Phytochemicals are defined as bioactive nutrient plant chemicals in fruits, vegetables, grains, and other plant foods that may provide desirable health benefits beyond basic nutrition to reduce the risk of major chronic diseases.

One of the three major groups of phytochemicals include Polyphenols – subcategorised as the flavonoids, phenolic acids and other non-flavonoid polyphenols.

Considerable differences exist between onion varieties in phytochemical content, particularly for polyphenols – onion polyphenols are under basic research to determine their possible biological properties in humans.

Shallots have the highest level of polyphenols, six times the amount found in Vidalia onions. Yellow onions have the highest total flavonoid content, an amount 11 times higher than in white onions.  Red onions have considerable content of anthocyanin pigments (are colored water-soluble pigments belonging to the phenolic group, responsible for the colors red, purple, and blue in fruits and vegetables) with at least 25 different compounds identified representing 10% of total flavonoid content.

(Sources of Information: sciencedirect.com, ncbi.nlm.nih.gov & Wikipedia)