In certain embodiments, the invention comprises the use of plant-derived substances to reduce body fat and/or to increase insulin sensitivity in mammals. In certain embodiments, the invention comprises a composition for reducing the amount of fat in the body of a mammal, the composition comprising at least one plant-derived substance which inhibits adipogenesis in the body of the mammal and at least one plant-derived substance which promotes lipolysis in the body of the mammal. In certain embodiments, the invention comprises a composition for reducing the amount of fat in the body of a mammal, the composition comprising at least one plant-derived substance which inhibits adipogenesis in the body of the mammal, at least one plant-derived substance which promotes lipolysis in the body of the mammal, and at least one plant-derived substance which inhibits lipogenesis in the body of the mammal. In certain embodiments, capsaicin may be used in combination with other plant-derived substances to reduce the body fat in a mammalian subject. In one embodiment, an effective amount of a composition comprising capsaicin in combination with grape seed extract and/or genistein and/or caffeine may be administered to a subject to reduce the body fat of the subject. In one embodiment, a composition comprising grape seed extract is administered to a mammalian subject to increase insulin sensitivity.