The dose-dependent bone-sparing effects of dietary isoflavones (IF) were in
vestigated in adult (7-month-old) Wistar rats. Forty animals were ovariecto
mised, allocated into four groups of ten rats each, and immediately treated
orally with IF at 0 (OVX), 20 (IF20), 40 (IF40) or 80 (IF80) mug/g body we
ight per d for 91 d; ten sham-operated (SH) controls received the same diet
without added IF. Animals were killed on day 91. Both femoral failure load
and total femoral, diaphyseal or metaphyseal bone mineral densities (BMD)
were lower in OVX animals than in SH animals. Urinary deoxypyridinoline (DP
D) excretion, a marker of bone resorption, and plasma osteocalcin (OC) leve
ls, a marker of osteoblast activity, were higher in OVX animals than in SH
animals. Total femoral and diaphyseal BMD and femoral failure load were sim
ilar in IF-treated rats and SH rats. Although metaphyseal BMD in IF40 or IF
80 rats was similar to that in SH rats, its value was lower in IF20 rats th
an in controls, The day 91 urinary DPD excretion in IF40 and IF80 rats, but
not in IF20 rats, was similar to that in SH rats. Day 91 plasma OC concent
rations in IF-treated rats were similar to day 45 values, but were decrease
d in OVX and SH rats. Thus, daily IF consumption prevented ovariectomy-indu
ced bone loss, both by depressing bone resorption and stimulating osteoblas
t activity. Moreover, as only the highest IF level induced a weak uterotrop
hic activity, the optimal IF dose which preserves both cancellous and corti
cal bone, but exhibits no oestrogen-like effects on the uterus, was 40 mug/
g body weight per d.