Effects of Soy Protein Intake on Low Bone Mass Caused by Energy Restriction in Young Female Rats

Abstract Objectives Eating disorders, such as anorexia nervosa, in young adulthood have been shown to lower bone mineral density (BMD), possibly due to the dysfunction of growth and gonadal hormones caused by reduced energy intake, which leads to bone loss. Isoflavones in soy protein have estrogen-like effects and are used as an alternative treatment for a wide range of hormonal disorders such as osteoporosis. The purpose of this study was to determine the effect of soy protein intake on bone fragility caused by energy restriction in young female rats. Methods Six-week-old Sprague–Dawley female rats were acclimated for 1 week and then divided into four groups: (1) ad libitum intake and casein diet group (Cas: n = 8), (2) ad libitum intake and soy diet group (Soy: n = 8), (3) energy-restricted and casein diet group (ER-Cas: n = 8), and (4) energy-restricted and soy diet group (ER-Soy: n = 8). The animals were maintained under these feeding conditions for 10.5 weeks. In the energy-restricted groups, the calorific intake was reduced by 40%. After rearing, viscera, blood, muscle, and bone samples were collected. Bone samples were evaluated for BMD by the dual-energy x-ray absorptiometry method. Two-way analysis of variance and multiple comparison tests were used to evaluate the differences among the four groups. Results The bodyweight and uterine weight results revealed a significant main effect of energy restriction on bone mineral content (BMC), bone area, and BMD. BMC and BMD results revealed a significant interaction of soy protein and energy restriction. BMD was highest in the Soy group, followed by the Cas group, ER-Cas group, and ER-Soy group. There was no difference in estradiol among the four groups. Energy restriction had a significant main effect on insulin-like growth factor I (IGF-I), which was significantly reduced in the ER-Soy group compared with the two ad libitum groups. Conclusions Soy protein showed a tendency of having a positive effect on BMD and IGF-I under ad libitum conditions, whereas under energy restriction, it had a negative effect and did not suppress the effect of energy restriction. Funding Sources This work was supported by the Research Grant of Fuji Foundation for Protein Research and the Cooperative Research Grant of Advanced Research Initiative for Human High Performance, University of Tsukuba.