BackgroundDespite the prior evidence of the impacts of sumac on glycemic indices, lipid profile and visceral fat, there is a lack of evidence regarding the efficacy of sumac in cases with metabolic syndrome (MetS). Therefore, we aimed to assess the effect of sumac supplementation on MetS markers among adults with this syndrome.MethodsIn this triple-blinded randomized placebo-controlled cross-over clinical trial 47 adults with MetS were randomly assigned to receive 500 mg sumac or placebo (lactose) capsule, twice a day. Each phase took 6 weeks and there was a 2-week washout between phases. All clinical evaluations and laboratory tests were conducted before and after each phase.ResultsAt the baseline of the study, mean (± SD) age, weight, and waist circumference of participants were respectively 58.7 (± 5.8) yr, 79.9 (± 14.3) kg, and 107.6 (± 10.8) cm. Intention to treat analysis (ITT) analyses revealed that sumac supplementation decreased systolic blood pressure by 5 mmHg (128.8 ± 21.4 at the baseline vs. 123.2 ± 17.6 after 6 weeks intervention,P= 0.001). The comparison of changes in two trial arms showed that sumac supplementation significantly reduced systolic blood pressure (sumac group -5.59 ± 10.6 vs. control group 0.76 ± 10.5,P= 0.004), but did not change anthropometric indices or diastolic blood pressure. Similar results were also found in the per-protocol analyses.ConclusionsThis cross-over trial revealed that sumac supplementation could reduce systolic blood pressure in men and women with MetS. Daily intake of 1000 mg sumac, as an adjuvant therapy, may be beneficial in management of MetS in adults.