The Role of Soy Isoflavones in Menopausal Health
Peri- and postmenopausal women can consume soy and soy isoflavones via two sources: their diet (in soy-containing foods, soy milk, and foods containing soy flour or soy oil) and dietary soy and isoflavone dietary supplements. Isoflavones are contained in many edible plants but among foods consumed in the United States, only in soybeans is their concentration sufficient to be physiologically relevant. Other nonfood sources (such as red clover) also contain notable amounts. Most soy and isoflavone supplements are derived from soybeans, and dozens of types of such supplements are marketed, sometimes in combination with vitamin and mineral supplements. Soy and isoflavone supplements are regulated in the United States under the Dietary Supplement Health and Education Act, under which their marketers cannot make health claims related to disease risk reduction but may make claims that they support the structure or function of the body. While the US government (Food and Drug Administration and Federal Trade Commission) has the power to inspect, they have insufficient personnel to effectively implement the regulations involving quality control or health claims.
Frequently used soy foods and their isoflavone content are listed in Table 2 . The isoflavone content of each soy food can vary considerably depending on growing conditions and processing. In Southeast Asia, many soy foods are manufactured from fermentation of soy beans (eg, miso and tempeh). This process tends to concentrate the isoflavones prior to consumption and produces metabolites not formed in the human body. Other processing that removes fats, taste, and color tends to remove isoflavones.
Functionally, in theory, isoflavones can exert both estrogenic and antiestrogenic effects, depending on their concentration, the concentration of endogenous sex hormones, and the specific end organ involved. Some effects of these molecules may result from interactions with pathways of cellular activity that do not involve the ERs. In addition, it is not clear whether the putative health effects in human beings are attributable to isoflavones alone or to isoflavones plus other components in whole foods.
Although soy-containing foods have been consumed by Asian populations for centuries, the best-known soy food, tofu, was only introduced on a large scale in US markets in the 1970s.
As more and more scientific publications over the last few decades suggested potential health benefits of dietary soy and isoflavones, US soy food sales increased-from $1 billion in 1996 to $4.5 billion in 2009. The most dramatic increase occurred between 1996 and 2003, with the biggest gains occurring in sales of soy milk and energy bars. Recently introduced categories of soy foods include soy-based drinks, drinkable cultured soy, soy dairy-free frozen desserts, and energy bars, all of which have shown strong and steady growth in sales. US food manufacturers introduced more than 2,700 new foods with soy as an ingredient from 2000 to 2007. Previously, most sales of soy food and drinks occurred in health food stores, but now 75% of these sales are from supermarkets.
From 2006 through 2009, approximately one third of Americans consumed soy foods or beverages once a month or more frequently. In addition, soy flour and soy oil are used in baked goods, entrees, cereal, pasta, meal replacements, powdered soy beverages, chips, snack foods, and low-carbohydrate foods, comprising about one third of total soy food sales in recent years. Soy isoflavones may also appear unexpectedly in many products in which soy protein is used for its textural properties.
Soy food intake has been assessed specifically among midlife US women. A 2002 telephone survey of 886 women ages 45 to 65 who were members of the Group Health Cooperative in Washington State found that 22.9% used dietary soy. Breast cancer survivors were six times as likely as the overall survey respondents to report use of dietary soy, while women taking hormone therapy (HT) were half as likely to report use of dietary soy. The Study of Women's Health Across the Nation (SWAN), a multisite, longitudinal US cohort study, found substantial variation in dietary isoflavone intake by race/ethnicity among 3,133 women who were premenopausal or early perimenopausal at baseline. Considerably lower isoflavone intakes were found at baseline in Caucasian, Hispanic, and African women (averaging <0.5 mg, a level of questionable biologic significance) than in Asian women (averaging >9.7 mg/d in women of Chinese ethnicity and >18 mg/d in women of Japanese ethnicity). Approximately 40% of non-Asian US women in SWAN consumed no daidzein or genistein, the most prevalent isoflavones.
To put these US dietary intakes into context, data from large Japanese cohort studies included in a comprehensive review published in 2006 indicated that soy protein intake among older Japanese adults was approximately 10 g/day and was nearly the same in women and men. In these studies, soy foods contributed from 6.5% to 12.8% of total protein intake. Mean estimates of isoflavone intake (expressed as aglycone equivalents) ranged from about 30 to 50 mg/day. In a prospective cohort study in Japanese adults ages 45 to 74, the mean fourth quartile isoflavone (excluding glycitein) intake was 78 mg/day in men (n = 9,044) and 77 mg/day in women (n = 10,121). According to food disappearance data from the United Nations' Food and Agriculture Organization, per capita soy protein intake in Japan has remained relatively constant during the past 40 years, although soy's share of total protein intake has decreased from about 13% to 10% because of the increased protein content (mostly from animal sources) of the Japanese diet.
Additional context for US isoflavone intake comes from mainland China, where diet is more heterogeneous than in Japan. The Shanghai Men's Health Study and the Shanghai Women's Health Study, prospective epidemiologic studies involving approximately 50,000 participants each between ages 40 and 70, have indicated that daily mean soy protein and isoflavone intakes in Shanghai are similar to or somewhat higher than those in Japan.
Studies on the prevalence of soy or isoflavone supplement use in the United States are very limited. The only data from a national sample, the 2002 National Health Interview Survey (NHIS), indicate that 9.4% of US adults reported use of soy supplements in the prior 12 months, but this survey report did not specifically address use among midlife women.
In the absence of further data specific to soy supplement use, it may be reasonable to look to use of complementary and alternative medicine (CAM) as a surrogate. Among all gender and age groups in the 2002 NHIS, midlife women had the highest prevalence of reported CAM use. Approximately 45% of women ages 40 to 59 in the 2002 NHIS reported any CAM use in the prior 12 months, and 28.4% reported use of biologically based CAM therapies. The latter percentage was up from 16.2% from the 1999 NHIS. In both the 1999 and 2002 NHIS, use of biologically based CAM therapies was highest among Asian and Caucasian women. Use of medicinal herbs, which include soy, comprised the largest proportion of these therapies, with 18.6% of women overall reporting such use in the 2002 NHIS. A similar level of use of medicinal herbs and teas was reported in a separate nationally representative sample of US women conducted in 2001, with complex and nuanced differences in use by race and ethnicity.
US dietary consumption of soy has increased severalfold over the past 15 years, with one third of Americans consuming soy food or beverages at least once a month. Despite this increase, dietary consumption of soy in the United States remains far below that in Asia. Although nearly 1 in 10 US adults reported use of soy supplements in a 2002 nationwide survey, further studies are limited. More research is needed to understand the health reasons for soy use among midlife women-specifically of soy isoflavone supplements. In addition, the interrelations of dietary intake and supplement use with equol production in terms of effects on health outcomes in midlife women require further study, as do potential interactions with prescription and over-the-counter medications.
Prevalence of Use
Peri- and postmenopausal women can consume soy and soy isoflavones via two sources: their diet (in soy-containing foods, soy milk, and foods containing soy flour or soy oil) and dietary soy and isoflavone dietary supplements. Isoflavones are contained in many edible plants but among foods consumed in the United States, only in soybeans is their concentration sufficient to be physiologically relevant. Other nonfood sources (such as red clover) also contain notable amounts. Most soy and isoflavone supplements are derived from soybeans, and dozens of types of such supplements are marketed, sometimes in combination with vitamin and mineral supplements. Soy and isoflavone supplements are regulated in the United States under the Dietary Supplement Health and Education Act, under which their marketers cannot make health claims related to disease risk reduction but may make claims that they support the structure or function of the body. While the US government (Food and Drug Administration and Federal Trade Commission) has the power to inspect, they have insufficient personnel to effectively implement the regulations involving quality control or health claims.
Dietary Intake of Soy and Soy Isoflavones
Frequently used soy foods and their isoflavone content are listed in Table 2 . The isoflavone content of each soy food can vary considerably depending on growing conditions and processing. In Southeast Asia, many soy foods are manufactured from fermentation of soy beans (eg, miso and tempeh). This process tends to concentrate the isoflavones prior to consumption and produces metabolites not formed in the human body. Other processing that removes fats, taste, and color tends to remove isoflavones.
Functionally, in theory, isoflavones can exert both estrogenic and antiestrogenic effects, depending on their concentration, the concentration of endogenous sex hormones, and the specific end organ involved. Some effects of these molecules may result from interactions with pathways of cellular activity that do not involve the ERs. In addition, it is not clear whether the putative health effects in human beings are attributable to isoflavones alone or to isoflavones plus other components in whole foods.
US Intake
Although soy-containing foods have been consumed by Asian populations for centuries, the best-known soy food, tofu, was only introduced on a large scale in US markets in the 1970s.
As more and more scientific publications over the last few decades suggested potential health benefits of dietary soy and isoflavones, US soy food sales increased-from $1 billion in 1996 to $4.5 billion in 2009. The most dramatic increase occurred between 1996 and 2003, with the biggest gains occurring in sales of soy milk and energy bars. Recently introduced categories of soy foods include soy-based drinks, drinkable cultured soy, soy dairy-free frozen desserts, and energy bars, all of which have shown strong and steady growth in sales. US food manufacturers introduced more than 2,700 new foods with soy as an ingredient from 2000 to 2007. Previously, most sales of soy food and drinks occurred in health food stores, but now 75% of these sales are from supermarkets.
From 2006 through 2009, approximately one third of Americans consumed soy foods or beverages once a month or more frequently. In addition, soy flour and soy oil are used in baked goods, entrees, cereal, pasta, meal replacements, powdered soy beverages, chips, snack foods, and low-carbohydrate foods, comprising about one third of total soy food sales in recent years. Soy isoflavones may also appear unexpectedly in many products in which soy protein is used for its textural properties.
Soy food intake has been assessed specifically among midlife US women. A 2002 telephone survey of 886 women ages 45 to 65 who were members of the Group Health Cooperative in Washington State found that 22.9% used dietary soy. Breast cancer survivors were six times as likely as the overall survey respondents to report use of dietary soy, while women taking hormone therapy (HT) were half as likely to report use of dietary soy. The Study of Women's Health Across the Nation (SWAN), a multisite, longitudinal US cohort study, found substantial variation in dietary isoflavone intake by race/ethnicity among 3,133 women who were premenopausal or early perimenopausal at baseline. Considerably lower isoflavone intakes were found at baseline in Caucasian, Hispanic, and African women (averaging <0.5 mg, a level of questionable biologic significance) than in Asian women (averaging >9.7 mg/d in women of Chinese ethnicity and >18 mg/d in women of Japanese ethnicity). Approximately 40% of non-Asian US women in SWAN consumed no daidzein or genistein, the most prevalent isoflavones.
Comparative Asian Intake
To put these US dietary intakes into context, data from large Japanese cohort studies included in a comprehensive review published in 2006 indicated that soy protein intake among older Japanese adults was approximately 10 g/day and was nearly the same in women and men. In these studies, soy foods contributed from 6.5% to 12.8% of total protein intake. Mean estimates of isoflavone intake (expressed as aglycone equivalents) ranged from about 30 to 50 mg/day. In a prospective cohort study in Japanese adults ages 45 to 74, the mean fourth quartile isoflavone (excluding glycitein) intake was 78 mg/day in men (n = 9,044) and 77 mg/day in women (n = 10,121). According to food disappearance data from the United Nations' Food and Agriculture Organization, per capita soy protein intake in Japan has remained relatively constant during the past 40 years, although soy's share of total protein intake has decreased from about 13% to 10% because of the increased protein content (mostly from animal sources) of the Japanese diet.
Additional context for US isoflavone intake comes from mainland China, where diet is more heterogeneous than in Japan. The Shanghai Men's Health Study and the Shanghai Women's Health Study, prospective epidemiologic studies involving approximately 50,000 participants each between ages 40 and 70, have indicated that daily mean soy protein and isoflavone intakes in Shanghai are similar to or somewhat higher than those in Japan.
Soy Supplement Use
Studies on the prevalence of soy or isoflavone supplement use in the United States are very limited. The only data from a national sample, the 2002 National Health Interview Survey (NHIS), indicate that 9.4% of US adults reported use of soy supplements in the prior 12 months, but this survey report did not specifically address use among midlife women.
In the absence of further data specific to soy supplement use, it may be reasonable to look to use of complementary and alternative medicine (CAM) as a surrogate. Among all gender and age groups in the 2002 NHIS, midlife women had the highest prevalence of reported CAM use. Approximately 45% of women ages 40 to 59 in the 2002 NHIS reported any CAM use in the prior 12 months, and 28.4% reported use of biologically based CAM therapies. The latter percentage was up from 16.2% from the 1999 NHIS. In both the 1999 and 2002 NHIS, use of biologically based CAM therapies was highest among Asian and Caucasian women. Use of medicinal herbs, which include soy, comprised the largest proportion of these therapies, with 18.6% of women overall reporting such use in the 2002 NHIS. A similar level of use of medicinal herbs and teas was reported in a separate nationally representative sample of US women conducted in 2001, with complex and nuanced differences in use by race and ethnicity.
Conclusions
US dietary consumption of soy has increased severalfold over the past 15 years, with one third of Americans consuming soy food or beverages at least once a month. Despite this increase, dietary consumption of soy in the United States remains far below that in Asia. Although nearly 1 in 10 US adults reported use of soy supplements in a 2002 nationwide survey, further studies are limited. More research is needed to understand the health reasons for soy use among midlife women-specifically of soy isoflavone supplements. In addition, the interrelations of dietary intake and supplement use with equol production in terms of effects on health outcomes in midlife women require further study, as do potential interactions with prescription and over-the-counter medications.
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