Scientific Name(s): Humulus lupulus L.
Common Name(s): Hops
Hops is a perennial climbing vine extensively cultivated worldwide. Male and female flowers are located on separate plants; the cone-shaped fruits are known as strobiles, which are collected in the fall and carefully dried.1, 2
Hops have been used for centuries to flavor and preserve beer. The bitter, aromatic taste of beer is mostly due to the hops content. Hops extracts are also used for other flavoring purposes in the food industry. Medical uses of hops and lupulin include aiding digestion, mild sedation, diuresis, and treating menstrual problems. Hops pickers have reported sedation during harvest, and hops flowers have been added to pillows for relief of nervous conditions.2, 3 Use of hops for the treatment of mood disturbances, such as restlessness, anxiety, and sleep disturbances, is approved in the German Commission E Monographs.4
The most characteristic constituents of hops are the bitter principles, known as alpha- and beta-acids. In the plant the alpha-acids occur as humulone, cohumulone, and adhumulone.5, 6 During the brewing process, these compounds are isomerized to the iso-alpha-acid series of compounds, that possess the bitter taste.7 The beta-acid series of compounds include lupulone and congeners;5, 6 this series is destroyed during brewing. The relative proportions of the bitter acids affect the quality of the hops, and many methods have been developed for quantifying hop acids in different varieties, including nuclear magnetic resonance (NMR)5, 8 and high-pressure liquid chromatography (HPLC).9, 10 The complex profile of hop acids is dependent on genetics, cultivation, and storage conditions. Long-term storage of hops leads to major deterioration in quality.
The essential oils of hops are less characteristic but are still important to hop quality. Over 100 volatile compounds have been identified, with gas chromatography and gas chromatography-mass spectroscopy (GC-MS) being key techniques for analysis.11, 12 Caryophyllene, beta-myrcene, and humulene are the most abundant constituents of hops volatile oils.
A third group of hops constituents is the prenylflavonoids. Xanthohumol is the dominant prenylflavonoid of hops,13 with 8-prenylnaringenin also of importance.14 A GC-MS method has been developed for the latter,14 while liquid chromatography-tandem mass spectrometry (LC-MS) has been used to directly quantify prenylflavonoids and their isomerization products in beer and hops extracts.15 The variation in prenylflavonoids between hops varieties has also been studied.16 The fate of xanthohumol as hops is processed into beer has been studied; 20% to 30% is converted to isoxanthohumol.17 The metabolism of xanthohumol in rat and human liver microsomes has also been characterized.18, 19
Uses and Pharmacology
The compound 2-methyl-3-buten-2-ol was isolated and found to reduce the spontaneous movement of rats when given intraperitoneally.20 The small amounts found in hops11 makes it unlikely that this compound completely explains hops sedation. A review of animal studies reports equivocal findings for sedative effects of hops.21
A randomized, placebo-controlled clinical trial evaluated 100 mg H. lupulus prepared in combination with soya and cade oil and soya lecithin in 101 patients with chronic primary insomnia. No difference over placebo was found.26 A smaller clinical study (n=17 nurses working shifts) evaluated the use of nonalcoholic beer containing hops and reported improved measures including actimeter-measured sleep latency and self-evaluated sleep anxiety.27
8-prenylnaringenin was found to be a potent estrogen receptor agonist in estrogen-responsive cells, while other hops phenolics were less active (isoxanthohumol, 6-prenylnaringenin) or had no activity (xanthohumol).28 The amounts present in beer were considered to be too small to cause estrogenic effects. Estrogenic effects in vivo were observed in mice given isolated 8-prenylnaringenin in drinking water at 100 mcg/mL, using uterine vascular permeability as an endpoint.29, 30
Estrogenic effects were also observed in evaluation of hops extract for the treatment of menstrual symptoms.31 A double-blind, randomized, placebo-controlled, crossover trial conducted in 36 women experiencing symptoms of natural menopause were treated with hop extract capsules (standardized to 8-prenylnaringenin at 100 mcg/day) or placebo daily for 16 weeks to investigate relief of menopausal discomforts. Both objective and subjective outcomes were measured. At 8 weeks, both the treatment and placebo groups experienced significant improvements compared to baseline; however, after the crossover, the treatment group continued to experience significant improvements in all outcome measures over the next 8 weeks, whereas all scores worsened slightly in the placebo group.32 Reviews of studies have been published.21, 30, 33 (Karabin Zanoli, keiler) Use of hops in breast enhancement products was cause for concern.34, 35
Colupulone adsorbed on brewers' yeast was found to induce cytochrome P-450 3A in mice, an enzyme capable of N-demethylation of ethylmorphine.36, 37 However, short-term assays for aflatoxin or benzpyrene activation through colupulone induction of CYP450 did not find a change in mutagen activation.38 While beer and other alcoholic beverages have been found to inhibit mutagenesis induced by carcinogens in an Ames test, the compounds responsible were not identified.39 In a later study, several hops prenylflavonoids inhibited carcinogenic amine activation by CYP1A2.40
Humulone was identified as the active hops constituent that inhibited phorbol ester-induced inflammation in mice.41 The same group later demonstrated that humulone was active in blocking tumor promotion in the classical two-stage model of carcinogenesis.42 Several different hops prenylflavonoids demonstrated antiproliferative and cytotoxic effects in breast, colon, ovarian, renal, and prostate human cancer cell lines.43, 44, 30 8-prenylnaringenin was shown to upregulate the cadherin and catenin genes in human breast cancer cells.45 A comprehensive evaluation of xanthohumol as a cancer chemopreventative agent found that it warranted clinical investigation because it had distinct activities at the initiation, promotion, and progression stages of carcinogenesis.46
Research reveals no clinical data regarding the use of hops for cancer chemoprevention.
The hop bitter acids have antibacterial and antifungal activity important for the preservative function of hops in beer.21(Zanoli) When tested at the normal pH of beer (4.0), isohumulone inhibited bacterial growth at concentrations at which it is normally found in beer.47 Polyphenols derived from hops reportedly reduced regrowth of dental plaque in a clinical study.48
The prenylflavonoids of hops were shown to be more effective antioxidants than nonprenylated flavonoids.49 Hops proanthocyanidins inhibited neuronal nitric oxide synthetase and efficiently scavenged reactive nitrogen species.50
Humulone potently suppressed COX-2 gene expression at the level of transcription.30, 51 (Karabin) Anti-inflammatory effects have been demonstrated for hops evaluated in knee osteoarthritis.52 (Hall) Xanthohumols inhibited diacylglycerol acyltransferase, an effect of possible importance in lipid metabolism.53 A clinical trial (n= 200) reported positive effects of matured hop extract in reducing body fat in healthy overweight trial participants.54 Limited studies suggest efficacy of hops extracts in preventing histamine release and reducing allergic symptoms.30, 55
Hops has been used as a mild sedative or sleep aid, with the dried strobile given in doses of 1.5 to 2 g. An extract combination with valerian, Ze 91019 (ReDormin, Ivel) has been studied at a hops dose of 60 mg for insomnia.56
Pregnancy / Lactation
Avoid use. In vitro antispasmodic activity on the uterus has been documented.3
None well documented. Flavonoids reportedly possess anticoagulant and antiplatelet activity.30
Research reveals little or no information regarding adverse reactions with the use of hops.
As an historical food constituent, hops has "generally recognized as safe" (GRAS) status by the FDA, however; use of medicinal quantities of hops may pose more risk than common levels of exposure in food use. Dogs appear to be somewhat sensitive to hops compounds. A malignant hyperthermic reaction was observed in 5 dogs who consumed boiled hops residues used in home brewing.57 A subchronic toxicity study of the hops alpha-acids was conducted in dogs; while high doses induced vomiting, the animals generally tolerated lower doses without ill effects. A wide safety margin for humans was extrapolated from this experiment. 58
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