Soy–tibolone combination—Effect on lipids in postmenopausal monkeys and women

ObjectivesTo determine whether co-administration of soy during tibolone treatment would prevent tibolone-induced dyslipoproteinemia in postmenopausal monkeys and women.MethodsSurgically postmenopausal cynomolgus monkeys (n = 18) were assigned randomly to one of four dietary regimens in a Latin Square crossover design, such that all animals received all diets for 14 weeks with a 4-week washout period: (1) casein/lactalbumin (CL); (2) tibolone (Tib, 1.25 mg/day women's equivalent); (3) soy (138 mg isoflavones/day women's equivalent); (4) Soy + Tib. Postmenopausal women on tibolone treatment were randomized to receive soy powder (52 g of soy protein containing 112 mg isoflavones) or placebo (containing 52 g of milk protein) daily in a crossover trial for 8 weeks with a 4-week washout period.ResultsMonkeys given Tib alone had ∼14% increase in plasma LDL + VLDL-C; whereas those given soy combined with tibolone had significant (∼22%) reductions. Tib treated monkeys had reductions in plasma HDL-C of about 48% vs. no reductions in Soy + Tib. In postmenopausal women using tibolone, soy reduced plasma LDL-C concentrations by ∼10% from baseline without a change in HDL-C.ConclusionsCo-administration of soy during tibolone treatment improved the lipoprotein profile in both monkeys and women; however, the effects were more robust in monkeys.