Poplar-type propolis is the widely used and studied type of propolis, commonly distributed in Europe, North America, most of Asia and parts of North Africa (19, 20). Propolis varies in compositions, with more than 300 compounds identified as (21): (a) free phenolic acids, such as caffeic acid, p-coumaric acid and ferulic acids; (b) esters of these acids, such as caffeic acid phenethyl ester (CAPE); (c) flavonoids (flavones and flavonols) such as crysin, luteolin, apigenin, and kaempferol (21).

Figure 1 shows representative polyphenol compounds in poplar propolis. Caffeic acid and p-Coumaric acid are the main phenolic acids in water extracts of poplar propolis, with the contents reaching 76.9 ± 0.6 and 61.4 ± 0.3 mg/g, respectively (22). It is reported that caffeic acid can prevent diet induced hyperlipidemia and obesity in mice by regulating the expression of intestinal microorganisms and liver lipogenic genes (23, 24). Besides, the study found p-coumaric acid can be used against obesity by promoting thermogenesis of brown adipose tissue (25). The esters of caffeic acid, CAPE is one of the most characteristic polyphenols in ethanolic extracts of poplar propolis (about 11.2 ± 0.2 mg/g) (22). CAPE is a promising propolis ingredient. Several studies have shown that CAPE has many beneficial biological properties, including antioxidation (22), improving radiation protection (26), insulin resistance (27), and anti-obesity (28). Cardinault et al. explored the differential effects of three types of propolis alcohol extracts (Poplar, Baccharis and Dalbergia) on obesity in high-fat-fed mice, and found that only poplar propolis extract exerted a significant anti-obesity effect, suggesting that this difference may be related to its unique polyphenol compound (such as CAPE) (29). Moreover, in vivo and in vitro studies have found that CAPE can inhibit the differentiation of mouse preadipocytes and prevent adipogenesis (30, 31), and its specific mechanism will be discussed in the next chapter. In addition to phenolic acids, flavonoids, such as chrysin, galangin, apigenin and quercetin, exhibit excellent anti-obesity effects through different mechanisms (32–35).

Baccharis type propolis, mainly produced in Minas Gerais, Sao Paulo, Rio de Janeiro and Paraná in southeastern Brazil, is also known as “green propolis” due to its color ranging from greenish yellow to dark green (14). Baccharis dracunculifolia has been shown to be the most important botanical source of propolis (36, 37). Therefore, its chemical composition is very different from that of poplar-type propolis.

Figure 2 shows representative components of Baccharis type propolis. Compared with poplar-type propolis, Brazilian green propolis was also rich in Artepillin C (Art-C) and chlorogenic acid, with contents of 91.84 ± 1.16 mg/g and 19.35 ± 0.37 mg/g, respectively (38). As one of the main chemical markers of green propolis, Art-C can regulate a variety of signaling pathways to prevent the development of chronic diseases, including anti-obesity (39), anti-inflammatory (40), gastroprotective (41) and immunomodulatory (42), etc. Studies have found that Art-C plays an anti-obesity role by regulating several pathways, including affecting brown adipocyte production and promoting white adipose tissue thermogenesis (discussed in the next chapter) (39, 43). Another phenolic acid, chlorogenic acid, has also been proved to improve obesity by preventing energy balance transfer and improving lipid metabolism in obese mice fed by high-fat diet (44, 45).

Dalbergia propolis is mainly produced in Brazil, Cuba, Venezuela in South America and Mexico in North America (46–48). The main plant source has been determined as Dalbergia ecastaphyllum (L.) Taubert (49). This type of plant produces a red resin, which is collected by bees and used to produce propolis. Therefore, the color of Dalbergia propolis is often bright red and highly recognizable, also known as red propolis (50). Figure 3 shows the chemical structures of the representative phenolic compounds found in ethanol extract of red propolis. The main components of Dalbergia propolis are flavonoids (including flavanids, isoflavones and dihydroflavonoids) (51, 52). Isoflavones are the most abundant characteristic compounds of Dalbergia propolis different from poplar type and green propolis, Bueno-Silva et al. indicated that formononetin is the most abundant compound in Brazilian red propolis and resin, at 112.78 ± 9.07 μg/g and 77.4 ± 1.05 μg/g, respectively (53). They play a role in different physiological processes and play a variety of functions, including anti-obesity (54), anti-bacterial (55), anti-inflammatory and anti-cancer effects (52). Marcelle F. Prata and co-workers explored the effect of extract of Brazilian red propolis (mainly contained isoflavones) on hypolipidemic and anti-obesity in dyslipidemia mice model, and found that the extract of Brazilian red propolis had a hypolipidemic effect on rodent models with dyslipidemia, and minimized the effects of high-fat diet on body weight parameters and abdominal fat accumulation in mice (56). In addition, in vitro experiments showed that the ethanolic extracts of Brazilian red propolis could affect the differentiation of 3T3-L1 preadipocyte cells and the expression of adipokine (54). Although there are few studies, it can be found that red propolis extract has a promising future as a dietary supplement for the prevention and treatment of obesity and obesity-related diseases.

So far, numerous in vitro and in vivo studies have demonstrated that propolis affects body weight and adipose tissue weight in obese animals by affecting adipogenesis, fat accumulation and lipid metabolism (Figure 4). Ichi et al. reported that feeding pellets containing 0.5% (w/w) Brazilian propolis for 8 weeks reduced fat accumulation and serum cholesterol and triglyceride levels in high-fat-fed rats (57). In addition, propolis intake also affected lipid metabolism-related proteins in mice, such as peroxisome proliferator-activated receptor alpha (PPAR α), sterol-regulatory element binding protein 1 (SREBP-1), and 3-Hydroxy-3-Methylglutaryl-Coenzyme A (HMG-CoA), therefore, propolis may improve body fat accumulation and dyslipidemia by affecting the expression of fat accumulation and lipid metabolism-related proteins (57), which was the same result as another report (59). Therapeutic effects of Brazilian propolis extract on anti-obesity and dyslipidemia have also been observed in a single-gene mutant obesity model (60). Besides Brazilian propolis, propolis from other geographical areas also has weight loss effects. For example, Chinese propolis reduced the body weight of high-fat diet mice in a dose-dependent manner and reversed the liver weight loss and triglyceride accumulation associated with liver steatosis (62); Oral administration of ethanolic extract of Croatian propolis (50 mg/kg, for 30 days) was also shown to reduce body weight and hepatic lipid accumulation in high-fat diet C57BL\6N mice, and improve mouse plasma atherogenic index (61). However, although the intervention of propolis is related to the increase of lipid metabolism related proteins, as a mixture, the details of how specific components of propolis affect the lipid metabolism of the body are still unknown. Consequently, the anti-obesity effect of propolis-derived chemicals (such as CAEP) was investigated. Several studies have found that CAPE inhibits adipocyte differentiation. For example, CAPE can significantly inhibit the expression of proteins related to lipid metabolism in adipocytes, such as PPAR-γ, adipocyte-specific fatty acid binding protein (aP2), C/EBP-α and fatty acid synthase, thereby reducing the deposition of triglycerides in 3T3-L1 cells after MDI stimulation (30, 31). The molecular mechanism by which CAPE inhibits adipocyte differentiation has also been investigated. Shin et al. found that 40 μM CAPE delayed the cell cycle progression of MDI-stimulated 3T3-L1 cells, thereby inhibiting mitotic clonal expansion (28). At the same time, CAPE also blocked the phosphorylation of ERK and Akt in 3T3-L1 cells, resulting in the inhibition of the expression of cyclin D1 downstream (31, 77). Therefore, CAPE can inhibit fat production by interfering with the early phase of fat production.

Although some characteristic compounds in propolis have been reported to ameliorate metabolic syndrome, it remains a serious challenge to identify new targets of propolis active compounds and explore their underlying mechanisms. Recently, a new molecular target of propolis to improve metabolic syndrome has been found, that is, Art-C protects mice from obesity induced by high-fat diet, enhances insulin sensitivity and improves glucose and lipid metabolism by inhibiting CREB/CRTC2-mediated both gluconeogenic and SREBP transcription (63). Excitedly, Art-C has been designed and modified to form a new compound A57, which shows higher inhibitory activity on CREB-CRTC2 association and better ability to improve insulin sensitivity in obese animals (63).

For a long time, it was thought that the fat is simply a place to store fat. Recent studies have shown that adipocytes have a secretory function and can secrete various physiologic active substances, which are collectively known as adipocytokine. At present, the recognized adipocytokine can be divided into two categories: one is specifically expressed in adipose tissue, such as leptin and adiponectin; The other is non-specific expression of adipose tissue, such as tumor necrosis factor (TNF α) and interleukin-6 (IL-6). Research has shown that propolis and its extract can play an anti-obesity role by influencing the secretion of adipocytokine (Figure 5).

Leptin acts on the weight regulation center of the hypothalamus, leading to decreased appetite and increased energy consumption, thereby reducing fat deposits and inhibiting body weight gain (78). Kohei et al. studied the effect of Brazilian green propolis ethanol extract (EEGP) on leptin expression in vivo and in vitro, and found that EEGP (100 μg/ml) significantly increased the leptin expression of 3T3-L1 adipocytes (64). Similarly, intraperitoneal injection of EEGP (100 mg/kg, twice a week for 5 weeks) strongly inhibited the feeding of C57BL/6 mice, and tripled the leptin expression in epididymal adipose tissue (64). However, another report describes the effect of CAPE on leptin expression in 3T3-L1 adipocytes (31). CAPE inhibited leptin expression in 3T3-L1 cells in a dose-dependent manner (31), and accompanied by the down-regulation of insulin receptor substrate-1 (IRS-1) (31), which is a prerequisite for adipocyte differentiation (79). Hence, the CAPE-induced leptin reduction in 3T3-L1 cells appears to be due to insufficient cell differentiation. Actually, obesity also has a persistent increase in leptin that produces leptin resistance, affects glucose intolerance, and will become a determinant of diabetes, so for this type of obesity, controlling leptin as an intervention target may have the opportunity to prevent various chronic diseases (66). In a quasi-experimental study, subjects with central obesity and normal weight were collected to determine whether honey and propolis can decrease leptin levels in patients with central obesity (15). The study found that honey and propolis can reduce leptin levels in participants with central obesity, indicating that these bee products may become dietary supplements for patients with central obesity (15).

Besides, propolis and propolis-derived chemicals also modulate the expression of other adipokines (Table 1). Adiponectin is a beneficial adipokine that can regulate energy homeostasis, glucose metabolism, and fat metabolism in organisms (80). It was found that ethanolic extract of Brazilian red propolis (EERP, 20 μg/ml for 3 days) may activate the adiponectin promoter through PPARγ, thereby promoting the expression of adiponectin mRNA in post-confluent 3T3-L1 preadipocytes (54). The same report also demonstrated that EERP also reversed the inhibiting effect of TNF-α on adiponectin expression in differentiated 3T3-L1 cells (54). Other studies have also reported the up-regulation of adiponectin in adipocytes by other propolis derivatives. For example, Art-C (10 or 25 μM) significantly enhanced adiponectin expression (1.5–2.0-fold) in 3T3-L1 cells (67, 68). Furthermore, although CAPE (10 μM) decreased leptin expression, it more than doubled adiponectin expression in human ASC-derived adipocytes (65). Taken together, several polyphenolic compounds in propolis positively regulate adiponectin expression in adipocytes.

Apart from the beneficial adipokines, propolis can affect harmful adipokine expression. CAPE (10 μM) attenuated LPS-mediated effects and significantly down-regulated the expression of pro-inflammatory cytokines (TNF-α, IL-1β, IL-6, and IL-8) in ASC-derived adipocytes (65). In addition, in differentiated 3T3-L1 cells, CAPE had a significant inhibitory effect on TNF-α as well, although a higher dose (50 μM) was required (31). It was found in the same report that CAPE (25 or 50 μM) also decreased resistin mRNA and intracellular protein levels in 3T3-L1 cells (31). Recently, studies have shown that propolis polyphenols may play an anti-obesity role with the Nrf2 pathway, I-kappa-B kinase epsilon (IKK ε) and TANK binding kinase 1 (TBK1). Cardinault et al. studied the preventive effect of ethanol extract of poplar propolis (PPEE) on obesity and related metabolic diseases (69). The results showed that PPEE could prevent diet induced obesity, improve glucose homeostasis, promote lipid metabolism and thermogenesis, and significantly reduce the expression of inflammatory genes (such as Tnfa, Ccl5 and Ccl2), accompanied by the activation of Nrf2 pathway (69). Another study found that Chrysin can reduce hepatic IKK ε/TBK1 expression, promote triglyceride hydrolysis and oxidation, inhibit fat production and inflammation, indicating IKK ε/TBK1 may be one of the pathways of chrysin’s anti-inflammatory and insulin sensitivity (70). Nevertheless, the propolis and its derivatives reported in the above study have a regulatory effect on the two pathways of Nrf2 and IKK ε/TBK1, but the specific targets it acts on, and how to exert anti-obesity effects through specific targets in the pathway, are unknown. Moreover, a triple-blind randomized clinical trial involving 54 male military cadets reported that propolis intervention effectively reduced the oxidative stress and inflammation (IL-6, IL-10) of the subjects after vigorous activities, demonstrate that supplementation with propolis might have beneficial effects on oxidative stress and inflammation status following intense physical activities while not affecting athletic performance in healthy active subjects (71).

Mammals have two types of adipose tissue with distinct physiological functions, namely white adipose tissue (WAT) and brown adipose tissue (BAT): WAT stores excess energy in the form of ATP, while BAT is characterized by the expression of thermogenic uncoupling proteins 1 (UCP1) promotes calorie production to consume excess energy, thereby inhibiting weight gain and metabolic disease (81). Recent studies have shown that brown-like adipocytes, also known as beige or brite cells, are an inducible form of brown adipocytes present in white adipose tissue that share many biochemical and morphological features with brown adipocytes (82, 83). Since these cells are innately capable of releasing excess energy, a new strategy to induce brown/beige adipocytes in WAT may be one of the most feasible approaches to prevent and treat obesity and related diseases (84, 85). This has also brought attention to dietary factors or other diet-derived factors that may contribute to the induction of brown/beige adipocyte proliferation. Recent studies have found that Brazilian propolis-derived such as Art-C, can achieve anti-obesity effects by inducing browning and thermogenesis of white adipocytes (Figure 6).

Through in vivo and in vitro experiments, Nishikawa et al. investigated the effect and mechanism of Art-C induced browning of white adipocytes (43). In vitro experiments, Art-C (1–10 μM) significantly increased the mRNA level of brown adipocyte markers and the protein levels of UCP1 and PRDM16 in a dose-dependent manner (43). Further studies found that this significant induction was achieved by activating PPARγ and promoting the stabilization of PRDM16 protein (43). Similarly, in animal experiments, oral administration of Art-C (5 or 10 mg/kg for 4 weeks) can significantly induce brown like adipocytes in mouse inguinal adipose tissue, accompanied by significant expression of UCP1 and PRDM16 proteins, which further verifies the results of in vitro experiments (43). Recently, the same group also found that co-administration of Art-C (5 mg/kg) and curcumin (1.5 mg/kg) synergistically promoted the induction of brown adipocytes in inguinal adipose tissue, and believed that this synergistic effect was associated with norepinephrine produced by murine macrophages (86), but the specific molecular mechanism needs to be further explored. In addition, recent research of Nishikawa team also proposed that Art-C induced thermogenesis is associated with the thermogenesis pathway related to creatine metabolism (39). The research found that Art-C (10 mg/kg, 28 days) significantly induces the thermogenesis of beige adipocytes in inguinal adipose tissue. However, this induction effect is blocked by creatine metabolism inhibitors, indicating that Art-C may achieve energy consumption by significantly up regulating the expression of enzymes related to creatine metabolism in thermogenesis pathway (39). However, the relative contributions of UCP1-dependent and Cr-metabolism-related pathways to the observed Art-C-induced thermogenesis remain unclear. Further studies and mechanism investigations are needed to clarify the intrinsic relationship between Art-C and the two pathways, and UCP1-KO and WAT-specific GATM KO mice may be a powerful tool for subsequent studies (87).

Gut microbiota are considered to be a metabolic “organ” involved in regulating energy balance, sugar and lipid metabolism (88, 89), and there is increasing evidence that microbiota regulation is associated with obesity (90). Polyphenolic compounds have been shown to have a very low absorption rate in the front end of the gut, only 5%–10%, and the rest of the unabsorbed polyphenols (90%–95%) reach the colon in high concentrations, where they are broken down by the gut microbiota and degraded into smaller phenolic compounds, which further exert a series of biological effects (91, 92). Therefore, polyphenolic compounds may exert biological effects such as regulating sugar and lipid metabolism, antioxidant and anti-inflammatory by regulating the composition and metabolism of gut microbiota (92–94). So, as the most abundant component in propolis, do propolis polyphenols exert anti-obesity effects by regulating intestinal microbes?

A few studies suggest that the anti-obesity effects of polyphenols in propolis may be related to changes in the gut microbiome (Table 1). Roquetto et al. investigated the effects of green propolis on gut microbiota composition and anti-inflammatory effects in mice fed a high-fat diet, and found that 0.2% crude propolis repaired high-fat diet-induced gut microbial disturbances and reduced circulating LPS levels and inflammatory response (72). Similarly, another study suggested that propolis may mediate anti-obesity effects by modulating gut microbiota composition and function (73). The study found that dietary supplementation with 1% or 2% ethanol extract of propolis reduced high-fat diet-induced weight gain and hepatic fat accumulation. Besides, it improved glucose tolerance and lipid profile, accompanied by an increase in anti-obesity and anti-inflammatory bacteria, such as Intestinimonas genera and Parabacteroides distasonis species and a decrease in pro-inflammatory bacteria, such as Faecalibaculum genera and Bacteroides vulgatus species (73). Recent study using Db/Db mice reported propolis can improve sarcopenic obesity by regulating lipid metabolism disorder and inflammation, regulate intestinal microecology, and increase the abundance of intestinal microbiota related to pentose phosphatase pathway and glycerol metabolism (75). Furthermore, a recent study first reported the therapeutic effect of Chinese propolis (CP) on obesity, and the results showed that dietary CP supplementation significantly improved obesity-related physiological indicators such as weight gain, insulin resistance, hepatic steatosis and triglyceride accumulation (62). Interestingly, the effect of CP on the microbiome structure and metabolism of mice varied by gender (62). It was found that, regardless of gender, CP significantly reduced the abundance of Alistipes, which is the main producer of LPS in mice, but only increased Lactobacillus and the level of propionic acid in male mice (62). In addition to the structure of intestinal microorganisms, the change of its metabolite short chain fatty acids (SCFA) is also very important for obesity (95). SCFA is absorbed in the intestine and plays a regulatory role in intestinal physiology, metabolism and immunity as a regulator of energy intake and inflammation (96). A recent study determined the effects of poplar propolis polyphenol mixtures on the composition and function of gut microbiota obtained from the stools of five different donors, including obese children (74). The results showed that propolis could significantly increase total gut microbial SCFA production in obese children, but did not result in a significant increase in propionic acid concentrations (74). Previous studies have shown that higher concentrations of propionic acid are associated with higher android-to-gynoid fat ratio, which is a risk factor for children’s metabolism and cardiovascular disease (97). Propolis polyphenol does not lead to propionic acid concentration, suggesting that propolis may play a protective role in obese subjects, or at least not worsen the situation (74). However, further studies are needed to understand how propolis polyphenols exert their anti-obesity effects by regulating the structure and metabolism of gut microbiota. A recent study has elucidated the therapeutic effect of CAPE on nonalcoholic fatty liver disease (NAFLD) through the regulation of gut microbes and its potential mechanism (76). It was found that oral administration CAPE inhibited bile salt hydrolase (BSH) activity by reducing the abundance of BSH-producing bacteria, such as Parabacteroides (76). The inhibition of CAPE in BSH lead to the increase of T-β-MCA, which inhibits intestinal FXR signal, reduces ceramide synthesis and promotes GLP-1 secretion. This pathway has been further verified in intestinal FXR-deficient mice (76). Thus, CAPE improves NAFLD by inhibiting bacterial BSH activity, altering bile acid composition and modulating the intestinal FXR signaling-ceramide axis. Nevertheless, although this study reveals the critical role of the gut microbiota during CAPE treatment, details about how CAPE specifically inhibits bacterial BSH activity and selectively modulates intestinal FXR signaling remain unknown. In addition, although intestinal FXR deficiency is associated with elevated serum GLP-1 levels, earlier studies have reported that TGR5 directly promotes GLP-1 secretion (98, 99), while FXR indirectly regulates GLP-1 secretion through mechanisms that rely on downstream TGR5 signaling. Therefore, more in-depth studies are needed to explore the mechanism of TGR5 in CAPE-mediated upregulation of Gcg and GLP-1.