What Are the Active Ingredients in Hops Flower Extract and Its Benefits?

Hops (Humulus lupulus L.), also known as Hubei, Xiangshema, Shema Hua, and Peiwa Guli (Uyghur), are plants belonging to the Moraceae family and the Humulus genus. They are a unique medicinal and edible plant resource native to Xinjiang, China. Additionally, hops are one of the key ingredients in beer brewing, not only imparting a distinctive flavor but also extending the shelf life of beer.

Humulus lupulus has been used by humans for over 2,000 years. As early as the 2nd century BCE, the ancient Babylonians cultivated and utilized it. In the 13th century, it began to be used as a herbal medicine. In 1516, Germany issued a decree designating Humulus lupulus as the sole bitter additive for beer [1]. Hops extract contains various chemical components such as flavonoids, resins, polyphenols, and polysaccharides, which confer antibacterial, antitumor, antioxidant, hypoglycemic, hypotensive, and estrogen-like pharmacological effects [2]. In recent years, hops have been extensively studied in the field of osteoporosis prevention, showing great potential. This paper provides a brief overview of the relevant research.

1 Active Components

1.1 Resins

Resin compounds are the primary components of hop extracts and exhibit broad biological activity. The European Brewing Convention (EBC) classifies resins into soft resins and hard resins based on their solubility in different organic solvents [3]. Among these, α-acids and β-acids are the most representative soft resin components in hops and are the primary sources of the unique flavor of hops. α-Acids primarily include humulone (Figure 1A) and its isomers, while β-acids primarily include cystine (Figure 1B) and its isomers [4]. In hops, β-acids are present in lower concentrations than α-acids and are easily oxidized to form β-resins. Under certain conditions, α-acids can be converted into iso-α-acids, which are the primary bitter components in beer [5].

1.2 Flavonoids

Flavonoids, represented by quercetin (Figure 2), are important chemical components in hop extracts. Based on the differences in their core structures, they can be classified into flavonoids, chalcones, and flavones [6]. Humulone is a unique isoprenylated flavonoid specific to hops, first isolated and identified by Power et al. [7], primarily concentrated in hop strobila glands. It is currently attracting significant attention due to its broad pharmacological activities.

1.3 Volatile oils

Volatile oil components in hops are secreted by the strobila glands of hops and are the source of the beer aroma. The volatile oils in hops primarily contain terpenes such as caryophyllene, geraniol, humulene, and pinene, as well as their esters, ketones, and alcohols [8]. Early studies generally believed that terpenes play a key role in the flavor of hops, but recent research indicates that terpenol compounds with higher hydrophilicity contribute more significantly to the flavor of hops [9].

2 Anti-osteoporosis effects

Osteoporosis is a systemic bone metabolic disease characterized by reduced bone mass and impaired bone microarchitecture. In Europe, hop extracts are used to treat postmenopausal osteoporosis. In recent years, the role of hops in preventing and treating osteoporosis has garnered significant attention. This may be achieved through estrogen-like effects, alleviating oxidative damage, and regulating bone formation-resorption balance to maintain bone homeostasis.

2.1 Estrogen-like effects

Estrogen deficiency is one of the primary factors contributing to the onset of osteoporosis [10]. The flavonoid component 8-isopentenylflavonol (8-PN) in hops is an isomer of demethylflavonol and is currently the most effective plant estrogen isolated [11]. As early as 2002, Miligan et al. [12] found that both natural and synthetic 8-PN exhibited similar biological activity in yeast cells transfected with human estrogen receptors and in estrogen-responsive human Ishikawa Var-I cells. 8-PN demonstrated good binding affinity with both forms of estrogen receptors (ER-α and ER-β). In vitro screening experiments revealed that its estrogenic activity was higher than that of several commonly used plant estrogens, including coumarins. As the most representative flavonoid component in hops, humulone also exhibits significant phytoestrogenic activity. Studies have shown that in ovariectomized mice, 30 and 90 mg/(kg·d) of humulone significantly inhibited estrogen deficiency-induced weight gain, increased estrogen (E2) levels, and suppressed the high expression of bone turnover markers such as alkaline phosphatase (ALP) and tartrate-resistant acid phosphatase (TRAP), and improve bone microstructural damage, enhance bone density, and prevent osteoporosis in ovariectomized mice [13].

Multiple resinous components in hop extract also exhibit estrogen-like effects. Holick et al. [14] found in a 14-week clinical trial that resin components, berberine, vitamin D, and vitamin K, when used in combination, can regulate bone metabolism levels in postmenopausal women, reduce bone turnover markers such as osteocalcin (OCN), and significantly increase serum 25-hydroxyvitamin D levels while reducing bone loss. Keiler et al. [15] used ovariectomized rats as study subjects, to investigate the preventive effects of standardized hop extract on bone loss in rats. The results showed that standardized hop extract significantly reduced the number of osteoclasts at the tibial metaphysis in rats and prevented the reduction in trabecular thickness caused by estradiol depletion, thereby preventing osteoporosis induced by estrogen deficiency.

2.2 Alleviating oxidative damage

Estrogen or androgen deficiency reduces the skeletal system's ability to defend against oxidative stress, leading to bone loss. Therefore, high oxidative stress levels and sex hormone deficiency are both important causes of osteoporosis [16]. Flavonoids generally contain phenolic hydroxyl groups and have significant spatial steric hindrance, thereby exhibiting varying degrees of antioxidant activity. Wu Jie et al. [17] found that in studies on the antioxidant effects of humic acid and other food components, humic acid exhibited synergistic antioxidant activity with food acidifiers such as citric acid, sodium citrate, and vitamin C in the DPPH system. Suh et al. [18] found that humic acid can reduce oxidative damage in MC3T3-E1 osteoblasts by activating the key oxidative stress pathway Nrf2, further confirming its antioxidant activity.

2.3 Maintenance of Bone Homeostasis

Osteoblasts and osteoclasts play complementary roles in bone metabolism by promoting bone formation and bone resorption, respectively, thereby maintaining bone homeostasis [19]. Studies have shown that at the MC3T3-E1 osteoblast level, humic acid significantly upregulates the expression of ALP and osteogenic marker genes bone morphogenetic protein-2 (BMP-2) and bone sialoprotein (BSP), and promotes bone formation by activating transcription factor RUNX2 through the regulation of p38 MAPK and ERK signaling pathways [20]. Previous studies by our research group have found that humic acid can both promote the proliferation of primary osteoblasts, ALP activity, and bone mineralization levels, as well as increase the expression levels of bone formation-related proteins BSP, BMP-2, and osteopontin (OPN) [13]. In terms of inhibiting osteoclast-mediated bone resorption, humic acid can disrupt the binding between RANK and TRAF6, thereby inhibiting the NF-κB and Ca²⁺/NFATc1 signaling pathways during osteoclast generation, and suppressing the expression of osteoclast-related marker genes such as cathepsin K (CtsK), nuclear factor kappa-light chain regulatory element-binding protein (NF-κB) (NF-κB), and TRAP, thereby inhibiting bone resorption [21].

Hops resin components also regulate bone metabolism balance. Humulone significantly promotes osteoblast proliferation, increases alkaline phosphatase (ALP) activity, promotes bone mineralization nodules, and enhances the expression of bone formation-related proteins such as OCN, BSP, and BMP-2. Humulone also significantly promotes osteoblast activity and increases the expression of bone formation-related proteins OCN, OPN, BSP, and BMP-2. At the osteoclast level, humulone and humulone both reduce osteoclast numbers and inhibit the expression of osteoclast-related proteins CtsK and matrix metalloproteinase 9 (MMP-9) [22]. Additionally, our research group previously found that ethanol extracts of hops significantly promote osteoblast proliferation, ALP activity, and bone mineralization nodules, enhance the expression of bone formation-related proteins OPN and BMP-2; and significantly inhibit the expression of osteoclast-related proteins TRAP, CtsK, and MMP-9, thereby maintaining bone metabolic balance, preventing bone loss caused by bone resorption exceeding bone formation [23].

3 Related Products and Applications

Beer brewing is the most traditional application of hops, and the humulone and related isoprenoid flavonoids in hops are primarily ingested through beer consumption [24]. In recent years, hops have gained increasing attention as a specialty traditional Chinese medicine due to their dual medicinal and edible properties. Related health products, such as American Citra and Czech Saaz hop granules, Australian Nature's Way hop capsules, and Finnish Menomax hop concentrate tablets, have emerged in succession. Modern research has identified some hop-related products with promising activity in preventing osteoporosis.

Ban et al. [25] used an ovariectomized rat osteoporosis model to study the effects of Lifenol® hop extract on the prevention and treatment of osteoporosis. The results showed that the product significantly improved weight gain caused by ovariectomy, regulated lipid levels and fat accumulation, reduced blood flow velocity, alleviated hot flashes in rats, and significantly increased bone density in the femurs of rats, and improve osteoporosis. Direct evidence suggests that drinking beer can prevent osteoporosis.

Kondo [26] used ovariectomized rats as an osteoporosis model to study the effects of beer on osteoporosis. The results showed that beer significantly inhibited bone loss in the femurs of ovariectomized rats, and this inhibitory effect was not observed in beer made with alcohol alone or without hops, indicating that the active components with anti-osteoporosis effects in beer originate from hops. Additionally, Ferk et al.[27] found that after humans consumed a yellow oxalic acid beverage for 14 days, the levels of oxidative purines in the body significantly decreased, oxidative damage was alleviated, and serum estrogen and bone calcium levels relatively decreased, with significant improvement in bone metabolism disorders.

4 Current Status of Hops Resources in China

There is no consensus among scholars regarding the origin of hops, with some suggesting that it originated in China [28-29]. In China, the birthplace of hops is located in Shangzhi City, southeastern Heilongjiang Province. In 1960, the Ministry of Light Industry decided to establish the Xinjiang Farm as a national hops production base and introduced hops from Qingdao, Shandong Province, and the Northeast region. After 40 years of efforts, the area under hop cultivation has reached 185.2 hectares, making it one of the main cash crops in the Xinjiang region.

The wild hop populations in China are primarily distributed near the Tianshan and Altai Mountains in Xinjiang. The Xinjiang region, with its abundant sunlight and significant day-night temperature differences, is highly suitable for hop cultivation and has gradually developed into a vast area with diverse habitats and distinct maturity periods for wild hop varieties [3]. However, in recent years, the development and production of hops have faced challenges. The hops cultivated in Xinjiang are primarily varieties introduced from the United States and Germany.

Due to prolonged cultivation in a foreign environment, their growth performance has declined, pest and disease incidence has increased, and variety degradation has become severe, leading to a decline in medicinal quality. Looking at the different varieties of high-quality aromatic, aromatic, bitter, and high α-acid type hops currently available, those with high α-acid content exceed 8% (high α-acid type), while those with lower content range from 3% to 4% (bitter and aromatic types). The ratio of α-acid to β-acid is higher than 2.0 (high α-acid type), while those with a ratio below 1.0 are classified as high-quality aromatic types. This indicates that the content and proportion of key active components, such as bitter acids, vary significantly among different types of hops [4]. Therefore, it is essential to strengthen breeding research from the source, improve disease resistance, increase yield, and enhance the content of bitter acid components. Additionally, cultivated varieties and breeding lines (or wild hops) should be utilized to combine superior traits under suitable conditions.

5 Outlook

Modern pharmacological studies have clearly demonstrated the anti-osteoporosis effects of hops and their extracts through in vivo and in vitro experiments. Humulone, humulone, and caryophyllene all promote bone formation and inhibit bone resorption; however, the underlying mechanisms of their anti-osteoporosis effects remain to be elucidated. Hops have preventive and therapeutic effects on postmenopausal osteoporosis in women, but their effects on senile osteoporosis remain unclear. Therefore, the authors believe that elucidating the mechanisms of action of hops and their active components is essential for providing a theoretical foundation for the clinical application and translation of hops, as well as for expanding the scope of their applications. Additionally, China has abundant hop resources, but the genetic resources, genetic background, and phylogenetic relationships are unclear, leading to inconsistent quality of herbal materials. Therefore, it is essential to establish a genetic resource inventory of hops, investigate the distribution frequencies of different genes within populations, clarify the phylogenetic relationships between populations, and establish quality standards for herbal materials to provide quality assurance for hop bioactive research and development.

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