Background — Recent studies have shown that ingestion of glucose or vitamin C lowers blood pressure in patients with autonomic dysfunction, and modestly reduce blood pressure in normal subjects. Whether oral glucose loading with vitamin C ingestion affects cardiovascular responses during orthostatic stress in normal young subjects is yet to be determined. We therefore hypothesized that co-administration of glucose and vitamin C will reduce orthostatic tolerance in normal young male volunteers. Methods: In a randomized controlled cross-over fashion, nine non-obese (19.3 ± 0.6 kg/m2), young (21.9 ± 0.6 yr) men ingested nothing (control), glucose (75g) alone and with Vitamin C (1g) 10 min before standing on three separate days of appointment. We measured supine and standing blood pressure, heart rate, haematocrit and blood glucose levels. Orthostatic tolerance was evaluated as the time to presyncope during motionless standing or until presyncopal symptoms were observed. Results: During the first 60 min of standing, 89%, 78% and 0% of subjects experienced presyncopal symptoms when ingested nothing, glucose alone and glucose+vitamin C, respectively. Ingesting glucose alone did not affect orthostatic tolerance (61.4 ± 1.4 vs 63.0 ± 1.7min; p < 0.05) while ingesting glucose with vitamin C significantly improved orthostatic tolerance (86.6 ± 2.2 vs 63.0 ± 1.7 min; p< 0.001). Increases in systolic blood pressure and heart rate induced by standing were significantly enhanced when glucose+vitamin C was ingested, while changes in diastolic blood pressure were not significantly affected. Ingestion of glucose alone or with vitamin C led to a comparable significant increase in blood glucose levels after standing. However, significant increase in haematocrit level was observed after standing when glucose+vitamin C but not when glucose alone was ingested. Conclusion: The results of the present study demonstrate that oral glucose loading with vitamin C ingestion improves orthostatic tolerance, and the improvement is associated with enhanced systolic blood pressure, heart rate and haematocrit level. Keywords: Blood pressure, glucose load, haematocrit, orthostatic tolerance, vitamin C