Caffeine as a Factor Influencing the Functioning of the Human Body—Friend or Foe?

1 Student Research Club, “Biomarkers in Medical Diagnostics”, Department of Laboratory Diagnostics, Division of Laboratory Diagnostics, Faculty of Pharmacy, Wroclaw Medical University, Borowska Street 211A, 50-556 Wroclaw, Poland

Izabela Kokot

2 Department of Laboratory Diagnostics, Division of Laboratory Diagnostics, Faculty of Pharmacy, Wroclaw Medical University, Borowska Street 211A, 50-556 Wroclaw, Poland; lp.corw.demu@tokok.alebazi

Ewa Maria Kratz

2 Department of Laboratory Diagnostics, Division of Laboratory Diagnostics, Faculty of Pharmacy, Wroclaw Medical University, Borowska Street 211A, 50-556 Wroclaw, Poland; lp.corw.demu@tokok.alebazi

1 Student Research Club, “Biomarkers in Medical Diagnostics”, Department of Laboratory Diagnostics, Division of Laboratory Diagnostics, Faculty of Pharmacy, Wroclaw Medical University, Borowska Street 211A, 50-556 Wroclaw, Poland

2 Department of Laboratory Diagnostics, Division of Laboratory Diagnostics, Faculty of Pharmacy, Wroclaw Medical University, Borowska Street 211A, 50-556 Wroclaw, Poland; lp.corw.demu@tokok.alebazi

Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

Associated Data

Abstract

Nowadays, caffeine is one of the most commonly consumed substances, which presents in many plants and products. It has both positive and negative effects on the human body, and its activity concerns a variety of systems including the central nervous system, immune system, digestive system, respiratory system, urinary tract, etc. These effects are dependent on quantity, the type of product in which caffeine is contained, and also on the individual differences among people (sex, age, diet etc.). The main aim of this review was to collect, present, and analyze the available information including the latest discoveries on the impact of caffeine on human health and the functioning of human body systems, taking into account the role of caffeine in individual disease entities. We present both the positive and negative sides of caffeine consumption and the healing properties of this purine alkaloid in diseases such as asthma, Parkinson’s disease, and others, not forgetting about the negative effects of excess caffeine (e.g., in people with hypertension, children, adolescents, and the elderly). In summary, we can conclude, however, that caffeine has a multi-directional influence on various organs of the human body, and because of its anti-oxidative properties, it was, and still is, an interesting topic for research studies including those aimed at developing new therapeutic strategies.

1. Introduction

Caffeine is one of the most popular and widely consumed beverages in the world. Its main source is coffee, however, it may also be present in other plants such as tea leaves, guarana berries, and cacao beans. It is worth pointing out that caffeine may also be found in energy drinks, soft drinks, gums, and medications [1,2]. Chawla et al. [3] stated that average caffeine consumption from all sources reaches 76 mg/person/day; in the United States and Canada, it is approximately 210–238 mg/person/day and exceeds 400 mg/person/day in Sweden and Finland. On the other hand, Conway [4] reported that in 2020–2021, coffee consumption was around 166.63 million 60 kg bags worldwide. This huge interest in coffee is due to its taste attributes and stimulating effects, and it additionally stems from the culture and habits of people in many countries. Due to the times and culture we live in, the presence of coffee rituals in our daily lives does not seem particularly surprising. However, it is amazing that a humble Yemeni bush has wrapped its branches around all continents. Interestingly, despite such a large consumption of caffeine, research shows that it is more a habit than a compulsive addiction [5]. Recently, caffeine has been of scientific interest because, as a bioactive molecule, it has been shown to have beneficial health effects (e.g., it is able to protect against oxidative stress in Alzheimer’s disease (AD)) [6]. However, people with hypertension, children, adolescents, and the elderly may be more susceptible to the negative effects of caffeine consumption [7]. Due to the widespread presence of caffeine, we present its impact (both positive and negative) on the human body, taking into account individual systems.

1.1. Caffeine—General Information

Caffeine (1,3,7-trimethylcanthine or 3,7-dihydro-1,3,7-trimethyl-1H-purine-2,6-dione), a well-known purine alkaloid, was described by Gennaro [8] as a white, odorless powder with a slightly bitter taste. Its chemical formula is C₈H₁₀N₄O₂. Caffeine occurs in more than 60 plant species globally [9]. This substance is produced by extraction from green coffee beans, tea leaves, and cola nuts, and also by synthetic procedures (e.g., methylation of various xanthines and theophylline) [8]. The most popular plants and products containing caffeine are shown in Figure 1 .

The most popular plants and products containing caffeine. Based on de Mejia et al. [10].

1.2. Metabolism of Caffeine

In the human body, caffeine is rapidly absorbed by the small intestine after oral administration into the body within 45 min and its average peak value occurs at 30 min [11], which directly depends on pH [8] and may be prolonged by food intake [12]. Its metabolic half-life is 3–5 h [13] and it readily penetrates the blood–brain barrier [12]. The first step in caffeine’s biotransformation is mediated by hepatic microsomal enzymes—selective catalysis by cytochrome P450PA in human liver microsomes [8]. Caffeine is primarily metabolized in the liver via the isoenzyme CYP1A2 (in about 80%), which causes its 3-demethylation to major metabolite, which is 1,7-dimethylxanthine (paraxanthine). Moreover, caffeine itself may increase CYP1A2 activity [14], and this isoenzyme is also responsible for the 1- and 7-demethylation of caffeine to 3,7-dimethylxanthine (theobromine) and 1,3-dimethylxanthine (theophylline), as shown in Figure 2 . These metabolites may be further primarily demethylated via CYP1A2, then acetylated via N-acetyltransferase 2, and oxidized via xanthine oxidase or CYP3A4 to yield major metabolites that are excreted primarily in the urine including 1-methyluric acid, 5-acetylamino-6-formylamino-3-methyluracil, 1-methylxanthine (e.g., after further demethylation of paraxanthine via CYP1A2), 1,7-dimethyluric acid, and 1,7-dimethylxanthine (paraxanthine) [14]. A low percentage (0.5–4.0%) of an ingested dose of caffeine is excreted unchanged in urine and in bile and is also found in saliva, semen, and breast milk [8].

The main pathways of caffeine metabolism in the liver.

1.3. Genetics and Caffeine

Individual variation in response to caffeine consumption is connected with genetic aspects. There are two genes especially linked with caffeine metabolism—CYP1A2 and ADORA2A [15]. The CYP1A2 gene, which codes CYP1A2, is mainly responsible for caffeine metabolism, as above-mentioned. A single nucleotide polymorphism (SNP) (–163 C > A, rs762551) in intron 1 is considered responsible for individual differences in caffeine biotransformation [16]. Sachse et al. [16] determined that there is a homozygous variant A (AA)—“fast metabolizers”, a heterozygous variant (CA), and a homozygous variant C (CC)—“slow metabolizers”. Womack et al. [17] examined 35 male cyclists (16 AA homozygotes and 19 C allele carriers) and showed that there was a significantly greater performance improvement among men with AA genotypes. On the other hand, Pataky et al. [18] reported that athletes with the C allele had a better response to caffeine. Overall, most studies found that response to caffeine was not associated with CYP1A2—163 C > A polymorphism [19,20,21].

Gene ADORA2A encodes adenosine receptor A_2AR, which plays a role in caffeine metabolism [22]. A 1976 T > C (rs5751876) SNP in the ADORA2A categorized people in TT—“high responders to caffeine” and CC/CT—“low responders to caffeine” [23]. Loy et al. [24] reported that TT athletes had higher improvements in cycling performance than C allele carriers. On the other hand, Carswell et al. [23] found that there were no differences in performance between TT and CT/CC genotypes. The results of the above investigations lead to the conclusion that further research into the influence of genetics on caffeine metabolism is needed. Future research should also be focused on detailed determination of which genes may affect the caffeine metabolism.

1.4. Effects on Receptors

1.5. Toxicity of Caffeine

A single dose consumption of 200 mg of caffeine, or less, by healthy people without comorbidities and pharmacokinetic disturbances, is usually not associated with toxic effects [31]. However, a dose above 300 mg at once can cause caffeine intoxication, the symptoms of which are mainly related to its stimulating effect. The most common ones are: restlessness, nervousness, excitement, insomnia, facial flushing, increased urination, gastrointestinal disorders, muscle tremors, chaotic flow of thoughts and speech, irritability, arrhythmia, tachycardia, and psychomotor agitation. The severity of the undesirable effects of caffeine consumption is dose dependent [32,33]. The threshold of caffeine toxicity appears to be about 400 mg/day in healthy adults (19 years or older), 100 mg/day in healthy adolescents (12–18 years old), and 2.5 mg/kg/day in healthy children (less than 12 years old) [34,35].

1.6. Adenosine

Caffeine is so close in structure to adenosine that it is able to bind to the receptors that are specific to adenosine, which plays an important role in understanding how caffeine acts in the human body. Adenosine is an endogenous purine nucleoside, which is able to modulate the release of excitotoxic mediators, limit calcium influx, hyperpolarize neurons, and exert modulatory effects on glial cells when high concentrations of this nucleoside are observed [36]. Enhanced nerve activity, hypoxia, ischemia, or central nervous system damage may increase its level from 30–300 nM (physiological conditions) to 10 μM or even higher [37]. Adenosine binds to specific receptors expressed on the cell surface—A₁R, A_2AR, A_2BR, A₃R, which are members of G protein-coupled family receptors [38]. The A₁ subtype is mainly localized in the brain, spinal cord, eye, adrenal gland, heart, and to a lesser extent in tissues such as skeletal muscle and adipose, while the A_2A subtype is mainly localized in the spleen, thymus, striatopallidal GABAergic neurons and to a lesser extent in the heart, lung, and blood vessels [39]. Caffeine causes most of its biological effects via antagonizing all types of ARs: A₁, A_2A, A₃, and A_2B and, similar to adenosine, exerts effects on neurons and glial cells of all brain areas. As a consequence, caffeine, when acting as a nonselective AR antagonist, is doing the opposite of the activation of adenosine receptors due to the removal of endogenous adenosinergic tone [25,40,41]. The similarities between the caffeine and adenosine chemical structures are shown in Figure 3 .

The comparison of caffeine and adenosine chemical structures.

2. The Role of Caffeine in Various Systems in the Human Body

2.1. Central Nervous System

Caffeine has multiple targets in the brain such as adenosine, ryanodine, γ-aminobutyric acid receptors, and cyclic nucleotide phosphodiesterase isoenzymes. Its action on A_2ARs may explain the psychomotor stimulant effect, mediated by dopaminergic mechanisms. Caffeine, through antagonism of ARs, affects brain functions such as sleep, cognition, learning, and memory, and modifies brain dysfunctions and diseases: Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, epilepsy, pain/migraine, depression, and schizophrenia [25]. Another possible mechanism of caffeine’s action on the nervous system is inhibition of the neurotransmitter acetylcholinesterase (AChE) [7]. This may be an important discovery for many studies on the effects of caffeine, clinically proven, but with unknown mechanisms of action. At low doses (

2.1.1. Caffeine Impact on Children and Adolescent

Caffeine intake has an impact not only on adults but also on children. In a subsequent study, Watson et al. [44] examined 309 Australian children (aged 8–12 years) on the basis of an interview conducted from the parents containing information about the consumption of caffeine (mostly in coffee, tea, and sodas) from 0 to 151 mg per day. This study showed that increased age, higher puberty scores, and higher morning tiredness values were associated with increased caffeine consumption. Otherwise, decreased total caffeine consumption was correlated with better internalization and decreased total behavior problems. Moreover, higher caffeine consumption was associated with a worse sleep routine, morning tiredness, and restless sleep. In another study, Richards and Smith [45] aimed to examine the connection between caffeine consumption and stress, anxiety, and depression in 3071 children from three academies in the southwest of England (aged 11–17 years). Participants were asked to complete a questionnaire about common dietary and caffeinated drink intake (total and separately) and well-being. Data analysis (made after adjusting additional dietary, demographic, lifestyle covariates, and sex) showed that caffeine consumption may be associated with stress, anxiety, and depression in secondary school children. Observed effects differed between men and women—effects relating to depression were strongest in females. Authors also reported that high caffeine intake (>1000 mg/week) was a risk factor associated with anxiety and depression in both sexes [45].

2.1.2. Caffeine and Taste Perception

Caffeine can also affect taste perception. Circulating adenosine intensifies the sweet-tasting signals in the taste buds [46]. Caffeine blocks adenosine receptors [47] and negatively affects the perception of sweet tastes [48]. The bitter taste of caffeine may have an influence on how the human body reacts. The results of some studies suggest that bitter products can enhance performance and also give the signal to our brain that the organism is ready for action [49]. These aspects are still to be examined, and seem to be potentially important for research in the field of sports medicine.

Gramling et al. [50] examined 12 males and 16 females (both caffeine consumers and non-consumers) using functional magnetic resonance imaging (fMRI), and showed differences in blood-oxygenation-level-dependent activation. The study was based on hedonic evaluation of caffeine, sucrose, or saccharin. During the hedonic evaluation of caffeine or sucrose, caffeine non-consumers had greater activation in neuronal areas associated with memory and reward, while during the hedonic evaluation of saccharin, greater activation in neuronal areas associated with memory, reward, and information processing occurred in the group of caffeine consumers. The results of the above studies support the observations on differential memory, reward, and information processing of taste between those who habitually consume caffeine and those who do not [50].

2.1.3. Caffeine and Alzheimer’s Disease

Alzheimer’s disease is the reason for 50–70% of neurodegenerative dementia cases and is characterized by a progressive decline in cognitive function, and pathologically by loss of synaptic integrity and neurons, amyloid plaques composed of amyloid beta (Aβ), and neuronal tangles composed of hyperphosphorylated tau protein [47,51]. Li et al. [52] described a mechanism whereby caffeine protects against Aβ generation. This mechanism includes suppression of LDL cholesterol-enhanced amyloidogenic processing of amyloid beta protein precursor (AβPP) by blocking AβPP internalization via its actions on adenosine receptors—A₃Rs. Additionally, neuronal cell cultures with Aβ in the presence of an A_2AR antagonist completely prevented Aβ-induced neurotoxicity [53].

The neuromodulatory effects mediated by caffeine rely on a balanced activation of the inhibitory A₁R and excitatory A_2AR receptors [54]. There are also premises that caffeine may have a neuromodulatory effect via receptors RyRs and inhibition of PDEs, and this applies to all doses of caffeine within the range of normal, habitual consumption, so the only molecular targets for caffeine at nontoxic doses are the adenosine receptors in the brain, especially the inhibitory A₁R and the faciliatory A_2AR [55]. Upregulation of adenosine receptors A₁R and A_2AR is observed in AD. Activation of ARs affects synaptic neurotransmission and the release of various neurotransmitters (acetylcholine, glutamate) [53].

The consumption of some caffeinated drinks, and caffeine itself, is correlated with lower risk of AD and dementia. The neuroprotective effect of caffeine has been demonstrated by Maia et al. [56] in a study among 54 patients with AD who consumed 73.9 ± 97.9 mg/day of caffeine during the 20 years before diagnosis of AD in relation to individuals without AD who consumed 198.7 ± 135.7 mg/day during the corresponding 20 years of their lifetimes. Authors showed that caffeine consumption was associated with lower risk for AD. Kolahdouzan and Hamadeh [6] also stated that caffeine is protective in AD and the dosages of caffeine at which this effect is noticeable is 3–5 mg/kg body weight. According to Ritchie et al. [57], consumption of at least three cups of coffee per day is associated with less decline in verbal memory. Eskelinen et al. [58] also studied the association between caffeine intake and AD and dementia. A study among 1409 subjects, aged 50 years, showed that midlife coffee drinking reduced risk of dementia and AD by 62–70% in people who drank 3–5 cups of coffee per day, compared to low coffee consumers (0–2 cups). However, it was also shown that tea drinking is not associated with risk of dementia/AD. Other studies by Lindsay et al. [59] in the Canadian community of 10,236 participants over the age of 65 years stated that coffee consumption was correlated with reduced risk (31%) of AD in the Canadian population. Overall, it should be emphasized that active constituents of coffee or tea, other than caffeine, may contribute to their effects on cognition/AD risk [59]. Recently, Dong et al. [60] investigated the association of intake of coffee (caffeinated and decaffeinated) and caffeine derived from caffeinated coffee with cognitive performance in older adults. They analyzed data from the National Health and Nutrition Examination Survey (NHANES) 2011–2014 that included a total of 2513 participants aged 60 years or older. The information on coffee and caffeine consumption were received from two 24-h dietary recalls, whereas cognitive performance was evaluated by the Consortium to Establish a Registry for Alzheimer’s Disease (CERAD) test, Animal Fluency test and Digit Symbol Substitution Test (DSST). The authors observed no significant association between decaffeinated coffee intake and various dimensions of cognitive performance. It was suggested that caffeinated coffee and caffeine from coffee were associated with cognitive performance, while decaffeinated coffee was not associated with them [60]. Confirmation of the above findings was a placebo-controlled study involving sixty elderly people that showed that there was no significant association between decaffeinated coffee and cognitive functions [61]. In contrast, in a randomized, placebo-controlled study provided by Haskell-Ramsay et al. [62], it was shown that decaffeinated coffee may have a protective effect on cognitive performance. It should be noted, however, that the final analysis was carried out on a relatively small number of male and female participants: older (61–80 years, N = 30) and young (20–34 years, N = 29).

The results of studies on the effect of caffeine on the nervous system and cognitive functions are very promising, thus it would be justified to include this substance in the algorithm of management in people exposed to neurodegenerative diseases. However, taking into account the above data, a detailed study on the effects of caffeine and caffeine-containing products must first be carried out, as these products can contain many various bioactive ingredients that can alter the primary effect of caffeine.

2.1.4. Caffeine and Parkinson’s Disease

Parkinson’s disease (PD) is a neurodegenerative disease with motor and non-motor symptoms [63]. Some studies reported that caffeine intake is beneficial for PD patients, similarly to Alzheimer’s disease. First of all, caffeine intake is associated with lower risk of Parkinson’s disease. Ross et al. [64] analyzed data on 8004 Japanese-American men (aged 45–68 years) drinking at least 28 ounces (421 mg of caffeine) of coffee, who were 5-times less exposed to PD, and this risk decreased as coffee consumption increased. Another study by Liu et al. [65] examining 187,499 men and 130,761 women showed that higher caffeinated coffee consumption was associated with lower PD risk in a dose-dependent manner, but consumption of other caffeine-containing beverages (soft drinks, hot tea, and iced tea) was not associated with the risk of PD. Moreover, Palacios et al. [66], after adjustment for age, smoking, and alcohol intake showed that the association of caffeine consumption with lower risk of PD was stronger for men compared to woman. These findings may be explained by the combined effects of caffeine and other coffee substances and should be further investigated. Although caffeine may reduce the risk of Parkinson’s disease, its intake does not improve the motor symptoms of the disease, as postulated by Postuma et al. [67], who treated 60 PD patients with caffeine and 61 with placebo. Patients with PD ongoing for 1–8 years consumed 200 mg of caffeine twice a day through 6–18 months. The authors found that caffeine does not influence sustained motor improvement in PD [67]. Caffeine not only reduces risk of PD, but may also be a promising diagnostic biomarker for early PD. Fujimaki et al. [63] examined 108 patients with PD and 31 age-matched healthy controls by using liquid chromatography-mass spectrometry to measure levels of caffeine and its 11 metabolites in serum. The authors showed that the absolute serum concentrations of caffeine and nine of its downstream metabolites were significantly lower in patients with PD than in healthy controls [63]. The above observations show new possibilities for laboratory diagnostics, but more research is needed for confirmation of those observations and complete evaluation of these findings.

There are also indications that caffeine may be an adjuvant in the treatment of PD. It was hypothesized that caffeine may increase excitatory activity in local areas by inhibiting astrocytic inflammatory processes, but evidence remains inconclusive. Roshan et al. [68] suggested that the co-administration of caffeine with currently available PD drugs helps to reduce drug tolerance and increases the effectiveness of the drug action. Despite the positive aspects, caffeine in combination with other substances can sometimes have a negative effect. Simon et al. [69] examined 1549 patients with early PD who completed a caffeine intake questionnaire. Half of the examined subjects were treated with creatine (10 g per day). Studies showed that higher caffeine intake was associated with significantly faster PD progression among subjects taking creatine [69], most probably caused by the caffeine action, which completely negates the effect of creatine on muscle contractions [70,71], potentially by counteracting the creatine-associated facilitation of calcium uptake by the sarcoplasmic reticulum [71].

2.1.5. Caffeine and Huntington’s Disease

Huntington’s disease (HD), an inherited neurodegenerative disorder caused by expanded CAG (cytosine, adenine, guanine) repeats, is characterized by motor, cognitive, and psychiatric disturbances [72]. HD is caused by a mutation in the IT15 gene that encodes for the protein huntingtin (Htt), which is widely distributed in the central nervous system, but to date, the exact cellular function of Htt is still not completely understood [73,74]. Htt is involved in many physiological functions including brain-derived neurotrophic factor expression/transport. When the Htt protein becomes mutated (mHtt), not only is normal Htt function impaired, but several mechanisms important for neuronal activity and survival become impaired, leading to a gain of function that can be toxic to the cells [75]. It is not clear whether HD is a prion-like disorder comparable to Alzheimer’s or Parkinson’s diseases, but experimental data suggests that mHtt triggers mis-conformation of wild-type Htt, and neuropathological observations in patients who received intracerebral allografts support the transfer of HD pathology from cell to cell [76].

In their review, Blum et al. [74] showed that many genetic, epidemiological, and experimental studies suggest that adenosine receptors, both A₁R and A_2AR, are linked to HD pathophysiology, although their exact involvement remains unclear. Additional investigations are needed to dissect the pre- and postsynaptic aspects of A_2AR, and the relationship between A_2AR and mHtt induced glial dysfunction, which has been largely underestimated. Since HD is a chronically progressive disease, there are multiple mechanisms along the degenerative process that may be affected by their interactions with A_2AR. The role of A₁R in HD pathogenesis also needs to be reconsidered.

Simonin et al. [72] assessed caffeine consumption in 80 patients with HD and it turned out that among people who consumed a caffeine dose greater than 190 mg per day, the risk of earlier HD was higher. As ARs, especially A₁R and A_2AR, are known targets for caffeine, the possible mechanism of caffeine action in HD pathophysiology may be associated with its activity as a nonselective AR antagonist. Chronic caffeine intake may reduce adenosine A_2AR activity [77]. In the 3-NP model of Huntington’s disease, A_2AR antagonism has been associated with worsening signs [78]. However, Tanner et al. [79], based on their study results, concluded that only caffeinated soda, but not other caffeinated beverages, was associated with Huntington’s disease risk, nor was a combined caffeine dose associated, but this finding may be spurious, or not related to caffeine.

2.1.6. Caffeine and Perception of Pain

Caffeine is a constituent of many over-the-counter pain relievers and prescription drugs because the vasoconstricting and anti-inflammatory effects of caffeine act as a compliment to analgesics, in some cases, increasing the effectiveness of pain relievers by up to 40% [80]. Studies conducted on 62 people (aged 19–77 years) by Overstreet et al. [81] showed that habitual dietary caffeine consumption was associated with a higher pain threshold, higher heat pain tolerance, and higher pressure pain threshold. The mechanism by which caffeine can reduce pain sensation appears to be closely related to its direct effects on adenosine receptors, especially through the central blocking of those receptors that influence pain signaling or block peripheral adenosine receptors on sensory afferents. The antagonism of adenosine receptors as well as the inhibition of cyclooxygenase activity in some places may explain the anti-nociceptive effect of caffeine and its supportive effect [82,83,84,85,86,87]. This is the reason why low-dose caffeine is present as an adjuvant in conjunction with antidepressants, acetaminophen, and non-steroidal anti-inflammatory drugs in many over-the-counter (OTC) pain medications [88,89].

2.1.7. Caffeine and Mental Health

Depression, anxiety, and suicide are becoming an increasingly common problem among children and adults. This prompts not only the search for new therapies, but also for the causes of these ailments more closely. Over the years, caffeine has been investigated as a potential protective or risk factor for psychiatric disorders [90]. It has already been documented that caffeine intake is associated with depressive symptoms. Lucas et al. [91] indicated that the risk of depression is associated with the dose of consumed caffeine. They showed that people who drank more than two cups of caffeinated coffee per day were 24% less exposed to depression than those who did not drink coffee. In addition, Richards and Smith [45] found that the incidence of depression decreased as the dose of caffeine intake increased, but they did not verify the differences between men and women. Botella et al. [92] investigated the effect of caffeine on depression in 2307 students aged 11–17 and showed that consuming less than 1000 mg of caffeine per week increased the risk of depression in girls, but not in boys. This conclusion has been confirmed by Iranpour and Sabour [93], who analyzed data derived from 4737 adults and found that women who consumed more caffeine had a lower risk of depression.

Kendler et al. [94] analyzed data from the Virginia Twin Registry of about 3600 adult twins and reported that the risk of developing anxiety increased after consumption of >6 cups of caffeinated coffee per day. Additionally, Bertasi et al. [95], in their study among 114 college students, showed that high caffeine intake is associated with higher levels of anxiety. Botella et al. [4] examined 3323 students (11–17 years old) and showed that the effect of caffeine on anxiety is significant mainly in boys—the anxiety increased with increased dose of caffeine intake. According to this, Hedstrom et al. [96] obtained similar results among 39 men and 60 women aged 18–31 years, and reported that at the same doses of caffeine consumption, men had a higher anxiety state than women.

Some studies have examined the effect of a single instance of caffeine intake on neurocognitive performance. Konishi et al. [97] investigated the effect on driving performance in 100 healthy Japanese volunteers (50 males, 50 females, aged 22–59 years) with a valid driver’s license. In this double-blinded, randomized, placebo-controlled study, half of the participants formed a caffeine intake group and the other a placebo group. Individuals did not consume caffeine for three days before the examination, then the “caffeine group” was given 200 mg of caffeine. Thirty minutes after administration, cognitive functions were evaluated via the Symbol Digit Coding Test (SDC), the Stroop Test (ST), the Shifting Attention Test (SAT), and the Four Part Continuous Performance Test (FPCPT). After these tests, the authors checked driving performance (brake reaction time and standard deviation of the lateral position) using a driving simulator. Research showed that the “caffeine group” had more appropriate responses than the placebo group on the SAT, made less mistakes, and had shorter times in the brake reaction time test. Studies conducted by Yu et al. [98] using long-term self-renewing neuroepithelial stem cells showed that consumption of caffeine (3 μM and 10 μM) activates immediate early genes after 1 h, while neuronal projection development processes were upregulated and negative regulation of axon extension processes were downregulated at 3 h.

It is known that caffeinated coffee consumption may be associated with a lower risk of suicide, depending on the amount of coffee consumed daily [99,100]. Lucas et al. [91] analyzed three prospective cohorts of American adults consisting of 43,599 men and 164,825 women, among which caffeine consumption was tested every four years. The authors documented 277 deaths from suicide and showed correlations between caffeine intake and deaths as an inverse relationship [91]. However, in a Finnish population study [101] of over 43,000 people who were followed for an average of 14.6 years, a J-shaped relation was found between daily coffee drinking and the risk of suicide. Compared to those who drank one cup of coffee per day, the risk of suicide was lower with moderate coffee consumption (2–3 cups per day to 6–7 cups per day), but increased with higher consumption (8–9 and 10 cups per day). The main effects and probable mechanisms of caffeine action on the nervous system are shown in Table 1 .

Table 1

The main effects of caffeine action on the nervous system and the most likely mechanisms of its action.

Aβ—amyloid beta, Ars—adenosine receptors, AChE—acetylcholinesterase, RyRs—ryanodine receptors. ↓—decrease, ↑—increase.

In conclusion, caffeine action in a variety of central nervous system diseases and disturbances is multi-directive, as caffeine has multiple targets in the brain and affects many brain functions such as sleep, cognition, learning, and memory, while on the other hand modifying brain dysfunctions and diseases such as Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, epilepsy, pain, and depression. The effect of caffeine depends mainly on the amount of the substance. At low concentrations, it has a positive effect on the human brain, but higher doses may be responsible for negative effects in mood and behavior. In some diseases, caffeine consumption in high doses may be a risk factor (AD, PD, Huntington’s disease), however, on the other hand, thanks to its therapeutic properties and ability to antagonize ARs, caffeine may be useful in the treatment of PD and pain symptoms or, in connection with some other medications, may delay the progression of the disease. The utility of caffeine and its metabolites in laboratory diagnostics as markers of certain diseases (e.g., PD) also seems promising.

2.2. The Immune System

The immune system is complex and is made up of various cells and proteins that are responsible for the body’s resistance, particularly through the immune response, which is a weapon against foreign antigens [103]. More and more attention is paid to the two-way relationship between the immune system and diet [104]. The role of caffeine in the functioning of the immune system reaches the cells themselves. Gibbs et al. [105] reported that caffeine in high concentrations (5 mM) inhibits the mammalian target of rapamycin (mTOR) in human myeloid leukemia cells, primary human acute myeloid leukemia cells, and primary human basophils, and affects glycolysis and the release of pro-inflammatory cytokines. Additionally, in monocytes, the effect of caffeine was potentiated by its ability to inhibit xanthine oxidase (purine catabolism), and in basophiles, caffeine increased intercellular cAMP levels [105].

Adenosine receptors are expressed on monocytes and macrophages and through these receptors, ligands (e.g., adenosine, caffeine) can modulate monocyte and macrophage function [38]. Chavez-Valdez et al. [106] used cord blood monocytes (CBM) from 28 full-term infants (>37 weeks’ gestation at birth) to investigate changes in the ARs mRNA profile (by qRT-PCR) and protein expression (by western blot) after in vitro culture, caffeine, or lipopolysaccharide (LPS) exposure, and the modulation of cytokine release and cAMP production (by ELISA) induced by caffeine and ARs antagonists. After 48 h in culture, it turned out that caffeine (50 μM) decreased tumor necrosis factor-alpha (TNF-α) from LPS activated-CBM by 20% and TNF-α gene expression by 30%, in conjunction with a minimum 2-fold increase in cAMP. This suggests that caffeine increases cAMP production and inhibits pre-transcriptional TNF-α production by CBM via A₁R blockade. These observations were later confirmed by Chavez-Valdez et al. [107], where the authors used LPS-activated CBM from 19 infants and exposed them to caffeine in various concentrations (0–200 μM) with or without previous exposure to ARs antagonists, and showed that caffeine at ≤100 μM reduced TNF-α levels by 25% and IL-10 levels by 17–25%. The results of the same study confirmed that caffeine concentration is directly positively correlated to toll-like receptor 4 (TLR4) gene expression, and therefore it promotes inflammation [107]. Steck et al. [108] demonstrated that caffeine may act as a phosphodiesterase inhibitor, suppressing phagocytosis in mononuclear phagocytes by promoting an anti-inflammatory response. Although the obtained results were promising, in the human body, it is a complex and dynamic process that involves many pathways that may contradict each other, resulting in inconsistent results dependent on dosage of caffeine and state of the exposed cell. In their study, Shushtari et al. [109] used mesenchymal stem cells (MSCs) isolated from bone marrow and treated them with different concentrations of caffeine (0, 0.1, 0.5, and 1 mM) for 72 h, out of which 24 h consisted of incubation with macrophages. The authors observed that MSCs treated with caffeine enhanced phagocytosis and the expression of reactive oxygen species, nitric oxide, and IL-12 by macrophages. These results proved that caffeine augments the instruction of anti-inflammatory macrophages and showed the potential mechanism of the immunomodulatory and anti-inflammatory effects of caffeine [109]. However, the presented results indicate the therapeutic properties of caffeine, high doses of caffeine necessary to obtain certain effects, may be toxic to the human body.

In summary, caffeine reaches cells by interaction with ARs, which are expressed on them. Its anti-inflammatory effect is important in the functioning of the immune system, however, more attention is required to the potential use of caffeine as a therapeutic agent in hematological diseases related to the malfunction of the immune system.

2.3. Digestive System

Caffeinated coffee consumption is one of the causes of gastrointestinal discomfort reported by patients as well as digestive system problems noted by doctors. The main pharmacologically active substance in coffee is caffeine, which may increase gastric acid secretion [8], relax smooth muscles by increasing gastrin concentration [110], and stimulate the secretion of hydrochloric acid [111], causing higher risk of inflammation of the intestinal mucosa and stomach. On the other hand, caffeine is said to be antioxidant and to have anti-inflammatory activity [112], thanks to which caffeine can reduce alanine aminotransferase (ALT), aspartate aminotransferase (AST), and bilirubin level in serum [112]. There is also a connection between coffee consumption and γ-glutamyltransferase (GGT) activity—together with higher coffee consumption, lower GGT activity was observed [113]. It should be noted, however, that coffee ingredients may disturb iron absorption [114], and zinc’s bioavailability [115]. It is suggested that coffee and its compounds may also affect intestinal microbiota, especially Bacteroides, and increase its level [116]. The effects of caffeine action on the digestive system are schematically shown in Figure 4 .

The scheme of the main effects of caffeine action on the digestive system. ALT—alanine aminotransferase, AST—aspartate aminotransferase, GGT—γ-glutamyltransferase. ↓↓↓—decrease, ↑↑↑—increase.

2.3.1. Caffeine Action on the Small and Large Intestine

Caffeine has an impact on net fluid movement and transit times, although the data in this respect are not conclusive. It has been reported that caffeine ingestions (75–300 mg) caused increased net secretion in jejunum for a minimum of 15 min, and 35 min later in ileum in the same doses of caffeine [117]. The results of another study showed that caffeine affects esophageal function by decreasing the pressure on the lower esophageal sphincter, leading to its relaxation [118]. Relaxed lower esophageal sphincter may be a reason for gastric reflux [119]. Moreover, it was also documented that caffeinated coffee stimulates gallbladder contraction and colonic motor activity, but there were no connections between coffee consumption and dyspepsia [119].

It is still unclear whether and to what extent the consumption of caffeinated coffee and caffeinated products affects gastrointestinal transit time and whether this is an effect of caffeine content. Boekema et al. [119] investigated the effect of caffeinated coffee consumption on gastrointestinal motility, on gastric emptying, and oro-cecal transit time. In a randomized, controlled, cross-over study gastric emptying and oro-cecal transit time, the authors studied 12 healthy volunteers using applied potential tomography and the lactulose hydrogen breath test. The lag-phase duration after coffee intake was not significantly different from that after water (median 19.8 min vs. 19.3 min, respectively), nor was the gastric half-emptying time (median 75.7 min vs. 83.4 min, respectively). Furthermore, coffee had no significant effect on oro-cecal transit time (median 135 min vs. 140 min, respectively). No significant correlation between any of the examined parameters and mean daily caffeinated coffee intake was found, which confirms that coffee consumption does not affect gastric emptying of a liquid meal or small bowel transit. On the other hand, Rao et al. [120], who investigated the effects of caffeinated coffee on colonic motor activity in healthy humans, revealed that coffee stimulates colonic motor activity. Its magnitude was similar to a meal, 60% stronger than water, and 23% stronger than decaffeinated coffee.

2.3.2. Caffeine and the Liver

Animal models (mice and rats) have shown that caffeine may play a potential role in the stimulation of β-oxidation in hepatic cells, intrahepatic lipid content reduction, and hepatic autophagy [5,6], which suggests that caffeine ingestion suppresses inflammation and lipogenesis [121], reduces lipid peroxidation [112], and is linked with lower risk for nonalcoholic fatty liver disease [122]. Caffeine is inversely associated with the risk of cirrhosis [123] and the risk of hepatocellular carcinoma, especially among people with the hepatitis C virus and other liver diseases [124]. Modi et al. [125] examined 177 patients (among which 99 were male, 104 Caucasian, 121 with hepatitis C) who underwent liver biopsy and completed the caffeine questionnaire. The authors analyzed caffeine consumption by coffee-cup equivalents, and revealed an association between caffeine consumption and lower risk of liver fibrosis, especially in patients with HCV infection when two coffee-cup equivalents were consumed per day [125]. Li et al. [126] examined the molecular aspects of the protective role of caffeine in fibrogenesis by using a LX-2 cell line (immortalized human hepatic stellate cells) and exposing it to various concentrations of caffeine. The results of the study showed that caffeine inhibits the viability and increases the apoptosis of the LX-2 cells in a dose- and time-dependent manner by autophagy of endoplasmic reticulum, via the inositol-requiring enzyme 1α signaling pathway. In another study, Salomone et al. [117] suggested that through stimulation of the nuclear factor erythroid 2-related factor 2 (Nrf2) (which reduces oxidative stress and inflammation), peroxisome proliferator-activated receptors alfa (PPARα) (which increase β-oxidation and reduce lipid accumulation), 5’ adenosine monophosphate-activated protein kinase (AMPK) (which increases autophagy and reduces lipid accumulation), and inhibition of sterol regulatory element-binding transcription factor (SREBP1c) (which reduces de novo lipogenesis and reduces lipid accumulation) and receptor A_2AR, coffee and its components suppress liver fibrogenesis and carcinogenesis [117]. Moreover, caffeine, depending on the dose, is said to increase apolipoprotein A1 and paraoxonase-1 protein levels in liver cells [127].

2.3.3. Caffeine and Glycaemia

High doses of caffeine increase glucose tolerance [128] and decrease insulin sensitivity [129]. It has been shown that decaffeinated coffee, in contrast to caffeinated, reduces HbA_1c levels [130] used in glycaemia monitoring, which suggests that coffee contains other substances that may improve glucose metabolism, and this needs to be examined. The studies on caffeine consumption among people with type 2 diabetes do not show any correlation between decaffeinated coffee and improvement in glycemic control [130], and should be confirmed by additional research, since the results of another study have shown that frequent (at least seven cups per day) coffee consumption may reduce the risk of type 2 diabetes by 50% [131], as confirmed in many studies [132,133,134]. It has also been documented that caffeine may affect the phosphatase activity of glycogen synthase [110]. When linked with glucose, there was only slight stimulation over a wide concentration range, thus it seems that there is no synergy between these two compounds in the activation of glycogen synthase phosphatase [135].

2.3.4. Caffeine and Digestive Tract Cancer

Some studies indicate that coffee with caffeine may reduce the probability of digestive tract cancer occurrence, especially liver cancer [117,136] and colon cancer [137]. Liu et al. [114] used human gastric cancer cells in in vitro studies to show that caffeine treatment suppressed gastric cancer cell growth and viability, and activated the caspade-9/-3 pathway, which induced apoptosis. Taking the above into consideration, it seems that caffeine may be useful as a sustained anticancer agent in the therapy of gastric cancer, however, these observations should be confirmed by additional studies. The meta-analysis conducted by Li et al. [138] consisting of 25 case-control and 16 cohort studies showed that the risk of colorectal cancer was reduced by 15% for the highest coffee drinkers compared to low or non-drinkers, and the authors concluded that the risk of colon cancer was reduced by 21%. In this meta-analysis, information on the classification of subjects to the group of high, low, or non-coffee drinkers was taken by the authors from the classifications described previously in other, original articles, and thus there was no possibility of assigning unambiguous values for these groups. Tian et al. [139] reported a significant decrease in the risk of colorectal and colon cancer among subjects consuming at least four cups of coffee a day. The results of meta-analysis conducted by Sang et al. [140] showed that the risk of liver cancer for high coffee drinkers was 50% lower than for non/almost never drinkers. Additionally, Bravi et al. [141] analyzed some studies and in their meta-analysis showed that the risk of hepatocellular carcinoma was reduced by 40% for any coffee consumption versus no consumption. The authors suggested that the effect on liver enzymes and development of cirrhosis may be responsible for the protective effect of caffeine against liver carcinogenesis [141].

As documented by the many studies above-mentioned, caffeine action covers a variety of points in the food pathway, starting from the mouth, through the stomach, small intestine, and liver to the large intestine. It increases the secretion of gastric acid, decreases the activity of liver enzymes, and improves the motor functions of the intestines. Caffeine’s antioxidant and anti-inflammatory properties reduce the risk of liver disease (cirrhosis, fibrogenesis, nonalcoholic fatty liver disease, etc.) and some cancers. Additionally, caffeine is considered to be a promising anticancer agent because of its antioxidant potential, which is especially important in cancer treatment because cancer progression and metastasis are accompanied by oxidative–antioxidant imbalance.

2.4. Respiratory System

The effect of caffeine on the respiratory system is mainly described as beneficial, regardless of whether it is administered individually, as a component of drugs, or synergistically with applied therapy [142]. Caffeine acute ingestion (3 mg/kg body weight) may improve peak aerobic performance and increase peak pulmonary ventilation. The role of caffeine in this effect may be explained by its ability to affect respiratory muscle [143]. Supinski et al. [144] examined the effect in six healthy people of single, orally administered doses of caffeine (600 mg) on diaphragmatic muscle, and showed that caffeine increased trans-diaphragmatic pressure, probably by increases in muscle contractility. This suggests that caffeine may be used in the treatment of patients with respiratory muscle weakness. Another possible mechanism of caffeine action is without a doubt its influence on receptors. For example, Bruce et al. [145] showed that caffeine (200 mg) caused a decrease in exhaled nitric oxide via AR antagonism, or by altering levels of cGMP. The main effects of caffeine action in respiratory system diseases are summarized in Table 2 .

Table 2

The effects of caffeine action in respiratory system diseases.