The hypolipidemic effects of soybean and soy components have been clarified through numerous epidemiological and experimental studies. Soybean and soy foods contain various nutritional and functional components; thus, this complexity impedes understanding of their global functionality. To address this issue, we attempted to clarify the major active component of kori tofu (freeze-dried soybean curd) and its mechanism of action on modulating lipid metabolism in rats. The results suggested that the protein component played the principal role in reducing serum lipid levels through suppression of hepatic lipogenesis. Moreover, we attempted to reveal the hypolipidemic properties of four soy foods by comparing several parameters involved in lipid metabolism. Although the differences among the soy foods were not always obvious, the quantity and quality of some components, such as protein and dietary fiber, seemed to determine their hypolipidemic properties. Therefore, the moderate hypolipidemic effects of soy foods and their habitual consumption may contribute to improved lipid metabolism.