SummaryWe have reported that soybean milk is an excellent source for increasing bone mineral density (BMD) and mechanical bone strength. However, it is not known what kind of components in soybean milk affect bone metabolism. Consequently, the purpose of this study is to fi nd the effective components in soybean milk on bone metabolism. It might be some peptide in soybean milk. Therefore, in this study the soybean milk was separated into two different preparations according to molecular weight: a high-molecular-weight soybean milk, and a low molecular-weight soybean milk. Then, the effect of the peptides in soybean milk on bone metabolism was examined. After producing the experimental osteoporotic model rats, they were divided into four groups. The BMD and the mechanical bone strength of the three experimental groups, whose diet contained a soybean milk, a high-molecular soybean milk, or a low-molecular soybean milk (soybean milk peptide) were significantly higher than those of the control group. Moreover, the intestinal Ca absorption for the three experimental groups was significant ly increased. From these results, the peptides in the soybean milk are effective for the acceleration of the intestinal Ca absorption. It is possible to assume that the mechanism for increasing the BMD and the mechan ical strength in the three experimental groups was due to increasing the intestinal Ca absorption by alimenting the soybean milk, the high molecular soybean milk or the low-molecular soybean milk. Key Words osteoporosis, soybean milk, soybean milk peptide, intestinal calcium absorption, bone mineral density, mechanical bone strength Soybean is well known as being an excellent source of protein, as well as minerals. Soybean and/or soybean products are popular in Japanese diet and particularly, many elderly Japanese people favor the taste, texture, and flavor. Calcium (Ca) intake of the ordinary Japanese had never been sufficient according to the national nutrition survey performed in 1992 (1). An inadequate intake of calcium is one of the risk factors for osteoporosis (2). Although milk is an excellent calcium source (3), some elderly Japanese are not used to drinking milk, and some have a lactase intolerance. This makes it more difficult for the elderly to have the correct amount of Ca intake. Since the soybean and the soybean products have been proved to contain Ca as high as one third of Ca in milk (4), soybean products could be a good source of Ca. We have reported that soybean milk is an excellent source for incerasing bone mineral density (BMD) and mechanical bone strength (5). However, the mechanism of increasing the BMD and the mechanical bone strength by alimenting the soybean milk is unclear. There are some reports that some peptides, such as a casein-derived phosphopeptide (CPP) (6), a peptide present in bovine bone (7), and globin preparation (8), enhance the intestinal Ca absorption and affect the bone metabolism. It is possible to assume that the mechanism for increasing the BMD and the bone strength in the...