveness to CPT.
Although CIT supplementation increased serum ARG levels and the ARG/ADMA ratio, it did not statistically improve FMD.
The authors conclude: “These data show that CIT+GSH supplementation has protective cardiovascular effects at rest and during sympathetic stimulation (CPT) in healthy postmenopausal women.”
The endothelium and vascular function
The effectiveness of short-term oral ARG supplementation for improving FMD has been observed in healthy older adults.
Evidence supports the benefit of CIT supplementation on the attenuation of BP reactivity to CPT in healthy adults. A placebo-controlled study examined the effect of CIT supplementation on FMD but failed to detect improvements in healthy adults.
Recent evidence supports the combined use of CIT and GSH (CIT+GSH) to improve eNOS activity in a mouse ischemia model and the long-lasting increase of plasma NO levels in young men. Although combined CIT+GSH supplementation increased post-exercise plasma NO metabolites (NOx), an indicator of improved endothelial function, vascular function measures were not evaluated. Therefore, it is unknown whether CIT+GSH supplementation can improve vascular function and BP reactivity in postmenopausal women.
Given that CIT supplementation increased circulating ARG levels and the ARG/ADMA ratio, an indicator of increased NO availability, improvements in brachial FMD are expected in postmenopausal women.
The study
This double-blind, placebo-controlled, parallel design study, funded by Kirin Holdings, recruited 44 healthy postmenopausal women aged 51–74 years.
Participants were randomly assigned to a daily supplementation with a placebo (crystalline cellulose, n = 17), CIT (6 g, (n = 13) or CIT+GSH (2 g + 200 mg: Setria,n = 14).
Participants consumed 9 capsules in the morning and evening for 4 weeks (Kyowa Hakko Bio Co., Ltd., Tokyo, Japan, owned by Kirin Holdings).
Participants were instructed to avoid ARG or CIT rich containing foods (e.g., watermelon, salmon, nuts, turkey breast) or supplements with NO precursors or antioxidants during the study period.
The primary endpoint was the change in endothelial function, as assessed by brachial FMD. The secondary endpoints were reductions in PWV at rest and BP responses to CPT. Tertiary endpoints were changes in serum glucose, insulin, ARG, CIT, ORN, ADMA, arginase I, and oxidative markers.
Participants completed a health-history questionnaire and were familiarized with the protocol. Height, weight, waist circumference, BMI and body composition were all measured before and after four weeks of intervention.
The authors note limitations to their study include the relatively small sample size.
FBG was not considered for randomization, which resulted to be higher in the CIT+GSH group. The authors note that a GSH dose greater than 200 mg daily for longer than four weeks of supplementation may reduce markers of oxidative stress, as previously shown.
Source: Nutrients
https://doi.org/10.3390/nu15071557
“Combined Citrulline and Glutathione Supplementation Improves Endothelial Function and Blood Pressure Reactivity in Postmenopausal Women”
Authors: Figueroa. A., et al