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Introduction
The cognitive ability is gradually and constantly reduced by aging reportedly.123456 A previous study showed that cognitive impairment was less likely in postmenopausal women with higher estradiol levels.7 Nevertheless, 2 longitudinal researches on menopausal transition have failed to find any change in cognitive function in this period, confounding the previous results.89 In another cross-sectional study on 189 postmenopausal women, the late menopause compared with early menopause showed a worsen executive function.1
There are contradictory results on the effect of hormone replacement therapy (HRT).10111213 However, a meta-analysis of 16 studies involving 10,114 women showed that estrogen replacement therapy (estrogens only) or HRT (estrogens combined with a progesterone) had no protective and improving effects on cognitive function.14 Prolonged HRT led to venous thrombosis, stroke and even increased risk of breast cancer. Consequently, menopausal women has been interesting herbal medicines and dietary supplements like soy, red clover and Gingko biloba.15 There are several systematic reviews assessing the effect of herbal medicines such red clover, Gingko biloba, black cohosh and soy.151617 Therefore, it is essential to study simultaneously several reviews to provide existing information for clinicians, policy makers, patients and researchers. The purpose of current overview is to display the effect of herbal medicines on cognitive function among menopausal women.
Materials and Methods
Various kinds of reviews, including systematic review, critical reviews, narrative reviews, rapid reviews, conceptual reviews, and meta-analysis, were assessed to detect the review that evaluated the effect of herbal medicines and soy on cognitive function in postmenopausal women. Two separate authors performed a systematic review search of the 3 databases of (the MEDLINE Scopus and the Cochrane Central Register Trials) inception up to January 2018. The search terms in English were as follow: (Herbal medicine OR herbal drug OR phytomedicine OR Gingko biloba OR phytoestrogen OR soy OR isoflavones) AND (Cognition OR cognitive OR memory) AND (menopause and postmenopuse). The manual search of references listed in review, meta-analysis and articles were also carried out to broaden the spectrum of studies. The same authors (the first and second authors) performed search of databases and other sources, assessment of study quality and data extraction (Table 1). A third researcher judged on disagreements prior to final decision. The methodological quality of systematic review was assessed by same authors using the 11-items AMSTAR developed by Oxman et al.18 (Table 2). Each item was responded by “Yes”, “No”, “Can't Answer”. A pre-designed form confirmed by research team was used to extract the study data, including type of review, year of publication, first author, study populations, sample size, and main outcomes. Quality of the meta-analysis was assessed s using the 12-items according to CEBMA Center for evidence-based management (Table 3).
Results
Figure 1 shows the process of selecting reviews included into the overview. As can be seen, 2 systematic reviews and one meta-analysis were included into the overview in Table 1.
1. The effect of isoflavones on the cognitive functions among menopausal women
One review assessed the effect of isoflavones and soy on cognition. In this systematic review, authors searched seven databases up July 2011 and found 5 studies with 414 menopausal women who examined for the efficacy of isoflavones supplementation on cognition among menopausal women. According to 3 studies, 6 weeks of treatment with isoflavones could improve some aspects of cognition, such as mental flexibility and planning. There is need for a longer period (6 months) to improve category digit span test, digit symbol test and visual scanning test score. Fourth and fifth study found no effect of isoflavones on memory recall and recognition, working memory, global cognitive function, memory span and executive function. The authors reported that most of studies had a low quality.15
2. The effect of phytoestrogens on the cognitive functions in men and women
A review assessed the effect of phytoestrogens on the cognitive function among women and men aged between 48 to 97 years. The result of the review suggested inconclusive findings of the effect of phytoestrogen on the cognitive function. Menopausal women might be affected by variable demographic profiles (age, gender, ethnicity, and menopausal status) and treatment variables (dosage, length of treatment, and type of cognitive test). The length of treatment might influence the results. Good short-term cogitative effect might reverse with treatment continues in elderly. The authors also reported that the type of populations (Asian/European) has impact on the result. European countries with low soy consumption showed no effects of phytoestrogen on cognition. In contrast to Asian countries with higher take of tofu (a non-fermented type) found a negative relation between soy intake and cognitive function, while tempe (fermented soy) had protective effects.17
3. The effect of soy on cognitive functions in men and women
In a systematic review and meta-analysis evaluating the herbal effect on cognition, the efficacy of soy on cognition was studied within a subgroup of 4 studies on 344 menopausal women. Among these, a good effect on the cognitive function was reported in only one trial, but not in 3 other studies. Most of studies had low quality.15
Second study was performed as meta-analysis to assess the effect of soy isoflavoes on the cognitive function in the menopausal women. Pooled standardized mean difference (SMD) was significant for summary cognitive function tests (SMD = 0.08; 95% confidence interval [CI] = 0.02–0.15; P = 0.014), which showed that the women in soy group received more beneficial effect than that of placebo group. Subgroup analysis was conducted based on the cognitive function domains (visual memory, verbal memory, executive function, attention, and processing speed), the countries (non-US/US), the age (less than 60 years/equal and more than 60 years), the dosage (<90 / ≥90 mg) and the treatment duration (<12 months / 12 months). In the subgroup analysis of the cognitive function domains, the pooled SMD was calculated for the visual memory (SMD = 0.01; 95% CI, 0.02 to 0.18; P = 0.016; Heterogeneity, I2 = 2%, P = 0.426). Administration of soy isoflavones compared to placebo did not alter significantly other cognitive function domains, including verbal memory (SMD = 0.02; 95% CI, −0.06 to 0.11; P = 0.606; Heterogeneity, I2 = 37.8%; P = 0.075), executive function (SMD = 0.02; 95% CI, −0.04 to 0.09; P = 0.522; Heterogeneity, I2 = 18.4%; P = 0.239), attention (SMD = −0.01; 95% CI, −0.08 to 0.06; P = 0.721; Heterogeneity, I2 = 0%; P = 0.593), processing speed (SMD = 0.03; 95% CI, −0.04 to 0.09; P = 0.431; Heterogeneity, I2 = 2.7%; P = 0.420) and language (SMD = −0.02; 95% CI, −0.08 to 0.05; P = 0.586; Heterogeneity, I2 = 0%; P = 0.464). The results of subgroup analysis showed that the SMD was not affected by age, dosage and length. The result of subgroup analysis for non-US countries was significant (SMD = 0.12; 95% CI, 0.00 to 0.25; P = 0.044; Heterogeneity, I2 = 70.9%; P = 0.008). In fact, studies showing good effect of soy on the cognitive function have mainly been performed in non-US countries.16
4. The effect of red clover on the cognitive functions
The second study, a systematic review assessed the efficacy of phytoestrogens such as red clover, Gingko biloba, isoflavones and other herbal medicines such as black cohosh on the cognitive function. Two studies evaluated the efficacy of red clover cognition among 61 menopausal women. According to the result of systematic review, the red clover showed no positive effect on the processing speed, memory, verbal ability, frontal cortex function compared to placebo in the menopausal women.15
5. The effect of Gingko and ginseng on the cognitive functions
Two studies included into systematic review assessed the effect of Gingko and ginseng on the cognition.
The first study evaluated the combination of Gingko with ginseng on the cognition and showed that this combination could not improve sustained attention, episode memory, category generation, or frontal lobe function in 6-week follow up. In another 6-week study, the effect of Gingko biloba on the cognitive function was assessed in the menopausal women. The subgroup analysis was performed based on the menopausal status: early stage (early menopause, equal to and less than 5 years after menopause) and late stage (more than 5 years after menopause). The Gingko biloba had no significant effect on planning, memory and sustained attention except for its limited effect on mental flexibility in late menopause.15
6. The effect of black cohosh on the cognitive functions
Based on one study, the black cohosh did not improve verbal memory and California verbal learning test.15
Discussion
In the current study, 3 systematic reviews in the overview studied the efficacy of herbal medicines, including soy, red clover, gingko biloba, combination of gingko biloba with ginseng on some aspects of memory and cognitive function.
Several mechanisms of action have been commonly found in the phytoestrogens and estrogen, boosting cognitive function, such as increased choline acetyltransferase and brain-derived neurotrophic factor in the hippocampus and frontal cortex.19 In line with these mechanisms of action, phytoestrogen, soy and isoflavones may have a limited beneficial effect on some aspects of cognition.
The red clover and combination of Gingko with ginseng had no positive effect on the cognition. The Gingko biloba had limited effect on the mental flexibility in late menopause. Moreover, other herbal medicines like black cohosh indicated no beneficial effect. According to 2 reviews, the soy seems to be effective in increasing the summery cognitive function and visual memory. The effect of phytoestrogen on the cognitive function of menopausal women may be modified by age, gender, ethnicity, menopausal status, dosage, length of treatment and types of cognitive test, location, types of soy and types of phytoestrogen. The effect of isoflavones on the cognition was affected by duration of treatment, as six weeks of treatment improved some aspects of the cognition such as mental flexibility and planning and longer period of treatment (6 months) improved category digit span test, digit symbol test, and visual scanning test score.
The result of current overview should be interpreted with cautions because almost all of included studies in systematic review and meta-analysis included in overview had a major methodological difficulty. Most of studies did not report or explained clearly randomization technique, rate of dropouts, attrition rate, no use of intention to treat, blinding method, sequence generation, and method of determining sample size. It is highly recommended that future studies should be designed and reported according to the consort criteria to provide high quality studies for improving the quality of systematic review and meta-analysis. Adequate sample size would be considered in future trials to perform specified subgroup by menopausal status, dosage, length, age, gender, and ethnicity.
Conclusion
Phytoestrogen, soy and isoflavones may have a limited beneficial effect on some aspects of cognition. The effect of phytoestrogens on the cognitive function may be altered by age, gender, ethnicity, menopausal status, dosage, length of treatment and types of cognitive test, location, types of soy, and types of phytoestrogen.
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