Abstract: The invention concerns the use of an onion extract comprising 20 wt. % to 40 wt. % of polyphenols of the flavonol family, said onion extract having the following composition characteristics, expressed in weight relative to the total extract dry weight: 60 wt. % to 90 wt. % of the total flavonols of said extract consist of glycosylated polyphenols; 85 wt. % to 98 wt. % of total flavonols of said extract consist of free-form (aglycon) or glycosylated quercetin; 5 wt. % to 20 wt. % of the total flavonols of said extract consist of quercetin-3,4?-diglucoside; 30 wt. % to 70 wt. % of the total flavonols of said extract consist of quercetin-3-monoglucoside and quercetin-4?-monoglucoside; and 20 wt. % to 30 wt. % of the total flavonols of said extract consist of quercetin, for making a composition to control weight gain.