Effects of isometric strength training followed by no exercise and Humulus lupulus L–enriched diet on bone metabolism in old female rats

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We investigated in female rats the effects on bone metabolism of a prolonged no-training period, subsequent to an isometric exercise program, performed during young adulthood and those of a long-term consumption of Humulus lupulus L–enriched diet (genistein 1.92 and daidzein 1.24 mg/kg diet) combined or not with isometric training. Forty-eight rats (4 weeks old) were randomly divided into 4 groups: trained (C-Tr) or nontrained rats (C-NTr) fed with control diet and trained (H-Tr) or nontrained rats (H-NTr) fed with Humulus lupulus L–enriched diet. The diets lasted 100 weeks. Training was followed over a 25-week period. Bone parameters were measured at week 100. Our results showed that no significant difference was observed among the 4 groups in uterine relative weight, calcium (Ca) intake, fecal Ca, urinary Ca excretion, net Ca absorption, plasma Ca, and bone Ca content. Calcium balance was significantly enhanced in H-NTr rats in comparison with C-NTr and C-Tr rats. Isometric strength training led to a significant increase in total bone mineral density (BMD), diaphyseal BMD, and osteocalcin-deoxypyridinoline ratio in C-Tr rats compared with the other groups. The main findings of the present study indicate that in female rats, a 25-week isometric strength training performed during young adulthood followed by a prolonged no-training period increases BMD values and osteocalcin-deoxypyridinoline ratio, whereas long-term consumption of Humulus lupulus L–enriched diet does not improve bone parameters. It suggests that bone gains induced by exercise do not decrease immediately after cessation of training and also confirms the importance of the practice of physical activity during puberty and young adulthood to maximize the achieved peak bone density.