Serum equol, bone mineral density and biomechanical bone strength differ among four mouse strains

The extent of conversion of daidzein to its metabolite, equol, by intestinal microflora may be a critical step that determines if a diet rich in daidzein protects against the deterioration of bone after estrogen withdrawal. The objective was to determine the extent that daidzein is converted to equol. In addition, bone mineral content (BMC), bone mineral density (BMD) and strength of femurs and lumbar vertebrae (LV) in four mouse strains were measured. Mice were ovariectomized and fed control diet (AIN93G) with or without daidzein (200 mg daidzein/kg diet) for 3 weeks, after which serum, femurs and LV were collected. Serum daidzein and equol were elevated in all mice fed daidzein. Among mice fed daidzein, the CD-1 and Swiss–Webster (SW) mice had higher (P<.001) serum equol than C57BL/6 (C57) and C3H mice. Differences due to mouse strain were observed for all bone outcomes. C57 mice had lower femur BMC (P<.001), BMD (P<.001) and peak load at femur midpoint (P<.001) and neck (P<.001) than other mouse strains. C57 mice also had a lower femur midpoint yield load (P<.001) and resilience (P<.001) than C3H mice. C57 mice had a lower LV1–4 BMC (P<.001) and BMD (P<.001) compared with all mouse strains and peak load of LV3 was lower than CD-1 and SW mice. Differences in serum equol, BMD and bone strength properties should be considered when selecting a mouse strain for investigating whether dietary strategies that include isoflavones preserve bone tissue after ovariectomy.