Baseline characteristics
We screened 157 patients with a definite diagnosis of VVS who were referred for HUTT to the syncope unit of Tehran Heart Center from February 2020 to February 2021. After exclusion of 107 patients due to taking vitamin B supplements, two due to taking carbamazepine, two due to taking phenobarbital, one due to a history of sleeve gastrectomy, and one due to a history of malignancy, we included 44 patients aged 37.9 (14.7) years consisting of 23 (52.3%) females, in the case group. For the control group, we had 44 age- and sex-matched individuals without any history of syncope from TeCS, aged 37.9 (13.9) years, comprising 23 (52.3%) females. The baseline characteristics of the participants are presented in Table 1. We found no significant difference between the study groups regarding characteristics that may potentially confound serum levels of vitamin B12 or folate, including smoking, alcohol consumption, related medication, and past medical history (Table 1). In the case group, 13 (29.5%) and 21 (47.7%) patients had >3 syncopal episodes in the last year and the lifetime, respectively. According to our definition, 28 (63.4%) patients had frequent VVS versus 16 (36.4%) patients with infrequent VVS in the case group. Furthermore, 17 (38.6%) patients had a positive HUTT with vasodepressor response as the most common response, 7 (41.2%) patients, followed by the mixed response, 5 (29.4%) patients (Table 1).