This invention aims to provide a bone-fortifying agent that is safe and useful for prevention against, or treatment for various bone diseases, such as osteoporosis, fracture, rheumatism and arthritis. This invention provides food and drink, nutritional composition, feeding stuff or medicine for fortifying bones, which contain the bone-fortifying agent. Basic protein fractions derived from milk and/or its decomposed products, and isoflavone and/or its metabolite act on fortifying bones synergistically by being taken simultaneously, compared with being taken solely.本發明的目的在於提供一種安全性高，對於骨質疏鬆症或骨折、風濕病、關節炎等之種種的骨疾病之預防或治療為有用的骨強化劑。又，提供一種摻合骨強化劑的骨強化用飲食品、骨強化用營養組成物、骨強化用飼料或骨強化用醫藥品。藉由同時攝取來自乳汁的鹼性蛋白質級分及/或來自乳汁的鹼性蛋白質級分分解物與異黃酮(isoflavone)及/或其代謝物，相較於各別單獨攝取的情況，更可展現相乘的骨強化作用。
