Background: High intake of soy products has been suggested to prevent breast cancer, osteoporosis, and cardiovascular diseases.
Aim of the study: To investigate the effects of isoflavone-containing soy on circulating sex hormones, biomarkers of bone turnover, and lipoprotein profiles.
Methods: Fourteen young women received in a randomized crossover design 5 soy cookies (52 mg isoflavones) or 5 soy-free cookies (no isoflavones) per day for one menstrual cycle starting one week before menstruation. Serum and urine analyses were performed on day 3 after onset of menstruation (t(1)), 3 days before ovulation (t(2)), 3 days after ovulation (t(3)), during the midluteal phase (t(4)), and again 3 days after onset of the next menstruation (t(5)).
Results: With the exception of higher progesterone levels at t(2), soy supplementation did not affect the physiologic fluctuations in circulating sex hormones. The ratio of C-telopeptide (a bone resorption marker) to osteocalcin (a bone formation marker) was slightly higher at t(4) during the soy period compared to t(4) during the control period (P < 0.05), indicating an uncoupling of bone resorption and formation processes. Serum levels of total cholesterol, LDL cholesterol, and HDL cholesterol were not influenced by soy intake.
Conclusions: High short-term isoflavone-containing soy intake slightly affects physiologic fluctuations in bone turnover, but has no significant effects on most circulating sex hormones and on lipoprotein parameters in young healthy women.