Systematic Review of Intervention Studies Using Isoflavone Supplements and Proposal for Future Studies
88Many studies have reported beneficial health effects of isoflavones (IFs) on estrogen-related cancer, cardiovascular disease, lipid profiles, climacteric symptoms, and osteoporosis in humans. However, results have been mixed and are difficult to compare due to differences in study populations (e.g., men vs. women, pre- vs. postmenopausal women, equol producers and nonproducers), IF origin (soy vs. clover), IF form (extracted IF in aglycone form vs. with protein and various precursors), IF dose, and lack of other important data such as dietary intake.
To more precisely characterize the various health effects and limitations of clinical intervention studies carried out to date, we performed a systematic review of studies using extracted IF supplements in humans. Of the 22 articles identified, 6 studies were further selected for lipid analysis. Several studies of IF supplementation reported significant effects on prostate cancer, cardiovascular disease, osteoporosis, antioxidant status, hot flashes, menstrual cycle and associated hormones, and selected cognitive functions. Other studies reported no significant effects on prostate cancer, cardiovascular disease, menstrual cycle and associated hormones, reproductive health, and insulin resistance.
These conflicting and nonsignificant findings most likely result from data and study design limitations, and thus we propose a set of minimum requirements and guidelines for future research to enable comparison between studies and metaanalysis of future results. These issues include subject selection and characteristics, equol producer status, IF source, IF dose, IF components, intervention duration, blood or urinary levels of IFs, and dietary sources of IFs. Selection of a proper end point and its biological plausibility are also important.
Keywords: isoflavones, supplement, intervention, epidemiology.