The consumption of fruits and vegetables is associated with a reduced risk of developing chronic diseases such as cancer, diabetes, coronary heart disease and Alzheimer's disease. A daily intake of at least five servings of fruits and vegetables has been recommended for health and the prevention of these illnesses.

Most of the potentially harmful effects of oxygen for organisms are associated with the formation and activity of reactive oxygen species, and the protective effects of fruits and vegetables against chronic diseases are attributed to their content of antioxidant compounds. Phenols (including flavonoids), carotenoids, ascorbic acid, thiols and tocopherols are all naturally occurring plant antioxidants.

Onions are one of the richest sources of flavonoids in the human diet and, as such, possess a high level of antioxidant activity. Pungency and flavour of onions are derived mainly from sulfur compounds, but may be influenced by the bitter, astringent flavour of flavonoids. Onion varieties may differ in their flavonoids composition and content, and consequently in their antioxidative activity.

Yang et al.¹ has investigated the total phenols content, flavonoids content, antioxidative activity and antiproliferative activity against human liver and colon cancer cells of shallots and 10 onion varieties differing in colour, pungency and bitterness. For all varieties, the total phenols content and flavonoids content were strongly correlated with total antioxidative activity. These results may be important in influencing consumer choice and in breeding programmes to enhance nutritional quality of onions.
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¹ Yang, J; Meyers, KJ; van der Heide, J; Liu, RH (2004). Varietal differences in phenolic content and antioxidant and antiproliferative activities of onions. Journal of Agricultural and Food Chemistry 52 (22) 6787–6793.

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