Spice plant Allium cepa: dietary supplement for treatment of type 2 diabetes mellitus.

OBJECTIVE Diabetes mellitus (DM) is associated with significant morbidity and mortality and its prevalence is increasing worldwide. Although conventional antidiabetic agents are known to ameliorate the symptoms of diabetes, they also may cause adverse effects. The purpose of this review was to organize and discuss various studies that have been previously conducted indicating the efficacy of Allium cepa in DM. METHODS A comprehensive English literature search was conducted using various electronic search databases. Different search terms were used and an advanced search was conducted by combining all the search fields in abstracts, keywords, and titles. RESULTS Allium cepa, a spice plant, is commonly known as onion and belongs to the family liliaceae. Since ancient times, it has been used traditionally for the treatment of different diseases. Among various activities of Allium cepa, regulation of hypoglycemic activity is considered one of its important effects in DM. Sulfur compounds including S-methylcysteine and flavonoids such as quercetin are mainly responsible for the hypoglycemic activity of Allium cepa. S-methylcysteine and flavonoids help to decrease the levels of blood glucose, serum lipids, oxidative stress, and lipid peroxidation, as well as increasing antioxidant enzyme activity and insulin secretion. Extracts of onion also have been shown to have hypoglycemic and hypolipidemic effects by normalizing the activities of liver hexokinase, glucose 6-phosphatase and HMG coenzyme-A reductase. In preliminarily clinical trials, patients with diabetes safely consumed slices of Allium cepa, exhibiting sufficient hypoglycemic activity. In the future, further studies must be conducted to investigate and confirm the hypoglycemic activities of Allium cepa and its constituents and/or their synthetic analogs. CONCLUSION This review will not only elucidate the nutritious facts of Allium cepa but may also help in understanding the molecular basis of its effects in DM. This review will explore in particular the medicinal characteristics of Allium cepa supporting that the consumption of dietary onion could lower blood glucose levels, thus contributing to the reduction of risk factors associated with DM.