While many onions contain anti-cancer chemicals, a Cornell University study shows certain varieties have more of these compounds, and more effectively prevent growth of some cancer cells.

Scientists studied cancer cells treated with extracts from ten onion varieties and shallots, which had the most benefits owing to high phenolic content. Higher phenolic and flavonoid content means stronger antioxidant activity, which mops up free radicals and hinders damage to normal cells. Western Yellow and New York Bold onions are particularly high in flavonoids with strong anti-proliferation protection against liver and colon cancer cells.

Flavonoid consumption is associated with reduced risks of several diseases, and onions - particularly the more pungent varieties - are among its richest sources in the human diet.