Plants and plant parts are not only used for its scent or flavor but also playing an important role in the development of metabolic disorders or therapeutic properties and it affects the quality of life. There are a number of advantages associated with plants and plant phytoconstituents which are proved for its biological activities such as antidiabetic, antihyperlipidemic, free-radical scavenging, and anti-inflammatory activities.

The production of physiological amount of free-radical is essential to fight against an unfavorable environment but excess production of free-radical alters the cellular functions and induces chronic and degenerative diseases. Vitamin A, Vitamin C, and Vitamin E are dietary antioxidants, commonly present in vegetables and fruits which help to inhibit the free radicals. Many plant flavonoids have rich antioxidant properties which are used for the treatment of inflammatory disorders.

Flavonoids are phenolic substances which exhibit biological activities including antiallergenic, antiviral, anti-inflammatory, anti-tumor and treatment of neurodegenerative disorders. Quercetin is one of the most widely used bioflavonoids for the treatment of metabolic and inflammatory disorders. These flavonoids are found in citrus fruits, green leafy vegetables as well as many seeds, nuts, flowers, broccoli, olive oil, apples, onions, green tea, red grapes, dark cherries and berries such as blueberries.

Quercetin, a plant pigment is mostly found in onions. Onion (Allium cepa L.) belongs to the Alliaceae family and contains many cultivars which are colored. Two flavonoid classes are mainly found in onion, the anthocyanins, which impart a red/purple color to some varieties, and flavonols such as quercetin and its derivatives, responsible for the yellow and brown skins of many other varieties. Onions also provide important nutrients and health-promoting phytochemicals like vitamin C, calcium, iron, sodium, organosulfur compounds, amino acid, dietary fiber, folic acid and contain no fat.

Quercetin has been used as a nutritional supplement and has beneficial effects on cardiovascular protection, anticancer, antitumor, anti-ulcer, anti-allergy, anti-viral, anti-inflammatory activity, anti-diabetic, gastroprotective effects, antihypertensive, immunomodulatory and anti-infective. Quercetin can also protect against environmental causes of free radicals such as smoking. Flavonoids also play a vital role in plant physiology, mainly related to plant resistance in defense mechanisms against herbivore and pathogen attacks, UV radiation protection, plant-microbe symbiosis etc. It is a versatile antioxidant known to possess protective abilities against tissue injury induced by various drug toxicities.

Onion is a multi-use vegetable that is consumed fresh as salad as well as in the form of a number of processed products. Onions contain flavonoids, is an excellent choice for human daily consumption. Onion with antioxidant properties play a vital functions in exhibiting beneficial effects and employed as an alternative source of medicine to mitigate the disease associated with oxidative stress.

Md. Ataur Rahman
Scientist, ASRBC