Pharmaceutical, cosmetic, nutraceutical and dietary compositions derived from Garcinia mangostana are rich in γ-mangostin and other demethylated xanthones. These compositions exhibit potent antioxidative activity and reduce inflammation. These compositions may be used in a method of treating TNF-α, and aP2 mediated disorders. The compositions are prepared by subjecting a hydroalcoholic extract of fruit pericarp (fruit hull) of Garcinia mangostana, wherein said hydroalcoholic extract comprises predominantly α-mangostin, to demethylation in the presence of a Lewis acid, an organic base and a catalyst.