The effects of Allium cepa L. (onion) and its active constituents on metabolic syndrome: A review

Metabolic syndrome as a clustering disorder includes excess abdominal fat distribution, abnormal insulin and glucose metabolism, disturbed blood lipids, pro-inflammatory state, and hypertension. Regarding to the adverse effects of synthetic medicines, the identification of appropriate healthcare approaches, such as herbal medicines, with fewer side effects is more favorable. Allium cepa L. (onion) is a culinary and medicinal herb belonging to the family of Amaryllidaceae. Flavonoids such as quercetin and kaempferol, alk(en)yl cysteine sulfoxides including S-methyl cysteine sulfoxide and S-propyl cysteine sulfoxide, cycloalliin, thiosulfinates, and sulfides are main compounds existing in the plant. A. cepa and its pharmacologically active constituents display broad-spectrum activities including anti-oxidant, anti-inflammatory, lipid-modifying, anti-obesity, antihypertensive, and antidiabetic effects. Our objective in this review was to find out the role of A. cepa and its bioactive phytochemicals as cardiovascular protective agents in different metabolic syndrome risk factors, including hyperlipidemia, high blood glucose, obesity, and hypertension.