Flavonoids are a class of water-soluble plant pigments found in fruits, vegetables, and certain beverages that have antioxidant effects. Antioxidants are compounds that protect cells against the damaging effects of reactive chemicals known as free radicals. Free radicals can cause oxidative stress, leading to cellular damage. Diets high in produce are high in flavonoids.
It turns out there may be another benefit to flavonoids in the diet–weight loss. A study published in the American Journal of Clinical Nutrition (2008 Nov; 88(5):1341-52) looked at food intake and lifestyle in 4,280 men and women and monitored the subjects for BMI over a period of 14 years. The BMI increased in both women and men over the course of the study. Women with the highest intake of flavonoids gained significantly less weight than those who consumed the lowest levels of flavonoids. This effect was found only in women; the researchers found no such connection in men.