Hops and Women’s Health

By Stuart Milligan, MA, DPhil

"While you are working here, it is very like- ly that your menstrual cycles will be disturbed." These words greeted my wife when she started work in a laboratory involved in hops processing: The subsequent truth of this prediction led me (as a reproductive biologist) into a fascinating new research world of hops, herbs, and beer.

Without some additional flavoring, beer is a sickly bland drink. The female flowers (strobiles, or cones) of hops (Humulus lupulus L.) have been used for centuries to add aroma and bittering to beer, although their initial use was to provide a bacteriostatic and fungistatic effect that extended beer’s shelf-life. The flavoring and preservative properties reflect the complex chemistry of the lupulin glands within the hop flowers. They contain hundreds of secondary metabolites, ranging from the hop acids, which provide the bittering activity, to volatile hop oils.1,2

Although the hop plant belongs to the same family (Cannabaceae) as marijuana, hops do not contain any cannabinoids. However, medicinal properties have been attributed to hops for centuries.2 Nicholas Culpeper, the famous astrologer-physician of the early 17th century, stated that: "Half a dram of the seed in powder taken in drink kills worms in the body, brings down women’s courses, cures the yellow jaundice, eases the headache that comes of heat, and tempers the heat of the liver and stomach."3 Belief in the medicinal properties of hops has faded with time, although hop pillows are still produced for their reputed soporific effects.

Estrogenic Activity

The suggestion that hops have estrogenic activity has a very long history. In Germany, hop baths were used for the treatment of gynecological disorders.4 Changes in menstrual cyclicity were reportedly common in women hop pickers,1 and in Belgium, this legendary effect was attributed to the "hop devil."5

More recently, hop extracts have been reported to be effective in reducing hot flashes in menopausal women.6 Early scientific studies confirmed that hops possess very high estrogenic activity,4,7,8 but the chemical nature of hops was unresolved until just a few years ago.

Following the reports of menstrual disturbances among women associated with hops, my laboratory undertook to find the cause. Fractionation of the polyphenol components of hops led us to the identification of a minor component, 8-prenylnaringenin, as the agent responsible for the estrogenic activity of hop flowers.9-11 The same compound also has been isolated from a crude Thai drug that is derived from the heartwood of Anaxagorea luzonensis A. Gray (Annonaceae).

In vitro, the estrogenic activity of 8-prenylnaringenin has been demonstrated in a yeast screen bearing the human estrogen receptor alpha (ERa), in a human endometrial cell line (Ishikawa Var 1), and in competitive binding assays using rat uterine cytosol and pure human estrogen receptors (both ERa and ERb).9-11 The compound also showed typical estrogenic activity on the reproductive tract of mice, after subcutaneous injection and when given in the drinking water.12 The compound also suppressed bone loss in ovariectomized rats when given by daily injection.13

In both in vitro and in vivo tests, 8-prenylnaringenin appears to be one of the most potent phytoestrogens, with an estrogenic activity considerably greater than that of established phytoestrogens such as genistein and daidzein.9-11 Even so, it should be emphasized that the estrogenic activity of 8-prenylnaringenin is considerably weaker than that of 17b-estradiol (< 1/100 in vitro). The weak estrogenic activity of phytoestrogens is one of the confounding factors in any interpretation of the bio- logical significance of these compounds in humans. Other confounding factors include limited data on other bioactivity, bioavailability, and metabolism after oral intake.14

Other Biological Properties of 8-Prenylnaringenin and Related Compounds

Prenylflavonoids are quite rare compounds in nature, but 8-prenylnaringenin is just one of a number of related prenylflavonoids in hops; others include xanthohumol, desmethylxanthohumol, isoxanthohumol, 6-prenylna-ringen, 6,8-diprenylnaringenin, 3’-geranylchalconaringenin, 6-geranylnaringenin, 8-geranylnaringenin, and 4’-O-methyl-3’-prenylchalconaringenin.15,16 Although the estrogenic activity of 8-prenylnaringen has received the most attention (reflecting the current general interest in phytoestrogens), other biological activities of hop prenylflavonoids have been described.

In vitro studies have revealed that the compounds show variable antiproliferative effects on breast and colon cancer cell lines, inhibit cytochrome P450-mediated activation of procarcinogens, and inhibit diacylglycerol acyltransferase activity.17-19 8-prenylnaringenin, like 17b-estradiol, also has been shown to stimulate E-cadherin-dependent aggregation and growth of breast cancer cells in vitro.20 Oral intake of freeze-dried beer extracts was anticarcinogenic in mice.21 Both hop alpha-acids and xanthohumol inhibit the synthesis of cyclo-oxygenase 2 and inhibit osteoporosis in rats.22,23

Hops provide a rich source of polyphenolic compounds with a wide variety of bioactivities. Is exposure to any of these, singly or in combination, of potential use or threat to humans?

Hop Compounds in Beer

Over the last few years, scientific and public interest in the potential role of estrogenic compounds in the environment and diet has intensified. Exposure to industrial chemicals with estrogenic activity has been suggested as a possible cause of reported reductions in sperm counts and reproductive tract abnormalities in men,24,25 while phytoestrogens have been implicated as having a variety of beneficial health effects.26,27 In both cases, although exposure to large amounts of these compounds may produce significant effects in humans, the effects of normal, low-level exposures are debatable.14,28

The identification of 8-prenylnaringenin as a potent phytoestrogen could provide an obvious explanation for the menstrual disturbances in female hop workers in the past, presumably due to exposure via transdermal absorption or dust inhalation. However, hop picking is now done mechanically and the only widespread human exposure to 8-prenylnaringenin is likely to be via beer consumption.

The amount of 8-prenylnaringenin in beer is dependent on the brewing process and the nature of the raw materials used.16,29 In beers that include extracts from processed hops, very little 8-prenylnaringenin may be present. Although most beers contain less than 30 ug/L of 8-prenylnaringenin, the compound can occur at levels of up to 240 mcg/L in beers made using whole hops.16 This is consistent with the amount of 8-prenylnaringenin in dry hops (100 mg/kg) and the small amount (only a few grams) of hops used per liter of beer in brewing.

The concentration of 8-prenylnaringenin in beer is considerably greater than that of the widely known isoflavonoid phytoestrogens (formononetin, daidzein, genistein: 0.1-15 Nm).30 However, despite the relatively high estrogenic activity of 8-prenylnaringenin, the total estrogenic activity of beer made using whole hops still is very low,31 and no detrimental health effects due to estrogens in beer have been reported or are to be expected. A study examining the effect of 8-prenylnaringenin in mice showed that levels of 100 mcg/mL were required to produce any sign of estrogenic activity, i.e., at least 400-fold greater than that found in any beer!11

Moderate intakes of isoflavonoid phytoestrogens (e.g., in soy) have been associated in some reports with a reduction in incidence of breast and prostate cancer, cardiovascular disease, and menopausal symptoms;26,27 however, the evidence for beneficial effects is debatable.14,28 It also has been suggested that resveratrol, an extremely weak phytoestrogen found in grapes and wine, may contribute to the beneficial effects of wine consumption (e.g., the French paradox),32 but again, critical experimental evidence is lacking. Whether the intake of 8-prenylnaringenin or any related compound contributes to the reported beneficial health effects of moderate beer consumption33 is an open question. Beers with a high polyphenol content also have high antioxidant activity and it may be this contribution to the total antioxidant environment of the body that is important.34

The sparse knowledge of the bioavailability of the hop compounds, their metabolic fates, and their in vivo bioactivity will allow speculation to continue until experimental evidence is available to clarify the situation.

Hops and Hot Flashes

A methodologically flawed French study in 25 women found a benefit after 30 days of treatment with large doses of hop extract (1,600-2,600 mg/d, titrating down to 1,200-1,600 mg).6 The study was not randomized, and the control group was apparently composed of five women who had been accidentally treated with a low dose of hops (300 mg), which, since it had no benefit, was considered a placebo. In 17 of the 20 treated subjects, reduction of hot flash score (intensity ´ frequency) was significantly better than control (neither statistical test nor significance level was stated). Some subjects apparently received another herb, hawthorn, as well as hops.

Reduction in hot flashes would be consistent with the estrogenic activity of 8-prenylnaringenin, but such an effect needs confirmation in a properly controlled clinical trial, with attention to possible side effects and identification of side effects of the active agent(s).

Hops as a Breast Enhancement Supplement

Although hops are used to impart flavor, bitterness, and other properties to beer, the amount of hops used in brewing is relatively small and 8-prenylnaringenin therefore is not a significant dietary component. However, hops are now incorporated into a number of herbal preparations for women, including some that claim the ability to enlarge breasts.

Claims for the effectiveness of at least some breast enlargement preparations are based partly on their phytoestrogen content. The concentration of 8-prenylnaringenin in one breast-enhancing preparation containing hops was 10.9 mcg/g, which would result in a daily intake of approximately 130 mcg/d.35 This value is less than that found in a liter of some beers; there is no evidence as yet that such an amount would have a significant estrogenic effect in humans. When tested in mice, the hops-containing supplement showed no evidence of estrogenic activity following administration either in the feed or by subcutaneous injection of the extract.

If breast enhancement products do contain 8-prenylnaringenin, the question must then be asked: Are such products either effective or safe? In terms of efficacy, a peer-reviewed clinical trial has yet to be published. However, even if an effect did occur, real fears over safety issues exist. The breast is an important site of hormone-dependent cancer and is very sensitive to estrogens. Inappropriate estrogenic stimulation of this tissue would be of considerable concern. In addition, estrogenic effects would not be expected to be limited to breast tissue alone and this is consistent with reports of menstrual disturbances associated with some of these supplements. Any disturbance of menstrual cyclicity is of obvious concern in relation to normal fertility and adverse interactions with contraceptives.

An additional concern is that women may be taking such breast enhancement products in the early stages of pregnancy and the fetus may be inadvertently exposed to relatively large amounts of bioactive compounds with unknown fetal effects. Hop phytoestrogens would not be expected to be problematic; they are far weaker than pharmaceutical estrogens, which (with the exception of DES) have not been linked to birth defects. However, hops, and other ingredients in these preparations, contain many compounds for which reproductive toxicology information is lacking.

The Need for Caution

There are many target sites for estrogenic activity in the body, ranging from the reproductive tract and breast, through bone, fat, and blood vessels, to the bladder and brain.36 The significance of estrogenic effects at many of these sites is still unclear. In 1996, researchers discovered that there are two, rather than one, estrogen receptors (ERa and ERb).37 These two estrogen receptors have differing affinities for various estrogens and differing tissue and cellular distributions, and may interact with the vast array of nuclear receptor coactivators and corepressors in varying ways.36 These observations have opened a surprising new dimension in our ignorance of how 17b-estradiol works. Understanding the molecular mechanisms and activity of the much weaker phyto-estrogens is a long way off.

It also is inappropriate to assume that phytoestrogens only have estrogenic activity. Each phytoestrogen may have multiple biological activities completely independent of its estrogenic activity. This is illustrated by the activities of genistein, which include inhibiting the activities of enzymes such as 5a-reductase, aromatase, 17b-hydroxysteroid dehydrogenase, tyrosine kinase, and topoisomerase II.14 The fact that genistein is a potent inhibitor of proliferation even in estrogen receptor-negative breast cancer cells illustrates that the estrogenic activity of the compound probably is not responsible for all of its biological effects.14

It would seem dangerously naïve to assume that human exposure to a complex combination of compounds with a wide spectrum of bioactivities (as exists in hops) would have a single target (e.g., the breast) and that the actions at this target would be solely beneficial. The reproductive tract and breast are important sites of hormone-dependent cancer and are very sensitive to estrogens.38 Estrogens have many effects on other tissues. Uncontrolled exposure to large amounts of unstudied hops compounds is inadvisable, to put it mildly!

Dr. Milligan is Senior Lecturer in Physiology, Centre of Reproduction, Endocrinology and Diabetes, School of Biomedical Sciences, King’s College, Guy’s Campus, London, United Kingdom.

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