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Soy smart: Investigating cognitive benefits from isoflavones

Isoflavones could boost cognition in theory by both slowing diseases that impact cognition and by impacting certain areas of the brain rich in estrogen receptors. But how much of an impact do they actually make?

Study under review: Effects of soy isoflavones on cognitive function: a systematic review and meta-analysis of randomized controlled trials

Introduction

It’s a pretty safe bet that most people want to improve their cognitive functioning. There are at least two ways of doing so: by improving cognition from a healthy baseline, and by preventing diseases that reduce cognitive health. There is a chance that isoflavones derived from soy could do both.

Isoflavones[1] are bioactive compounds found predominantly in soy products. They are considered phytoestrogens because they are natural, plant-derived (hence the Greek root “phyto-”) molecules that exhibit estrogen-like action in the human body. It is important to note that their estrogenic activity is comparably low[2] and whether they increase or decrease (competition) estrogen signaling depends on environmental factors.

However, isoflavones’ estrogenic activity could be important for enhancing cognition, since estrogen receptors are found in pretty high quantities in two important brain areas: the hippocampus[3], which plays a major role in memory formation, and the prefrontal cortex[4], which is where a lot of higher reasoning capacity takes place. The theory that isoflavones can improve cognitive function has been bolstered in mouse models[5], where isoflavones have been shown to improve aging- and drug-related memory problems in otherwise healthy mice.

There is also reason to suspect that isoflavones could help ameliorate cognitive decline related to disease. This is because they have been shown to exert anti-inflammatory and antioxidant effects, as well as anti-apoptotic[6] (programmed cell death) effects in mitochondria. Preclinical studies have shown that soy isoflavones have the ability to remove amyloid beta[7] and decrease tau phosphorylation[8] in Alzheimer’s disease models. All these reported benefits would seem to explain reported improvements[9] in cognitive function associated with soy isoflavone intake in humans, but the verdict remains inconclusive[10].

Some studies report positive associations[11] between soy isoflavones and cognitive function, suggesting their use to combat cognitive decline[12] in elderly people, while one study[13] did not report any influence and suggests differences in metabolism or basal levels of estrogen as important factors. Systematic reviews of RCTs have also reported both positive[9] and no effects[14]. One meta-analysis[15] reported a very small effect, but did not include all studies available.

The authors of the study under review conducted a systematic review and meta-analysis to determine the influence of soy isoflavones on overall and domain-specific cognitive functions. They also evaluated the adverse effects of soy isoflavones reported.

Soy isoflavones have demonstrated actions that support cognitive function in animal studies, but human studies have yielded rather conflicting results. The authors of the study under review designed a meta-analysis with the aim of resolving the conflicting results.

Who and what was studied?

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The big picture

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