*2.2. Experimental Protocol*

This was a double-blind, placebo-controlled, parallel design study. The randomization was performed by a researcher not involved in laboratory measurements using a block scheme stratified by age and systolic BP (SBP) with a computer program [37]. Forty-four 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) (Figure 1). Participants consumed 9 capsules in the morning and evening for 4 weeks (Kyowa Hakko Bio Co., Ltd., Tokyo, Japan). Each capsule containing CIT, CIT+GSH, and placebo was indistinguishable by size, shape, and taste. Both research staff and participants were blinded to the group allocation until the completion of data analysis. Participants were instructed to avoid ARG or CIT rich containing foods (e.g., watermelon, salmon, nuts [walnuts, almonds], turkey breast) or supplements with NO precursors or antioxidants during the study period. Participants were recommended to maintain their usual diet and physical activity throughout the study. The last capsules were consumed 10–12 h before their laboratory visit at the end of the study. Participants returned their bottles during the last visit and adherence was assessed by capsule count.

**Figure 1.** CONSORT flow chart of participants through the study. CIT, citrulline, CIT+GSH, and CIT+glutathione.

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 and weight were measured using a stadiometer and beam scale (freestanding portable height rod and weigh beam, Detecto, Webb City, MO, USA), and waist circumference was measured using a non-elastic tape. Body mass index was calculated as weight (kg) divided by height squared (m2) and body composition was assessed using a total body dual-energy X-ray absorptiometry scanner (GE Lunar Prodigy; GE Healthcare, Madison, WI, USA).
