Soy isoflavones are polyphenols found in soy products and other plants. They preferentially interact with a type of estrogen receptor involved in cognitive functions. Because they interact with estrogen receptors, soy isoflavones have also been studied for preventing menopausal symptoms and premenstrual syndrome. Some clinical studies have suggested that soy isoflavone treatments may improve a few cognitive functions, though the benefits appear to depend on age, gender, and ethnicity. Soy isoflavone intake via diet or supplementation is generally regarded as safe.
Numerous randomized controlled trials have tested the effects of soy isoflavones, though results appear to depend on age, gender, and ethnicity. Our search identified:
Clinical studies of soy isoflavones suggest they benefit some cognitive functions, though benefits appear to vary by age, gender, and region . A large meta-analysis of 10 randomized controlled trials reported that soy isoflavone supplementation significantly improved overall cognitive function and visual memory in people under 60 years old from non-US countries .
In a large double-blind randomized controlled trial of postmenopausal women, treatment with isoflavone-rich soy protein for several years improved visual memory, but not other cognitive functions compared to control . More benefits were seen in women between 5–10 years of menopause than those 10 years post-menopause. Other smaller randomized controlled trials in postmenopausal women reported that isoflavone treatment improved episodic memory, attention, and category fluency, but not nonverbal memory, category generation, planning, or rule-learning .
In older men and women, soy isoflavones treatment resulted in improved spatial memory and construction, verbal fluency, and dexterity, but worse executive function . In young healthy adults, high soy diet for 10 weeks resulted in significant improvements in short-term and long-term memory and in mental flexibility, but not in attention or category generation compared to those in the control diet . Women, but not men, on the high soy diet also improved in letter fluency and planning.
Soy isoflavones bind to an estrogen receptor called ERβ, which is expressed in brain regions important for executive function and memory and in preclinical models can lead to improved cognitive functions . Soy isoflavones also exhibit antioxidant and anti-inflammatory activities . See Genistein rating for more details on this specific soy isoflavone.
In a randomized controlled trial of 59 people with Alzheimer's disease, APOE4 genotype did not affect the response to soy isoflavone treatment . For more information on what the APOE4 gene allele means for your health, read our APOE4 information page.
A randomized controlled trial of 59 Alzheimer's disease patients reported that soy isoflavone treatment (100 mg/day) for 6 months did not significantly improve cognitive function over placebo, despite increased plasma levels of isoflavones .
Soy isoflavones are generally considered safe . Numerous randomized controlled trials in menopausal women reported that side effects were not significantly different between soy isoflavone and placebo groups . Adverse events were generally mild  and included gastrointestinal and musculoskeletal complaints . One systematic review of over 100 studies in patients with or at risk of breast cancer suggested that while there is no clear evidence of harm, better evidence confirming safety is required before use of high dose (over 100 mg/day) isoflavones can be recommended for breast cancer patients . Drug interactions with soy isoflavones are not well-documented , but because they bind to estrogen receptors, it may interact with drugs that target the estrogen system.
NOTE: This is not a comprehensive safety evaluation or complete list of potentially harmful drug interactions. It is important to discuss safety issues with your physician before taking any new supplement or medication.
Soy isoflavones are present in soybeans, tofu, fava beans, and other plants. It is also available as a supplement in tablet and capsule forms. Doses that showed improvement in some cognitive functions in clinical studies ranged from 60–100 mg/day .
Evaluation of soy isoflavones' potential biological effects from Examine.com
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