Caffeine: a potential mechanism for anti-obesity

Abstract

Obesity refers to the excessive accumulation of fat caused by a long-term imbalance between energy intake (EI) and energy expenditure (EE). Over recent years, obesity has become a major public health challenge. Caffeine is a natural product that has been demonstrated to exert anti-obesity effects; however, the mechanisms responsible for the effect of caffeine on weight loss have yet to be fully elucidated. Most obesity-related deaths are due to cardiovascular disease. Recent research has demonstrated that caffeine can reduce the risk of death from cardiovascular disease; thus, it can be hypothesized that caffeine may represent a new therapeutic agent for weight loss. In this review, we synthesize data arising from clinical and animal studies over the last decade and discuss the potential mechanisms by which caffeine may induce weight loss, focusing particularly on increasing energy consumption, suppressing appetite, altering lipid metabolism, and influencing the gut microbiota. Finally, we summarize the major challenges associated with caffeine and anti-obesity research and highlight possible directions for future research and development.

Introduction

Obesity is a serious health problem that is strongly associated with an increased overall risk of death and is defined as a body mass index greater than or equal to 30 [1]. The World Health Organization defines obesity as the excessive accumulation of body fat that may be hazardous to health; the main cause of this condition is an imbalance between the body’s energy intake (EI) and energy expenditure (EE) [2]. Excessive body weight and obesity are becoming increasingly prevalent in some countries and showing a clear trend towards a global epidemic [3, 4]; consequently, obesity represents one of the most significant medical problems of the twenty-first century. Given the excessive mortality, high morbidity, and consequential economic burdens associated with obesity, this disease represents a major concern to the medical community. There are many factors that contribute to obesity, including genetics, poor dietary habits, and the lack of adequate physical activity [5, 6]. Obesity can lead to potentially life-threatening comorbidities. Excessive body weight and obesity increase the risk of an individual developing a variety of diseases, including hypertension, type 2 diabetes (T2D), cardiovascular disease (CVD), metabolic syndrome, degenerative joint disease, and certain cancers [7], thus complicating the assessment of drug safety. Obesity and obesity-related diseases cause a reduction in an individual’s quality-of-life and life expectancy, and the incidence of these diseases has risen dramatically over the last few years, thus becoming a major public health problem that places a significant burden on global health and the world economy [8–11]. Addressing this multifaceted issue necessitates a comprehensive and interdisciplinary approach that emphasizes preventive strategies, lifestyle modifications, and innovative interventions to mitigate the pervasive impact of obesity on global health.

Current medical treatment for obesity includes medication and surgery. The main medications approved by the Food and Drug Administration (FDA) for the treatment of obesity are orlistat, lorcaserin, phentermine, and topiramate [12]. With regard to the specific mechanism of action, orlistat is a reversible pancreatic and gastric lipase inhibitor that reduces fat digestion and absorption [13], lorcaserin is a selective 5-hydroxytryptamine 2 C (5-HT2C) receptor agonist that acts on 5-HT2C receptors in the brain and reduces body weight by reducing appetite and increasing satiety, and phentermine reduces appetite by increasing the level of hypothalamic norepinephrine, whereas topiramate may reduce appetite by acting on GABA receptors [12]. Significant progress has been achieved over recent years in the use of glucagon-like peptide-1 (GLP-1) receptor agonists to treat obesity, including exenatide and semaglutide. These drugs promote weight loss by mimicking the effects of GLP-1, increasing satiety, and by reducing appetite. In particular, semaglutide is being evaluated as a weight loss drug in obese subjects without diabetes [14]. However, there are many side effects associated with the therapeutic options available for the treatment of obesity at present, including paresthesia, dizziness, dysgeusia, insomnia, nausea, constipation, dry mouth, allergic reactions, and liver problems [15–18]. Bariatric surgery is the most effective method for inducing weight loss and can result in a significant reduction in mortality from CVD [19] or cancer [20]. However, due to economic, technological, and individual differences, this type of surgery is unlikely to be able to meet global medical needs. Most obesity-related deaths are caused by CVD [21] while stroke is the second leading cause of global death; furthermore, statistics show that obesity increases the risk of stroke [22]. In addition, obesity leads to increased risk factors for CVD, including high blood pressure, high cholesterol, and diabetes, and can directly influence the health and functionality of blood vessels while increasing the risk of thrombosis [23], all of which increase the incidence of stroke. Therefore, improving cardiovascular health is the main goal of weight loss therapy. Unfortunately, most of the clinical drugs designed to reduce food intake ultimately failed in clinical trials due to insufficient cardiovascular safety [24]. Therefore, there is a growing need to identify and apply safe and effective natural alternatives to prevent or suppress obesity in order to reduce the impact and cost of the metabolic consequences of this disease [25]. This underscores the importance of advancing research and development in pursuit of holistic and globally applicable solutions to address the intricate challenges posed by obesity.

Previous research suggested that weight maintenance or loss may be associated with increased EE, thermogenesis, and the oxidation of fat [26–28]. Furthermore, researchers have previously demonstrated that caffeine may reduce body weight by maintaining or enhancing EE, improving exercise capacity, increasing fat oxidation, and stimulating the thermogenic response [29–33]. Caffeine has also been shown to suppress a variety of obesity-related abnormalities, including hypometabolism, dyslipidemia, systemic/tissue inflammation, and insulin resistance [34]. Interestingly, individuals who successfully maintain weight tend to consume higher volumes of coffee and caffeinated beverages [35]. Regular coffee consumption is associated with moderate weight loss [36] and caffeine has been shown to exert effect on energy balance by increasing EE and decreasing EI [37]. Given the side effects of the drugs that are currently on the market that can be used to treat obesity, efficacy and safety are two major concerns for the development of new anti-obesity drugs. Some studies have shown that caffeine, a natural component of coffee, tea, guaraná, and mate, can all exert therapeutic and anti-obesity effects but without adverse effects [38–40]. While 400 mg/day of caffeine is considered safe for the average person [41], previous research demonstrated that the long-term consumption of 400–600 mg/day of caffeine can prevent CVD and significantly reduce the risk of cardiovascular mortality [42–44]. It is evident that caffeine represents a potential new target for the clinical management of body weight; however, there is a significant lack of knowledge relating to the specific mechanisms involved in the effects of caffeine on body weight [45]. In this paper, we review the literature relating to the anti-obesity effects of caffeine and provide an overview of the basic mechanisms involved, discuss the advantages of caffeine with regard to anti-obesity effects, highlight the challenges that still need to be addressed, and discuss the potential direction of future research.

Caffeine

Caffeine is an alkaloid extracted from tea and coffee and represents the most common psychoactive substance in the world [46]. Tea and coffee beans are also the main sources of dietary caffeine globally and the highest consumption rates of caffeine are associated with beverages, including coffee (71%), soft drinks (16%), and tea (12%) [47]. Caffeine is a methylxanthine substance that stimulates the central nervous system, promotes metabolism, and enhances blood circulation [48, 49]. The half-life of caffeine in adults is usually 4 to 6 h, although this varies from person to person [50]. Caffeine can exert a range of physiological effects, including the improvement of endurance, physical performance, cognition, resting EE, and the improvement of mood [48]. Furthermore, previous research has investigated the potential therapeutic effects of caffeine on various health issues, including neurodegenerative diseases [51–54], diabetes [55], kidney stones [56], aging [57], and obesity [34, 58], while improving exercise and relieving fatigue [33]. Four basic mechanisms have been associated with the pharmacological effects of caffeine. The pharmacological actions of caffeine involve multiple targets, including adenosine receptors, phosphodiesterase, intracellular calcium, and γ-aminobutyric acid (GABA) receptors [59–62]. However, the physiological impacts of caffeine are primarily attributed to its antagonistic action on adenosine receptors. Previous research showed that caffeine can block the activation of sterol regulatory element-binding protein 2 (SREBP2), thus inhibiting the expression of proprotein convertase subtilisin/kexin type 9 (PCSK9) to effectively reduce the levels of low-density lipoprotein cholesterol (LDLc) in the blood [61]. This research highlighted a potential mechanism for the cardiovascular protective effects of caffeine. Although a large number of studies have been conducted on the efficacy of caffeine, these have predominantly focused on caffeine refreshing, anti-depressant, and the treatment of neurodegenerative diseases. Consequently, we know relatively little about the specific mechanisms by which caffeine can promote weight loss. It is vital that we gain such knowledge if we are to develop caffeine as a potential clinical therapeutic.

Mechanisms

Increasing energy consumption in the body

Mendelian randomization studies have indicated that higher plasma levels of caffeine may reduce the risk of obesity and the development of T2D. The increased EE induced by caffeine intake may be modulated by increasing thermogenesis in the brown fat. Further clinical studies are now necessary to investigate the translational potential of caffeine with regard to reducing the burden of metabolic disease [63].

Caffeine is a xanthine alkaloid that is found in coffee, tea, soft drinks, and chocolate. The main mechanism of action involves the antagonism of adenosine receptors [64, 65]. Adenosine receptors are members of the G protein-coupled receptor family, which includes four isoforms in mammals: A1, A_2A, A_2B, and A3. These isoforms are widely expressed and play a wide range of physiological and pathobiological functions [65–67]. Of these, the A1 receptor (A1R) is expressed at higher levels in the brain, A_2AR is predominantly expressed in the striatum at high levels, and A_2BR is widely expressed, but has a lower overall abundance. A3R is expressed at lower levels and exhibits only low affinity for caffeine [68]. Caffeine antagonizes A1R, A_2AR, and A_2BR in a non-selective manner [69]. In vitro, A2BR has been shown to activate only at high pathological concentrations of adenosine. Therefore, it is generally accepted that A1R and A_2AR are the main targets of caffeine action in the central nervous system (CNS) [66]. While A1R is known to play an important role in analgesia [70] and sleep disorders [71], attempts to develop adenosine/A1R as a potential analgesic have failed due to lack of sufficient on-target selectivity and off-tissue adverse reactions [72]. A_2AR plays an important role in cancer [73], Parkinson’s disease [74], Alzheimer’s disease [75], sleep regulation [76], and depression [77, 78]. Moreover, animal studies have demonstrated that adenosine receptors, especially A_2AR, represent the main pharmacological targets of caffeine [68, 79], and that caffeine can block approximately 50% of A_2AR sin the coronal striatum [74]. Istradefylline, a selective A_2AR antagonist, is used as an adjunct to levodopa to alleviate episodes of Parkinson’s disease and first received FDA approval in 2019 [80, 81]. Collectively, these lines of evidence suggest that adenosine receptors are potential drug targets for several conditions, and a number of clinical development studies are already underway.

Previous research has also demonstrated that caffeine can exert thermic effects; for example, the intake of caffeinated coffee has been shown to increase EE [82]. Since brown adipose tissue (BAT) is dedicated to EE, increasing the activity of BAT represents a potential target for the development of anti-obesity strategies. A_2AR signaling has been shown to activate BAT and induce the browning of white adipose tissue, leading to increased thermogenesis, thereby protecting experimental mice from diet-induced obesity [83]. A_2ARs are the most abundant form of adenosine receptor in the BAT of humans and mice. In addition, A_2ARs are known to be associated with the activation of human BAT; therefore, targeting A_2ARs in BAT provides a potential pathway to enhance the functionality of BAT and improve human obesity [84]. Experiments have also shown that caffeine can promote the functionality of BAT at doses that are compatible with human consumption and mobilize the brown form of fat that is actively involved in metabolism; consequently, caffeine can exert influence on metabolism in humans [85]. Both the central and peripheral administration of caffeine has been shown to inhibit appetite and increase thermogenesis in BAT and EE in dietary obese (DIO) mice. These effects have been shown to be related to the antagonistic effects of caffeine on A1Rs in the hypothalamus, which suppressed appetite and promoted EE, thereby reducing diet-induced obesity [58]. Caffeine-induced BAT thermogenesis in rodents is a consequence of central activation. However, whether caffeine-induced BAT thermogenesis in humans occurs as a result of systemic or central activation remains unclear; it is possible that caffeine may influence human BAT by acting on metabolic neural pathways [86]. It is considered that this process involves the activation of orexinergic neurons in the perifornical lateral hypothalamus and lateral hypothalamus, and the broader antagonism of the adenosine receptors [86]. The activation of lateral hypothalamic neurons has been shown to significantly increase sympathetic activity in the BAT of rodents. Furthermore, neurons in the lateral hypothalamic region can influence sympathetic activity and thermogenesis in the BAT, and heart rate via pathways that are dependent of the activation of the dorsal hypothalamus and pallidocyte neurons [87]. The neural pathways controlling the regulation of thermogenesis in the BAT have been studied and described previously [88].

Collectively, these findings indicate that adenosine increases thermogenesis in BAT and that A_2ARs antagonists can block these effects. A1Rs appear to exert opposite effects to A_2ARs and A_2BRs. Previous research demonstrated that a selective A_2ARs antagonist, KD-64-A, exerts anti-inflammatory activity but does not reduce diet-induced obesity in mice when compared to caffeine, a non-selective antagonist [89]. Thus, selective A_2ARs antagonists are very different from non-selective adenosine receptor antagonists in terms of weight loss. The precise mechanisms responsible for these differences have yet to be elucidated. Many studies have demonstrated that caffeine can play a significant role in weight loss [46–48, 58]; consequently, the studies described in this section may provide evidence that A1Rs are a preferred target for caffeine, an adenosine receptor antagonist, and play a key role in its anti-obesity effects [90]. In addition, caffeine is a non-specific antagonist of adenosine receptors, and may also exert effects on a range of other peripheral mechanisms. Therefore, it is necessary to investigate the effects of caffeine combined with selective adenosine A_2A or A_2B agonists on thermogenic anti-obesity. Such combination therapies may increase the efficacy of caffeine [86].

The activation of BAT thermogenesis is regulated by the sympathetic nervous system via β-adrenergic receptors (β-ARs) in rodents and is thought to occur in a similar manner in humans. The pathway by which caffeine stimulates a thermogenic response may be similar to the pathway by which an organism reacts to cold stimuli. Caffeine can increase excitability in the sympathetic nervous system (SNS) to exert effects on the regulation of energy balance and lipolysis [37, 48]. The effect of low-dose caffeine on tissue thermogenesis has been shown to be entirely dependent on intact sympathetic innervation [89]. Norepinephrine (NE) is a major regulator of the SNS and can increase the utilization of adenosine triphosphate (ATP) via ion pumps and substrate cycling, and increase the oxidation rate of mitochondria in a manner that is only weakly coupled with ATP synthesis, thus resulting in an increase in thermogenesis at the organismal level. The BAT is responsible for consuming energy to generate heat via the mitochondrial uncoupling protein 1 (UCP1) to resist obesity [91, 92]. UCP1, a thermogenic protein located on the inner mitochondrial membrane, orchestrates mitochondrial respiratory chain uncoupling, ultimately increasing metabolism and heat production [93]. NE is also known to activate the β-ARs on BAT to induce lipolysis [94]. The β3-adrenergic receptors (β3-ARs) are the major regulators of thermogenesis in the BAT of rodents [95]. Furthermore, NE interacts with β3-ARs to promote thermogenesis [96]; the activation of β3-ARs promotes the increased expression of cyclic adenosine monophosphate (cAMP) which activates protein kinase A (PKA), stimulates lipolysis, releases free fatty acids, and induces the expression of UCP1 [91, 93, 94, 97, 98].

Research has also shown that the β-ARs can also activate mitogen-activated protein kinase (MAPK) in adipocytes. The activation of p38 mitogen-activated protein kinase (p38 MAPK) is indirectly dependent on PKA. p38 MAPK directly targets peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α) and activating transcription factor-2 (ATF-2) to induce expression of the UCP1 gene and enhance mitochondrial oxidative activity [99]. PGC-1α is a transcriptional coactivator involved in the control of energy metabolism and is expressed at high levels in BAT. The expression levels of the PGC-1α gene are up-regulated by cAMP-mediated pathways in BAT [100]; this process is crucial for the cAMP-dependent activation of BAT thermogenesis and plays a key role in inducing UCP1 gene expression and enhancing overall mitochondrial oxidative activity [101]. Therefore, increasing the levels of PGC-1α may represent a reasonable strategy with which to treat obesity. Caffeine induces PGC-1α expression and mitochondrial biosynthesis [102, 103], thus, increasing the levels of cellular cAMP may represent the mechanism by which caffeine increases UCP1 activity and thus thermogenesis. BAT generates heat rapidly and metabolizes glucose and lipids by activating UCP1. Therefore, caffeine may induce fat thermogenesis via the PGC-1α-UCP1 signaling pathway, thus increasing EE and ultimately exerting anti-obesity effects.

The suppression of appetite

Caffeine can reduce food intake by stimulating the CNS and by suppressing appetite. Previous studies have established that peripheral and central injections of adenosine can inhibit food intake in rats, and that mechanisms by which the peripheral and central nervous systems inhibit food intake are different [104]. One study found that the increased levels of adenosine in the cerebrospinal fluid and the overexpression of A1Rs in neurons of the hypothalamus paraventricular nucleus (PVN) in experimental mice were related to the development of obesity. Previously, researchers investigated the mechanism of action of caffeine by intracerebroventricular injection and by feeding caffeine, and found that both the central and peripheral administration of caffeine inhibited appetite and increased thermogenesis and EE in the BAT of DIO mice, thus reducing body weight. These studies in DIO mice showed that the anti-obesity effect of caffeine was predominantly mediated by antagonizing the expression of A1Rs in PVN neurons, inhibiting the expression of A1Rs in PVN oxytocin (Oxt) neurons, and negatively regulating the energy balance. A1Rs are expressed in adipose tissue and their activation can inhibit lipolysis. Thus, peripherally administered caffeine may promote lipolysis primarily by directly antagonizing the A1Rs. The central administration of caffeine has been shown to negatively regulate energy balance by facilitating the release of Oxt. Research has shown that the caffeine-A1R signaling system utilizes Oxt to suppress appetite and reduce weight, thus suggesting that Oxt acts as a key mediator of the caffeine-induced negative regulation of energy balance in DIO mice. Targeting the A1Rs in the PVN via caffeine or its derivatives may represent a relevant strategy with which to combat obesity and related comorbidities [58]. The downregulation of striatal dopamine D2 receptors (D2Rs) is required in addiction-like reward dysfunction and compulsive eating in obese rats [105], and A_2ARs expressed in the striatum may be involved in the pathogenesis of DIO. The antagonistic effect of caffeine on A_2ARs may increase the activity of D2Rs, thereby attenuating food addiction in DIO animals [58]. However, studies have found that A_2ARs play an important role in binge eating behavior, and their activation may help reduce food intake [106, 107], thus suggesting that A_2ARs may have a complex and dual role in regulating food intake behavior. Therefore, there is a clear need for in-depth research to investigate the mechanism of its action in the nervous system.

Studies have demonstrated that the intake of coffee can reduce body fat, increase the plasma levels of serotonin, reduce the plasma levels of ghrelin, and protect DNA integrity [108]. Increased serotonin secretion by intestinal neuronal cells in response to luminal stimulation has been reported to reduce food intake [109], whereas increased ghrelin secretion by gastric cells has been shown to stimulate appetite and food intake in both rodents and humans [110]. Thus, the intake of coffee can reduce food intake. Caffeine can increase the excitability of the SNS [48], suppress hunger, and enhance satiety [111]. Corticotropin-releasing factor (CRF) mediates the anorexic effects of caffeine in a manner which does not necessarily involve the sympathetic-adrenal system [112]. Therefore, caffeine may cause the effect of CRF on food intake to be independent of the sympathetic-adrenal system. One study has revealed that the blockade of A_2ARs requires the activation of CRF₂R [113]. This interaction between adenosine and CRF receptors could explain the possible mechanism of action of how caffeine could be used to combat obesity. Furthermore, the effects of caffeine on food intake are known to be mediated by multiple pathways. Research has shown that the stimulation of astrocytes can reduce ghrelin-induced food intake via the inactivation of A1Rs-mediated agouti-related peptide neurons [114]. Therefore, caffeine and astrocytes may co-regulate appetite through different but related pathways, although the specific interactions and mechanisms of action still need to be determined by further in-depth investigation.

Leptin is a hormone secreted by adipose tissue that has been reported to suppress appetite, and regulate both body weight and energy balance [115]. In obesity, elevated blood levels of leptin result in leptin resistance, which may be one of the main factors underlying obesity [116]. Several studies have investigated the impact of caffeine intake on leptin levels. Research indicates that caffeine may alleviate endoplasmic reticulum stress and improve leptin resistance in neurons [117]. In addition, an experimental study demonstrated that a combination of choline, carnitine, and caffeine reduced the serum levels of leptin in a rat model [118]. Although coffee consumption has been associated with lower leptin levels [119], observational studies have failed to establish a significant association between leptin and coffee intake [120]. Overall, caffeine has the potential to modulate the effects of leptin by influencing the activities of the nervous system and metabolic pathways that regulate appetite and energy metabolism. Nevertheless, further research is still needed to fully comprehend the relationship between caffeine and leptin, which may be influenced by individual variances and environmental factors.

Previous research demonstrated that the typical consumption of caffeinated coffee has no short-term effects on appetite, EI, glucose metabolism, or inflammatory markers, thus requiring long-term intake [121]. It has also been shown that moderate coffee intake can effectively reduce EI at the next meal and throughout the day, but has no significant effect on appetite [122]. Drinking decaffeinated coffee can significantly reduce hunger; however, caffeine does not produce this effect alone [123]. Furthermore, while many studies have shown that caffeine can suppress appetite and reduce weight, some other studies have demonstrated that caffeine can increase appetite [124]. Consequently, there is clear ambiguity in the literature relating to the effects of caffeine on appetite. Therefore, further research is needed to identify the precise mechanisms involved, as well as to further consider the potential of incorporating caffeine-containing substances into weight loss diets as a dietary behavior.

The alteration of lipid metabolism in the body

Previous studies have reported that caffeine can inhibit adipogenesis [125] and the differentiation of adipocytes [126], inhibit the intracellular accumulation of lipid [127, 128], and promote lipolysis, fat oxidation [129], and fat thermogenesis [130].

Caffeine may affect weight loss by inhibiting adipogenesis and adipocyte differentiation, thereby inhibiting lipid deposition. Obesity is a disease characterized by the excessive accumulation of triglycerides (TG). Adipose tissue is predominantly composed of adipocytes and 3T3-L1 adipocytes are widely considered as a classical model of fat metabolism [131]. The inhibition of TG accumulation in 3T3-L1 cells may be crucial for the clinical treatment of obesity. Caffeine is an anti-adipogenic and bioactive compound that plays a role in the regulation of mitotic clonal expansion during adipocyte differentiation via the AKT/GSK3 pathway, thereby inhibiting the expression of two major adipogenic transcription factors: CCAAT/enhancer-binding protein (C/EBPα) and peroxisome proliferator-activated receptor γ (PPARγ) [125]. However, it is worth noting that the concentrations of caffeine used in these particular experiments were well outside the physiological range. Therefore, it is important to investigate the effects of caffeine in vivo with full consideration of both dose and exposure time. Similarly, a treatment combining catechins and caffeine was shown to inhibit the mRNA expression of transcription factors and adipogenesis-related enzymes in 3T3-L1 cells and increased the protein expression of lipolysis-related enzymes in the presence of NE, thereby inhibiting fat deposition [132]. In addition, the combined action of chlorogenic acid (CGA) and caffeine was shown to retard adipogenesis by modulating lipid metabolism and by inhibiting the differentiation of adipocytes in 3T3-L1 cells. In summary, the combination of CGA and caffeine inhibited the middle to late stages of differentiation in 3T3-L1 cells and attenuated adipogenesis by regulating the expression of adipose metabolism-associated enzymes such as adipose triglyceride lipase and hormone-sensitive lipase (HSL) in 3T3-L1 cells, thereby reducing the accumulation of adipose via the AMP-activated protein kinase (AMPK) pathway [133].

Previous research involving high-fat diet (HFD)–induced mice showed that caffeine reduced serum levels of lipid, promoted lipolysis, and inhibited the deposition of fat [127]. AMPK is an important therapeutic target for the treatment of obesity, and in mature adipocytes, AMPK was shown to exert significant anti-lipolytic effects [134]. SREBP-1c is a major transcriptional regulator of lipogenic enzymes and can be inhibited by AMPK via the phosphorylation of serine 365 [135]. Caffeine has also been reported to reduce the mRNA levels of genes related to adipogenesis, such as SREBP-1c and FAS by phosphorylating AMPK in human hepatocellular carcinoma HepG2 cells [136]. The mRNA expression levels of AMPK, CAT, and ACO in the combined CGA and caffeine group were significantly up-regulated [137], thus suggesting that the increased expression of AMPK may promote the expression of ACO and CAT, the major contributors to β-oxidation, while the expression level of PPARg2 mRNA was down-regulated. These researchers demonstrated that an 8% dose of caffeine reduced the accumulation of lipid in the livers of zebrafish larvae by upregulating the expression of the lipid β-oxidation gene ACO and by down-regulating the lipogenesis-related genes SREBP1, acetyl-CoA carboxylase 1 (ACC1), and fatty acid translocase [138]. Caffeine inhibits lipogenesis and stimulates lipolysis by modulating the AMPK-SREBP signaling pathway, effectively depleting triglyceride and cholesterol levels.

Caffeine may reduce weight by promoting lipolysis and fat oxidation. Research indicates that caffeine promotes lipolysis [139]. Furthermore, studies comparing the effects of exercise and caffeine intake on serum fatty acid profiles have demonstrated that the combination of exercise and caffeine can lead to a heightened lipolytic response [140]. A1Rs and A_2ARs are expressed abundantly in adipose tissue and participate in lipolysis [141]. A1Rs are expressed in adipose tissue and their activation inhibits lipolysis. Thus, the peripheral administration of caffeine may promote lipolysis by directly inhibiting the expression of A1Rs in adipocytes while the central administration of caffeine may negatively regulate energy balance by promoting the release of Oxt [58]. The antagonistic effect of caffeine on A1Rs leads to the activation of adenylate cyclase (AC), followed by the increased activity of cAMP and PKA [142]. The caffeine-A1R signaling pathway may contribute to weight loss by promoting lipolysis. Extracellular adenosine is known to increase the intracellular levels of cAMP by activating AC to bind to the A_2ARs and A_2BRs [65, 143], while an increase of intracellular cAMP can promote lipolysis. Agonists of A_2ARs and A_2BRs can increase lipolysis and BAT thermogenesis, and in brown adipocytes, the expression of A_2ARs has been shown to increase with increased levels of NE or intracellular cAMP [83].

Obesity is associated with the overgrowth and expansion of adipose tissue mass; thus, the number of adipocytes is a major determinant of fat mass in adults [144]. White adipose tissue (WAT) is known to be innervated by the SNS and needs to be activated for lipolysis. PKA can activate HSL in the adipocytes of WAT, promote the hydrolysis of stored triglycerides into free fatty acids, and play a key role in lipolysis [145]. This is the main pathway by which fat cells release stored energy. In long-term studies of rodents, the intake of caffeine was shown to reduce the size of fat pads and the number of fat cells [146], and has also been shown to enhance anti-inflammatory responses in human exercise trials [147]. In addition, caffeine increases the excitability of the SNS [48], and SNS-NE-mediated WAT lipolysis is completely dependent on the stimulation of β-ARs, ultimately depending on HSL and the phosphorylation of perilipin A [148, 149]; therefore, the intake of caffeine may promote the catabolism of white fat via the SNS-NE pathway.

Previous research showed that consumers of high amounts of caffeine lost more weight, fat mass, and waist circumference than those who consumed low amounts of caffeine; this was associated with relatively large heat production and fat oxidation [150]. Additionally, a randomized controlled trial revealed that caffeine consumption alone increased fat oxidation compared to a placebo [151]. In addition, caffeine may enhance athletic performance by promoting the oxidation of fat. The combination of moderate caffeine intake before exercise and moderate intensity exercise in the afternoon was shown to significantly increase the so-called maximal rate of fat oxidation, which appears to be optimal for individuals seeking to increase fat utilization during continuous aerobic exercise [152, 153]. Therefore, the combination of moderate caffeine consumption and exercise may be more conducive to weight loss.

In addition to acting as an antagonist of the adenosine receptor, caffeine also acts as a non-selective phosphodiesterase (PDE) inhibitor [60]. Caffeine is metabolized in the liver and the most abundant primary metabolite is paraxanthine. Methylxanthines (theophylline and/or caffeine) have been reported to act as thermogenic and lipolytic stimulators, inducing fat oxidation in adipocytes and the release of glycerol and fatty acids into the blood [130]. Methylxanthines increase fat oxidation by elevating the serum levels of catecholamine and prolonging the half-life of cAMP. Caffeine regulates cAMP levels by inhibiting PDE [154], thereby inhibiting the breakdown of cAMP in adipocytes to reduce the threshold of lipid solubility [155]. Pilot human intervention studies have clearly demonstrated that normal levels of daily coffee consumption have a significant effect on PDE activity in the platelets [156]. Caffeine acts by inhibiting PDE, leading to a subsequent increase in intracellular cAMP concentration [157]; this is responsible for the catabolism of cAMP and can rapidly hydrolyze intracellular cAMP into adenosine monophosphate (AMP). Increases in cAMP usually promote NE secretion [158]. Caffeine inhibits the ability of PDE to induce the degradation of cAMP, thus maintaining NE at a certain level [154] can stimulate lipolysis and induce the expression of UCP1. Caffeine promotes lipid metabolism through multiple mechanisms. It activates AMPK to attenuate lipogenesis and enhance lipolysis, while also inhibiting the differentiation and proliferation of preadipocytes to reduce lipid accumulation [159].

Additionally, caffeine may contribute to weight reduction by promoting fat thermogenesis. A previous study investigated the impact of caffeine on the thermogenesis of BAT in vivo, assessing thermographic temperature in the supraclavicular region. The results revealed an increase in BAT region temperature following caffeine ingestion, indicating enhanced BAT activity after the intake of a relatively low dose of caffeine [85]. These findings suggest that the rise in metabolic rate subsequent to caffeine consumption may be attributed to the augmentation of BAT function preceding changes in skin temperature [160].

Both beige and brown adipocytes have the ability to burn glucose and fat to produce heat [161]. Beige adipocytes are unique thermogenic adipocytes with extremely low basal expression levels of UCP1 [92]. UCP1 plays a central role in the non-shivering thermogenesis of brown fat but is dispensable for thermogenesis in beige adipocytes in vivo [162]. Research has shown that mechanisms exist in organisms that do not rely on UCP1 for thermogenesis [163]. The extracellular clearance of calcium has been shown to have no effect on the EE of NE-stimulated beige fat, while the intracellular clearance of calcium significantly inhibited the EE of NE-stimulated beige fat. Collectively, these results suggested that intracellular calcium is essential for the non-UCP1-dependent thermogenesis of beige fat, and also confirmed the involvement of sarcoplasmic/endoplasmic reticulum calcium ATPase (SERCA) in regulating intracellular calcium cycling [162]. The sarcoplasmic/endoplasmic reticulum calcium ATPase (SERCA2) calcium pump is one of several Ca²⁺-ATPase enzymes in the sarcoplasmic reticulum (SR) and endoplasmic reticulum (ER) that are responsible for regulating the transmembrane transport of calcium [164]. There are two major isoforms: sarcoplasmic/endoplasmic reticulum calcium ATPase 2 A (SERCA2A), which is primarily localized in the SR of muscle cells, and sarcoplasmic/endoplasmic reticulum calcium ATPase 2B (SERCA2B), which is primarily localized in the ER of most cell types [165]. By experimenting with cAMP agonists, researchers found that cAMP can specifically regulate SERCA2B. The application of SERCA2B inhibitors, knockouts, and the overexpression of SERCA2B further showed that SERCA2B is essential for the thermogenesis of beige fat [162]. The SERCA-ryanodine receptor (RyR) null-cycling thermogenesis mechanism is known to occur in the ER of thermogenic fats. This promotes thermogenesis through the null-cycling process of Ca²⁺ in and out of the ER; in other words, SERCA2B transports Ca²⁺ into the ER while ryanodine receptor 2 (RyR2) releases Ca²⁺. This process is coupled with the hydrolysis of ATP by SERCA2B, which mediates thermogenesis. Research has suggested that non-UCP1-dependent thermogenesis in beige fat acts through muscle/ER calcium cycle 2+-SERCA2B and the RYR2. Increased levels of Ca²⁺ act by activating the α1-ARs and β3-ARs or SERCA2B-RyR2 pathways to circulate and stimulate UCP1-independent thermogenesis in beige adipocytes [162]. These results suggest that enhanced calcium cycling promotes non-UCP1-dependent thermogenesis in beige adipose tissue. Cytoplasmic Ca²⁺ signaling is usually amplified by the significant release of calcium from the ER. Research has demonstrated that coffee, and the components of coffee, mobilizes intracellular calcium, causing PC-12 cells to release dopamine, which increases the levels of Ca²⁺ in hepatic ER [61, 166]. Caffeine acts as an agonist for the ryanodine receptor and can trigger the RyR-sensitive calcium pool at basal cytoplasmic levels of Ca²⁺ to promote Ca²⁺ release [167], thus increasing the levels of intracellular Ca²⁺ while enhancing the sensitivity of muscle fibers to Ca²⁺ [168]. Therefore, caffeine may promote non-UCP1-dependent thermogenesis in beige fat via the Ca²⁺-SERCA2b-RyR pathway, thereby increasing EE. In addition, cytoplasmic calcium has been shown to directly stimulate AC activity during adrenergic stimulation in brown adipocytes, thereby increasing cAMP production and PKA activation to induce lipolysis and thermogenesis [169]. Second, Ca²⁺ is able to achieve transcriptional control of metabolism through its dependent kinases and phosphatases, such as Ca²⁺/calmodulin-dependent protein kinase (CAMK) and calcineurin [170], thus initiating the increased phosphorylation of p38 MAPK and ATF-2. The kinases and phosphatases acting within this pathway increase PGC-1α expression and mitochondrial biogenesis in muscle [171], thus increasing thermogenesis and exerting anti-obesity effects.

In addition, elevated levels of GABA in the brain may contribute to increased thermogenesis in rats [172]. Caffeine can significantly increase the levels of GABA [62], which is known to promote the production of UCP1 and PGC-1α, facilitate lipid metabolism, promote fatty acid oxidation, and inhibit hepatic triglyceride levels [173].

Effects on the gut microbiota

The gut microbiota is now recognized as one of the key factors regulating host health. Deviations in the gut microbiota have been associated with many diseases, including obesity, T2D, hepatic steatosis, inflammatory bowel disease, and several cancers [174]. Recent studies have also demonstrated that the gut microbiota can influence energy metabolism [175, 176]. Tea and coffee have been shown to alter the gastrointestinal microbiota in both animals and humans [45, 177]. In addition, caffeine may play a role in anti-obesity by regulating the gut microbiota.

Coffee is initially absorbed in the stomach and small intestine, but further fermented in the colon by the gut microbiota [177]. The proportions of Firmicutes and Bacteroidetes were found to be increased in the ileum of rats fed with high-calorie diets, while the proportions of Enterobacteriaceae or Clostridium decreased [45]. Furthermore, the consumption of coffee altered the gut microbiota of rats fed with a HFD, and attenuated the ratio of Firmicutes to Bacteroides [177]. The combination of epigallocatechin-3-gallate (EGCG) and caffeine in low doses led to alterations in the gut microbiota, reducing the levels of Firmicutes and increasing the levels of Bifidobacterium [178]. Similarly, the in vitro colonic metabolism of coffee and CGA led to selective changes in the growth of human fecal microbiota, thus inducing an increase in the proportion of Bifidobacterium [179]. Researchers have hypothesized that individuals who possess the relevant gut microbiota had lower levels of cholesterol in their blood [180]. Drinking coffee may alter the gastrointestinal tract and reduce the absorption of lipids and glucose [181, 182]. High-doses of caffeine were shown to reduce body weight and liver weight in mice suffering from obesity induced by a HFD, while improving hyperlipidemia and insulin resistance. It is possible that the underlying mechanism may be related to changes in the intestinal flora, along with the metabolism of bile acids, lipids, and energy [183]. The synthesis and excretion of bile acids (BA) represent major pathways for the metabolism of cholesterol and lipids. The combination of low doses of EGCG and caffeine produced synergistic anti-obesity effects that were comparable to those of high-doses of EGCG; analysis suggested that these synergistic effects may be attributed to modulation of the gut microbiota and the metabolism of BA [178].

Nuclear receptor peroxisome proliferator-activated receptor α (PPARα) and liver X receptors (LXRα and LXRβ) are major regulators of cholesterol homeostasis; the activation of these receptors leads to reduced intestinal cholesterol absorption [184]. Niemann-Pick C1-like 1 (NPC1L1) is a protein that plays a critical role in the absorption of intestinal cholesterol. Previous in vivo studies have shown that the activation of LXRα reduces the intestinal expression of NPC1L1, thus leading to reduced cholesterol absorption [184, 185]. The effective hypolipidemic effect of Coffea arabica pulp aqueous extract (CPE) on hyperlipidemia was clearly demonstrated in a rat model of hypercholesterolemia. CPE reduced the expression of intestinal NPC1L1 while activating LXRα and reducing the intestinal absorption of cholesterol. When considering the two main components of CPE, CGA, and caffeine, only the latter was able to exert an inhibitory effect on cholesterol uptake in Caco-2 cells, thus suggesting that caffeine may be involved in NPC1L1 alterations, at least in part [186].

Drinking coffee is known to change the composition of the gut microbiota [177] and can exert beneficial effects [187]; however, the specific effects of coffee have yet to be clarified [188]. The gut microbiota can directly or indirectly influence obesity via microbiome-gut-brain-liver interactions. In future, research needs to investigate the specific effects of caffeine from the perspective of microbiome-gut-brain-liver interactions.

Challenges remaining

Low doses of caffeine can induce behavioral stimulation and achieve weak benefits [69]; however, at higher doses (> 400–500 mg/day), caffeine may lead to anxiety, aversion, irritability, and malaise, although these effects appear to be subject to large individual differences [68] (Table 1). An important issue when investigating the effect of caffeine on weight loss is determining the effective and optimal dose of caffeine.

Table 1.

Dosage of caffeine in animal models or human clinical studies

Species	Dosage
Human	600 mg [26]
Human	100 mg, 200 mg, 400 mg [30]
Rats	20 mg [34]
Mice	80 mg [38]
Human	100 mg, 200 mg [52]
Mice	1.5 mg [53]
Mice	30 mg/kg [54]
Human	200 mg [55]
Mice	60 mg/kg [58]
Rats	50 mg/kg, 100 mg/kg [62]
Human	129.5 mg, 259 mg [74]
Human	100 mg, 200 mg, 400 mg [111]
Rats	50 mg/kg [112]
Rats	0.1 g/kg [118]
Human	3 mg/kg [121]
Human	3 mg/kg, 6 mg/kg [122]
Human	6 mg/kg [123]
Mice	6 mg/kg, 12 mg/kg, 24 mg/kg [124]
Human	8 mg/kg, 4 mg/kg [129]
Human	2.5 mg/kg, 4 mg/kg [139]
Human	5 mg/kg [140]
Human	6 mg/kg [147]
Human	25 mg [150]
Human	100 mg [151]
Human	3 mg/kg [152]
Mice	10 mg/kg, 20 mg/kg [159]
Rats	20 mg/kg [178]
Mice	0.3 g/kg, 0.6 g/kg [189]
Human	100–400mg [191]
Human	25 mg, 50 mg, 75 mg [192]
Human	77 mg [193]
Human	5 mg/kg [195]
Human	6 mg/kg [197]
Human	100 mg [198]
Human	100 mg, 200 mg, 300 mg, 400mg [199]

Astilbin and CGA were previously shown to enhance the anti-obesity effect of caffeine in an HFD-induced mouse model of obesity, thereby reducing the minimum effective dose of caffeine in vivo [189, 190]. Caffeine can amplify the anorectic effects of other substances, particularly when combined with soluble fiber, catechin, or glucosyl hesperidin (G-hesperidin), resulting in a heightened synergistic effect compared to their individual actions [191, 192]. Moreover, when caffeine is mixed with tea catechin, known as tea polyphenols, the oxidizing and fat-activating effect was even stronger than for caffeine alone [31, 193].

Caffeine may play a role in weight loss via the many mechanisms described herein. However, this does not mean that individuals need to drink a lot of coffee and other caffeinated beverages each day to lose weight. The dose of caffeine ingested by experimental mice in previous research was equivalent to the dose of caffeine that would be ingested by individuals who drink 24 to 36 cups of coffee per day; this is far higher than the dose that is generally considered to be safe [58]. The chronic overconsumption of caffeine can also lead to toxicity, as characterized by an increase in certain symptoms, such as anxiety or depression. Therefore, reducing the effective dose of caffeine is important for the long-term use of caffeine in the body. The anti-obesity effect of caffeine is not effective in the long-term and large differences can occur between individuals. However, chronic or excessive caffeine consumption can result in addiction, insomnia, migraines, and other side effects. Moreover, high intake of caffeine may adversely affect the growth and development of children and could lead to cognitive deficits in the offspring of pregnant women, thereby increasing their susceptibility to diseases in adulthood. Hence, it is advisable for special populations and individuals sensitive to caffeine to restrict or reduce their caffeine intake to mitigate potential negative consequences [194]. When using caffeine as an aid for weight loss, it should be taken in moderation and combined with a healthy diet and exercise habits. The combination of caffeine therapy with calorie shifting diets may also represent an effective alternative for the loss of both weight and fat [195]. Therefore, caffeine and its derivatives, or combinations, with other products may represent an effective candidate for the development of therapeutic strategies to combat obesity.

The effects of caffeine on exercise performance indicate that acute caffeine intake may exert a greater effect on anaerobic performance in males than in females [196]; thus, there are notable gender differences in terms of caffeine metabolism [197, 198]. The consumption of caffeine caused a greater increase in plasma caffeine concentration (mg/L) in females, and was reduced in stress myocardial blood flow and myocardial blood flow reserve assessments in males; in contrast, no changes were reported in females [199]. Therefore, gender difference is an important factor to be considered in future studies related to caffeine.

Regular coffee consumption is associated with moderate weight loss [36]. Furthermore, the caffeine content of coffee is known to enhance satiety [150]; there is also evidence that caffeine-free coffee can also reduce hunger [123]. Caffeine has not been shown to reduce body fat, and its effect on fat metabolism is generally considered to be negligible [200]. The effect of caffeine on appetite remains unclear; therefore, future research should investigate the specific mechanisms involved.

Conclusions

Obesity and obesity-related diseases pose a serious threat to human health and have become a major public health problem. Globally, obesity has become a major epidemic, placing an enormous burden on the world economy and global health. Both human and animal studies have shown that caffeine may play a role in weight loss by increasing energy consumption, suppressing appetite, altering lipid metabolism, and impacting the gut microbiota (Fig. 1). Most of the biological effects of caffeine are mediated via antagonistic effects on adenosine receptors, although its ability to enhance sympathetic stimulation of the metabolic rate and thermogenesis occurs mainly by inhibiting the activity of phosphodiesterase inhibitors. Although the specific mechanism of action remains largely unknown, its low cost and widespread availability make caffeine a potentially promising candidate for the development of a new weight management strategy. Considering our ability to regulate energy balance and the dosage of caffeine, it is now necessary to conduct further animal and clinical studies to identify the optimal intake dose of caffeine to achieve weight loss, as well as to investigate the method of intake, combination treatments, the duration of treatment, and gender differences. The results of such studies will be of great significance for the development of caffeine-containing functional foods and their applications in health care, as well as for the development of complementary medicines.

Fig. 1.

Possible mechanisms of the anti-obesity of caffeine. A This picture depicts the possible anti-obesity effects of caffeine through pathways that increase body energy expenditure, suppress appetite, alter body lipid metabolism, and influence the gut microbiota. Caffeine acts as an adenosine receptor blocker, and its antagonistic effect on A1Rs leads to activation of AC, followed by an increase in the activity of cAMP and PKA, which phosphorylate lipid droplet proteins, including HSL and periplasmic proteins, thereby activating ATGL, and ultimately hydrolysis of the triglycerides stored in lipid droplets to FFAs; as antagonism of A_2AR may increase D2R activity, thereby suppressing appetite, caffeine intake increases plasma serotonin levels and decreases plasma gastric hunger hormone levels, thereby reducing food intake. Caffeine acts on the SNS, and NE, which is released from SNS endings, binds to β-ARs. The latter couples with G proteins that activate AC, increasing the cellular concentration of cAMP, which in turn activates PKA. Caffeine is an inhibitor of PDE, which prevents catabolism of cAMP; caffeine regulates the expression of adipose-metabolism-related enzymes such as HSL in 3T3-L1 cells, inhibits the expression of lipogenic transcription factor (C/EBPα and PPARγ), and inhibits 3T3-L1 cell differentiation, and reduces fat formation, thereby reducing fat accumulation through the AMPK pathway. Caffeine can increase the levels of GABA, which is known to promote the production of PGC-1α and UCP1, thereby inhibiting hepatic triglyceride levels. FFAs released from lipolysis activate existing UCP1 in mitochondria and their oxidation generates heat. As a calcium mobilizer, caffeine mobilizes intracellular Ca²⁺, elevated Ca²⁺ levels circulate and stimulate UCP1-independent thermogenesis in beige adipocytes through activation of the SERCA2B-RyR pathway, and secondly, Ca²⁺ is able to cause an increase in p38 MAPK and ATF-2 through its dependent kinases and phosphatases, such as CAMK increased phosphorylation, leading to PGC-1α expression and thus increased thermogenesis. Caffeine leads to alterations in the gut microbiota, decreasing the levels of Firmicutes and Bacteroidetes and increasing the levels of Bifidobacterium; caffeine may activate LXRα thereby decreasing the expression of NPC1L1 in the gut, leading to a decrease in cholesterol absorption. B The chemical structure of caffeine, NE, and cAMP. A1R and A_2AR, adenosine receptors; cAMP, cyclic adenosine phosphate; AC, adenylate cyclase; PKA, protein kinase A; HSL, hormone-sensitive lipase; ATGL, adipose triglyceride lipase; FFAs, free fatty acids; D2R, dopamine D2 receptors; SNS, sympathetic nerves; NE, norepinephrine; β-ARs, β-adrenergic receptors; PDE, phosphodiesterase; CAMK, calcineurin-regulated kinase; C/EBPα, CCAAT/enhancer-binding protein; PPARγ, peroxisome proliferator-activated receptor γ; AMPK, AMP-activated protein kinase; GABA, γ-aminobutyric acid; PGC-1α, peroxisome proliferator-activated receptor γ coactivator-1α; UCP1, uncoupling protein 1; SERCA2B, sarcoplasmic/endoplasmic reticulum calcium ATPase 2B; RyR, ryanodine receptor 2; p38 MAPK, p38 mitogen-activated protein kinase; ATF-2, activating transcription factor-2; CAMK, Ca²⁺/calmodulin-dependent protein kinase; LXRα, liver X receptor α; NPC1L1, Niemann-Pick C1-like 1

Meng Wang

is a master’s student at Chengdu University of Traditional Chinese Medicine. She currently focuses on the study of neuro-endocrine-immune mechanisms regulated by acupuncture.

Author contribution

Meng Wang conceived and wrote the article. Wei Guo reviewed the content and edited the manuscript. Jiang-Fan Chen provided supervision on the article. All authors critically read and commented on the final manuscript.

Funding

This study was funded by the National Natural Science Foundation of China (Grant No. 82151308 to J.-F.C.); the Research Fund for International Senior Scientists (Grant No. 82150710558 to J.-F.C); Pro-gram Project from the State Key Laboratory of Ophthalmology, Optometry and Vision Science, Wenzhou Medical University (Grant No. J01-20190101 to J.-F.C); and Key Research Project (Grant No. 2023C03079 to J.-F.C) from Zhejiang Provincial Administration of Science & Technology.

Data availability

No new data were created or analyzed in this study. All data referenced in this article are obtained from previously published studies, which are cited in the References section.

Compliance with ethical standards

Competing interests

The authors declare no competing interests.

Ethical approval

This article does not contain any studies with human participants or animals performed by any of the authors. 

Informed consent

Not applicable.